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Commit de6dba53 authored by Jim Fulton's avatar Jim Fulton Committed by GitHub
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Merge pull request #102 from zopefoundation/merge-zodbdocs 

Merge zodbdocs
parents f4cb94e8 5d15e377
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.travis.yml
View file @ de6dba53
language: python
sudo: false
python:
- pypy
- pypy3
- 2.7
- 3.3
- 3.4
- 3.5
matrix:
include:
- os: linux
python: pypy
- os: linux
python: pypy3
env: BUILOUT_OPTIONS=sphinx:eggs=
- os: linux
python: 2.7
- os: linux
python: 3.3
- os: linux
python: 3.4
- os: linux
python: 3.5
install:
# First install a newer pip so that it can use the wheel cache
# (only needed until travis upgrades pip to 7.x; note that the 3.5
# environment uses pip 7.1 by default).
- travis_retry pip install -U pip
# A newer wheel is also needed under Python 3, but only after we have
# a newer pip to take advantage of the cache.
- travis_retry pip install -U wheel
# Then start installing our deps so they can be cached. Note that use of --build-options / --global-options / --install-options
# disables the cache.
- travis_retry pip install -U manuel zope.testing zope.testrunner
- travis_retry pip install -U -e .
- pip install zc.buildout
- buildout $BUILOUT_OPTIONS
script:
- zope-testrunner -u --test-path=src --auto-color --auto-progress
- zope-testrunner -f --test-path=src --auto-color --auto-progress
- bin/test -v1j99
- if [[ $TRAVIS_PYTHON_VERSION != 'pypy3' ]]; then cd doc; make html; fi
notifications:
email: false
# cache: pip seems not to work if `install` is replaced (https://github.com/travis-ci/travis-ci/issues/3239)
cache:
directories:
- $HOME/.cache/pip
before_cache:
- rm -f $HOME/.cache/pip/log/debug.log
- eggs
README.rst
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......@@ -16,53 +16,32 @@ use ZoneAlarm.
Compatibility
=============
ZODB 4.3 requires Python 2.7 or Python >= 3.3.
ZODB 5 requires Python 2.7 (>= 2.7.9) or Python >= 3.3.
Travis: |buildstatus|_
winbot: |winbotstatus|_
Prerequisites
Documentation
=============
You must have Python installed. If you're using a system Python
install, make sure development support is installed too.
See http://zodb-docs.readthedocs.io/en/latest/
You also need the transaction, BTrees, persistent, six, zc.lockfile, ZConfig,
zodbpickle, zope.interface packages, and optionally manuel and zope.testing.
If you don't have them and you can connect to the Python Package Index,
then these will be installed for you if you don't have them.
Installation
============
ZODB is released as a distutils package. The easiest ways to build
and install it are to use `easy_install
<http://peak.telecommunity.com/DevCenter/EasyInstall>`_, or
`zc.buildout <http://www.python.org/pypi/zc.buildout>`_.
To install by hand, first install the dependencies listed in `Prerequisites`_.
These can be found in the `Python Package Index <http://www.python.org/pypi>`_.
To run the tests, use the test setup command::
python setup.py test
It will download dependencies if needed. If this happens, ou may get
an import error when the test command gets to looking for tests. Try
running the test command a second time and you should see the tests
run.
For developers of ZODB
======================
::
Building
---------
python setup.py test
Bootstrap buildout, if necessary using ``bootstrap.py``::
To install, use the install command::
python bootstrap.py
python setup.py install
Run the buildout::
bin/buildout
Testing for Developers
======================
Testing
-------
The ZODB checkouts are `buildouts <http://www.python.org/pypi/zc.buildout>`_.
When working from a ZODB checkout, first run the bootstrap.py script
......@@ -97,6 +76,19 @@ Running all the tests takes much longer.::
OK
Generating docs
---------------
cd to the doc directory and::
make html
Contributing
------------
Almost any code change should include tests.
Any change that changes features should include documentation updates.
Maintenance scripts
-------------------
......@@ -117,32 +109,3 @@ More information
================
See http://zodb.org/
There is a Mailman mailing list in place to discuss all issues related
to ZODB. You can send questions to
zodb-dev@zope.org
or subscribe at
http://lists.zope.org/mailman/listinfo/zodb-dev
and view its archives at
http://lists.zope.org/pipermail/zodb-dev
Note that Zope Corp mailing lists have a subscriber-only posting policy.
Bugs and Patches
================
Bug reports and patches should be added to the Launchpad:
https://launchpad.net/zodb
.. |buildstatus| image:: https://api.travis-ci.org/zopefoundation/ZODB.png?branch=master
.. _buildstatus: https://travis-ci.org/zopefoundation/ZODB
.. |winbotstatus| image:: http://winbot.zope.org/buildstatusimage?builder=ZODB_dev%20py_270_win64&number=-1
.. _winbotstatus: http://winbot.zope.org/builders/ZODB_dev%20py_270_win64/builds/-1
buildout.cfg
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......@@ -3,6 +3,7 @@ develop = .
parts =
test
scripts
sphinx
[versions]
# Avoid breakage in 4.4.5:
......@@ -40,3 +41,15 @@ arguments = ('${buildout:directory}/coverage',
recipe = zc.recipe.egg
eggs = ${test:eggs}
interpreter = py
[sphinx]
recipe = zc.recipe.egg
eggs =
Sphinx
docutils
ZODB
j1m.sphinxautointerface
j1m.sphinxautozconfig
scripts =
sphinx-build
interpreter = stxpy
doc/Makefile 0 → 100644
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# Makefile for Sphinx documentation
#
# You can set these variables from the command line.
SPHINXOPTS =
SPHINXBUILD = ../bin/sphinx-build
PAPER =
# Internal variables.
PAPEROPT_a4 = -D latex_paper_size=a4
PAPEROPT_letter = -D latex_paper_size=letter
ALLSPHINXOPTS = -d build/doctrees $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
.PHONY: help clean html web pickle htmlhelp latex changes linkcheck
help:
@echo "Please use \`make <target>' where <target> is one of"
@echo " html to make standalone HTML files"
@echo " pickle to make pickle files"
@echo " json to make JSON files"
@echo " htmlhelp to make HTML files and a HTML help project"
@echo " latex to make LaTeX files, you can set PAPER=a4 or PAPER=letter"
@echo " changes to make an overview over all changed/added/deprecated items"
@echo " linkcheck to check all external links for integrity"
clean:
-rm -rf build/*
html:
mkdir -p build/html build/doctrees
$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) build/html
@echo
@echo "Build finished. The HTML pages are in build/html."
pickle:
mkdir -p build/pickle build/doctrees
$(SPHINXBUILD) -b pickle $(ALLSPHINXOPTS) build/pickle
@echo
@echo "Build finished; now you can process the pickle files."
web: pickle
json:
mkdir -p build/json build/doctrees
$(SPHINXBUILD) -b json $(ALLSPHINXOPTS) build/json
@echo
@echo "Build finished; now you can process the JSON files."
htmlhelp:
mkdir -p build/htmlhelp build/doctrees
$(SPHINXBUILD) -b htmlhelp $(ALLSPHINXOPTS) build/htmlhelp
@echo
@echo "Build finished; now you can run HTML Help Workshop with the" \
".hhp project file in build/htmlhelp."
latex:
mkdir -p build/latex build/doctrees
$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) build/latex
@echo
@echo "Build finished; the LaTeX files are in build/latex."
@echo "Run \`make all-pdf' or \`make all-ps' in that directory to" \
"run these through (pdf)latex."
changes:
mkdir -p build/changes build/doctrees
$(SPHINXBUILD) -b changes $(ALLSPHINXOPTS) build/changes
@echo
@echo "The overview file is in build/changes."
linkcheck:
mkdir -p build/linkcheck build/doctrees
$(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) build/linkcheck
@echo
@echo "Link check complete; look for any errors in the above output " \
"or in build/linkcheck/output.txt."
doc/README.rst 0 → 100644
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==================
ZODB documentation
==================
``zodbdocs`` is the source documentation for the website http://zodb.org. It
contains all ZODB relevant documentation like "ZODB/ZEO Programming Guide",
some ZODB articles and links to the ZODB release notes.
Building the documentation
--------------------------
All documentation is formatted as restructured text. To generate HTML using
Sphinx, use the following::
python bootstrap.py
./bin/buildout
make html
doc/articles/ZODB-overview.rst 0 → 100644
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An overview of the ZODB (by Laurence Rowe)
==========================================
ZODB in comparison to relational databases, transactions, scalability and best
practice. Originally delivered to the Plone Conference 2007, Naples.
Comparison to other database types
----------------------------------
**Relational Databases** are great at handling large quantities of homogenous
data. If you’re building a ledger system a Relational Database is a great fit.
But Relational Databases only support hierarchical data structures to a
limited degree. Using foreign-key relationships must refer to a single table,
so only a single type can be contained.
**Hierarchical databases** (such as LDAP or a filesystem) are much more
suitable for modelling the flexible containment hierarchies required for
content management applications. But most of these systems do not support
transactional semantics. ORMs such as `SQLAlchemy
<http://www.sqlalchemy.org>`_. make working with Relational Databases in an
object orientated manner much more pleasant. But they don’t overcome the
restrictions inherent in a relational model.
The **ZODB** is an (almost) transparent python object persistence system,
heavily influenced by Smalltalk. As an Object-Orientated Database it gives you
the flexibility to build a data model fit your application. For the most part
you don’t have to worry about persistency - you only work with python objects
and it just happens in the background.
Of course this power comes at a price. While changing the methods your classes
provide is not a problem, changing attributes can necessitate writing a
migration script, as you would with a relational schema change. With ZODB
obejcts though explicit schema migrations are not enforced, which can bite you
later.
Transactions
------------
The ZODB has a transactional support at its core. Transactions provide
concurrency control and atomicity. Transactions are executed as if they have
exclusive access to the data, so as an application developer you don’t have to
worry about threading. Of course there is nothing to prevent two simultaneous
conflicting requests, So checks are made at transaction commit time to ensure
consistency.
Since Zope 2.8 ZODB has implemented **Multi Version Concurrency Control**.
This means no more ReadConflictErrors, each transaction is guaranteed to be
able to load any object as it was when the transaction begun.
You may still see (Write) **ConflictErrors**. These can be minimised using
data structures that support conflict resolution, primarily B-Trees in the
BTrees library. These scalable data structures are used in Large Plone Folders
and many parts of Zope. One downside is that they don’t support user definable
ordering.
The hot points for ConflictErrors are the catalogue indexes. Some of the
indexes do not support conflict resolution and you will see ConflictErrors
under write-intensive loads. On solution is to defer catalogue updates using
`QueueCatalog <http://pypi.python.org/pypi/Products.QueueCatalog>`_
(`PloneQueueCatalog
<http://pypi.python.org/pypi/Products.PloneQueueCatalog>`_), which allows
indexing operations to be serialized using a seperate ZEO client. This can
bring big performance benefits as request retries are reduced, but the
downside is that index updates are no longer reflected immediately in the
application. Another alternative is to offload text indexing to a dedicated
search engine using `collective.solr
<http://pypi.python.org/pypi/collective.solr>`_.
This brings us to **Atomicity**, the other key feature of ZODB transactions. A
transaction will either succeed or fail, your data is never left in an
inconsistent state if an error occurs. This makes Zope a forgiving system to
work with.
You must though be careful with interactions with external systems. If a
ConflictError occurs Zope will attempt to replay a transaction up to three
times. Interactions with an external system should be made through a Data
Manager that participates in the transaction. If you’re talking to a database
use a Zope DA or a SQLAlchemy wrapper like `zope.sqlalchemy
<http://pypi.python.org/pypi/zope.sqlalchemy>`_.
Unfortunately the default MailHost implementation used by Plone is not
transaction aware. With it you can see duplicate emails sent. If this is a
problem use TransactionalMailHost.
Scalability Python is limited to a single CPU by the Global Interpreter Lock,
but that’s ok, ZEO lets us run multiple Zope Application servers sharing a
single database. You should run one Zope client for each processor on your
server. ZEO also lets you connect a debug session to your database at the same
time as your Zope web server, invaluable for debugging.
ZEO tends to be IO bound, so the GIL is not an issue.
ZODB also supports **partitioning**, allowing you to spread data over multiple
storages. However you should be careful about cross database references
(especially when copying and pasting between two databases) as they can be
problematic.
Another common reason to use partitioning is because the ZODB in memory cache
settings are made per database. Separating the catalogue into another storage
lets you set a higher target cache size for catalogue objects than for your
content objects. As much of the Plone interface is catalogue driven this can
have a significant performance benefit, especially on a large site.
.. image:: images/zeo-diagram.png
Storage Options
---------------
**FileStorage** is the default. Everything in one big Data.fs file, which is
essentially a transaction log. Use this unless you have a very good reason not
to.
**DirectoryStorage** (`site <http://dirstorage.sourceforge.net>`_) stores one
file per object revision. Does not require the Data.fs.index to be rebuilt on
an unclean shutdown (which can take a significant time for a large database).
Small number of users.
**RelStorage** (`pypi <http://pypi.python.org/pypi/RelStorage>`_) stores
pickles in a relational database. PostgreSQL, MySQL and Oracle are supported
and no ZEO server is required. You benefit from the faster network layers of
these database adapters. However, conflict resolution is moved to the
application server, which can be bad for worst case performance when you have
high network latency.
BDBStorage, OracleStorage, PGStorage and APE have now fallen by the wayside.
Other features
--------------
**Savepoints** (previously sub-transactions) allow fine grained error control
and objects to be garbage collected during a transaction, saving memory.
Versions are deprecated (and will be removed in ZODB 3.9). The application
layer is responsible for versioning, e.g. CMFEditions / ZopeVersionControl.
**Undo**, don’t rely on it! If your object is indexed it may prove impossible
to undo the transaction (independently) if a later transaction has changed the
same index. Undo is only performed on a single database, so if you have
separated out your catalogue it will get out of sync. Fine for undoing in
portal_skins/custom though.
**BLOBs** are new in ZODB 3.8 / Zope 2.11, bringing efficient large file
support. Great for document management applications.
**Packing** removes old revisions of objects. Similar to `Routine Vacuuming
<http://www.postgresql.org/docs/8.3/static/routine-vacuuming.html>`_ in
PostgreSQL.
Some best practice
------------------
**Don’t write on read**. Your Data.fs should not grow on a read. Beware of
setDefault and avoid inplace migration.
**Keep your code on the filesystem**. Too much stuff in the custom folder will
just lead to pain further down the track. Though this can be very convenient
for getting things done when they are needed yesterday...
**Use scalable data structures** such as BTrees. Keep your content objects
simple, add functionality with adapters and views.
doc/articles/ZODB1.rst 0 → 100644
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Introduction to the ZODB (by Michel Pelletier)
==============================================
In this article, we cover the very basics of the Zope Object
Database (ZODB) for Python programmers. This short article
documents almost everything you need to know about using this
powerful object database in Python. In a later article, I will
cover some of the more advanced features of ZODB for Python
programmers.
ZODB is a database for Python objects that comes with
`Zope <http://www.zope.org>`_. If you've ever worked with a
relational database, like PostgreSQL, MySQL, or Oracle, than you
should be familiar with the role of a database. It's a long term
or short term storage for your application data.
For many tasks, relational databases are clearly a good solution,
but sometimes relational databases don't fit well with your object
model. If you have lots of different kinds of interconnected
objects with complex relationships, and changing schemas then ZODB
might be worth giving a try.
A major feature of ZODB is transparency. You do not need to write
any code to explicitly read or write your objects to or from a
database. You just put your *persistent* objects into a container
that works just like a Python dictionary. Everything inside this
dictionary is saved in the database. This dictionary is said to
be the "root" of the database. It's like a magic bag; any Python
object that you put inside it becomes persistent.
Actually there are a few restrictions on what you can store in the
ZODB. You can store any objects that can be "pickled" into a
standard, cross-platform serial format. Objects like lists,
dictionaries, and numbers can be pickled. Objects like files,
sockets, and Python code objects, cannot be stored in the database
because they cannot be pickled. For more information on
"pickling", see the Python pickle module documentation at
http://www.python.org/doc/current/lib/module-pickle.html
A Simple Example
----------------
The first thing you need to do to start working with ZODB is to
create a "root object". This process involves first opening a
connection to a "storage", which is the actual back-end that stores
your data.
ZODB supports many pluggable storage back-ends, but for the
purposes of this article I'm going to show you how to use the
'FileStorage' back-end storage, which stores your object data in a
file. Other storages include storing objects in relational
databases, Berkeley databases, and a client to server storage that
stores objects on a remote storage server.
To set up a ZODB, you must first install it. ZODB comes with
Zope, so the easiest way to install ZODB is to install Zope and
use the ZODB that comes with your Zope installation. For those of
you who don't want all of Zope, but just ZODB, see the
instructions for downloading StandaloneZODB from the `ZODB web
page <http://www.zope.org/Wikis/ZODB/FrontPage>`_.
StandaloneZODB can be installed into your system's Python
libraries using the standard 'distutils' Python module.
After installing ZODB, you can start to experiment with it right
from the Python command line interpreter. For example, try the
following python code in your interpreter::
>>> from ZODB import FileStorage, DB
>>> storage = FileStorage.FileStorage('mydatabase.fs')
>>> db = DB(storage)
>>> connection = db.open()
>>> root = connection.root()
Here, you create storage and use the 'mydatabse.fs' file to store
the object information. Then, you create a database that uses
that storage.
Next, the database needs to be "opened" by calling the 'open()'
method. This will return a connection object to the database.
The connection object then gives you access to the 'root' of the
database with the 'root()' method.
The 'root' object is the dictionary that holds all of your
persistent objects. For example, you can store a simple list of
strings in the root object::
>>> root['employees'] = ['Mary', 'Jo', 'Bob']
Now, you have changed the persistent database by adding a new
object, but this change is so far only temporary. In order to
make the change permanent, you must commit the current
transaction::
>>> import transaction
>>> transaction.commit()
Transactions group of lots of changes in one atomic operation. In
a later article, I'll show you how this is a very powerful
feature. For now, you can think of committing transactions as
"checkpoints" where you save the changes you've made to your
objects so far. Later on, I'll show you how to abort those
changes, and how to undo them after they are committed.
Now let's find out if our data was actually saved. First close the
database connection::
>>> connection.close()
Then quit Python. Now start the Python interpreter up again, and
connect to the database you just created::
>>> from ZODB import FileStorage, DB
>>> storage = FileStorage.FileStorage('mydatabase.fs')
>>> db = DB(storage)
>>> connection = db.open()
>>> root = connection.root()
Now, let's see what's in the root::
>>> root.items()
[('employees', ['Mary', 'Jo', 'Bob'])]
There's your list. If you had used a relational database, you
would have had to issue a SQL query to save even a simple Python
list like the above example. You would have also needed some code
to convert a SQL query back into the list when you wanted to use
it again. You don't have to do any of this work when using ZODB.
Using ZODB is almost completely transparent, in fact, ZODB based
programs often look suspiciously simple!
Keep in mind that ZODB's persistent dictionary is just the tip of
the persistent iceberg. Persistent objects can have attributes
that are themselves persistent. In other words, even though you
may have only one or two "top level" persistent objects as values
in the persistent dictionary, you can still have thousands of
sub-objects below them. This is, in fact, how Zope does it. In
Zope, there is only *one* top level object that is the root
"application" object for all other objects in Zope.
Detecting Changes
-----------------
One thing that makes ZODB so easy to use is that it doesn't
require you to keep track of your changes. All you have to do is
to make changes to persistent objects and then commit a
transaction. Anything that has changed will be stored in the
database.
There is one exception to this rule when it comes to simple
mutable Python types like lists and dictionaries. If you change a
list or dictionary that is already stored in the database, then
the change will *not* take effect. Consider this example::
>>> root['employees'].append('Bill')
>>> transaction.commit()
You would expect this to work, but it doesn't. The reason for
this is that ZODB cannot detect that the 'employees' list
changed. The 'employees' list is a mutable object that does not
notify ZODB when it changes.
There are a couple of very simple ways around this problem. The
simplest is to re-assign the changed object::
>>> employees = root['employees']
>>> employees.append('Bill')
>>> root['employees'] = employees
>>> transaction.commit()
Here, you move the employees list to a local variable, change the
list, and then *reassign* the list back into the database and
commit the transaction. This reassignment notifies the database
that the list changed and needs to be saved to the database.
Later in this article, we'll show you another technique for
notifying the ZODB that your objects have changed. Also, in a
later article, we'll show you how to use simple, ZODB-aware list
and dictionary classes that come pre-packaged with ZODB for your
convenience.
Persistent Classes
------------------
The easiest way to create mutable objects that notify the ZODB of
changes is to create a persistent class. Persistent classes let
you store your own kinds of objects in the database. For example,
consider a class that represents a employee::
import ZODB
from Persistence import Persistent
class Employee(Persistent):
def setName(self, name):
self.name = name
To create a persistent class, simply subclass from
'Persistent.Persistent'. Because of some special magic that ZODB
does, you must first import ZODB before you can import Persistent.
The 'Persistent' module is actually *created* when you import
'ZODB'.
Now, you can put Employee objects in your database::
>>> employees=[]
>>> for name in ['Mary', 'Joe', 'Bob']:
... employee = Employee()
... employee.setName(name)
... employees.append(employee)
>>> root['employees']=employees
>>> transaction.commit()
Don't forget to call 'commit()', so that the changes you have made
so far are committed to the database, and a new transaction is
begun.
Now you can change your employees and they will be saved in the
database. For example you can change Bob's name to "Robert"::
>>> bob=root['employees'][2]
>>> bob.setName('Robert')
>>> transaction.commit()
You can even change attributes of persistent instaces without
calling methods::
>>> bob=root['employees'][2]
>>> bob._coffee_prefs=('Cream', 'Sugar')
>>> transaction.commit()
It doesn't matter whether you change an attribute directly, or
whether it's changed by a method. As you can tell, all of the
normal Python language rules still work as you'd expect.
Mutable Attributes
------------------
Earlier you saw how ZODB can't detect changes to normal mutable
objects like Python lists. This issue still affects you when using
persistent instances. This is because persistent instances can
have attributes which are normal mutable objects. For example,
consider this class::
class Employee(Persistent):
def __init__(self):
self.tasks = []
def setName(self, name):
self.name = name
def addTask(self, task):
self.task.append(task)
When you call 'addTask', the ZODB won't know that the mutable
attribute 'self.tasks' has changed. As you saw earlier, you can
reassign 'self.tasks' after you change it to get around this
problem. However, when you're using persistent instances, you have
another choice. You can signal the ZODB that your instance has
changed with the '_p_changed' attribute::
class Employee(Persistent):
...
def addTask(self, task):
self.task.append(task)
self._p_changed = 1
To signal that this object has change, set the '_p_changed'
attribute to 1. You only need to signal ZODB once, even if you
change many mutable attributes.
The '_p_changed' flag leads us to one of the few rules of you must
follow when creating persistent classes: your instances *cannot*
have attributes that begin with '_p_', those names are reserved
for use by the ZODB.
A Complete Example
------------------
Here's a complete example program. It builds on the employee
examples used so far::
from ZODB import DB
from ZODB.FileStorage import FileStorage
from ZODB.PersistentMapping import PersistentMapping
from Persistence import Persistent
import transaction
class Employee(Persistent):
"""An employee"""
def __init__(self, name, manager=None):
self.name=name
self.manager=manager
# setup the database
storage=FileStorage("employees.fs")
db=DB(storage)
connection=db.open()
root=connection.root()
# get the employees mapping, creating an empty mapping if
# necessary
if not root.has_key("employees"):
root["employees"] = {}
employees=root["employees"]
def listEmployees():
if len(employees.values())==0:
print "There are no employees."
print
return
for employee in employees.values():
print "Name: %s" % employee.name
if employee.manager is not None:
print "Manager's name: %s" % employee.manager.name
print
def addEmployee(name, manager_name=None):
if employees.has_key(name):
print "There is already an employee with this name."
return
if manager_name:
try:
manager=employees[manager_name]
except KeyError:
print
print "No such manager"
print
return
employees[name]=Employee(name, manager)
else:
employees[name]=Employee(name)
root['employees'] = employees # reassign to change
transaction.commit()
print "Employee %s added." % name
print
if __name__=="__main__":
while 1:
choice=raw_input("Press 'L' to list employees, 'A' to add"
"an employee, or 'Q' to quit:")
choice=choice.lower()
if choice=="l":
listEmployees()
elif choice=="a":
name=raw_input("Employee name:")
manager_name=raw_input("Manager name:")
addEmployee(name, manager_name)
elif choice=="q":
break
# close database
connection.close()
This program demonstrates a couple interesting things. First, this
program shows how persistent objects can refer to each other. The
'self.manger' attribute of 'Employee' instances can refer to other
'Employee' instances. Unlike a relational database, there is no
need to use indirection such as object ids when referring from one
persistent object to another. You can just use normal Python
references. In fact, you can even use circular references.
A final trick used by this program is to look for a persistent
object and create it if it is not present. This allows you to just
run this program without having to run a setup script to build the
database first. If there is not database present, the program will
create one and initialize it.
Conclusion
----------
ZODB is a very simple, transparent object database for Python that
is a freely available component of the Zope application server.
As these examples illustrate, only a few lines of code are needed
to start storing Python objects in ZODB, with no need to write SQL
queries. In the next article on ZODB, we'll show you some more
advanced techniques for using ZODB, like using ZODB's distributed
object protocol to distribute your persistent objects across many
machines.
ZODB Resources
- `Andrew Kuchling's "ZODB pages" <http://web.archive.org/web/20030606003753/http://amk.ca/zodb/>`_ (archived)
- `Zope.org "ZODB Wiki" <http://www.zope.org/Wikis/ZODB/FrontPage>`_
- `Jim Fulton's "Introduction to the Zope Object Database" <http://www.python.org/workshops/2000-01/proceedings/papers/fulton/zodb3.html>`_
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Advanced ZODB for Python Programmers
====================================
In the first article in this series, "ZODB for Python
Programmers":ZODB1 I covered some of the simpler aspects of Python
object persistence. In this article, I'll go over some of the more
advanced features of ZODB.
In addition to simple persistence, ZODB offers some very useful
extras for the advanced Python application. Specificly, we'll cover
the following advanced features in this article:
- Persistent-Aware Types -- ZODB comes with some special,
"persistent-aware" data types for storing data in a ZODB. The
most useful of these is the "BTree", which is a fast, efficient
storage object for lots of data.
- Voalitile Data -- Not all your data is meant to be stored in the
database, ZODB let's you have volatile data on your objects that
does not get saved.
- Pluggable Storages -- ZODB offers you the ability to use many
different storage back-ends to store your object data, including
files, relational databases and a special client-server storage
that stores objects on a remote server.
- Conflict Resolution -- When many threads try to write to the same
object at the same time, you can get conflicts. ZODB offers a
conflict resolution protocol that allows you to mitigate most
conflicting writes to your data.
- Transactions -- When you want your changes to be "all or nothing"
transactions come to the rescue.
Persistent-Aware Types
----------------------
You can also get around the mutable attribute problem discussed in
the first article by using special types that are "persistent
aware". ZODB comes with the following persistent aware mutable
object types:
- PersistentList -- This type works just like a list, except that
changing it does not require setting _p_changed or explicitly
re-assigning the attribute.
- PersistentMapping -- A persistent aware dictionary, much like
PersistentList.
- BTree -- A dictionary-like object that can hold large
collections of objects in an ordered, fast, efficient way.
BTrees offer a very powerful facility to the Python programmer:
- BTrees can hold a large collection of information in an
efficient way; more objects than your computer has enough
memory to hold at one time.
- BTrees are integrated into the persistence machinery to work
effectively with ZODB's object cache. Recently, or heavily
used objects are kept in a memory cache for speed.
- BTrees can be searched very quickly, because they are stored
in an fast, balanced tree data structure.
- BTrees come in three flavors, OOBTrees, IOBTrees, OIBTrees, and
IIBTrees. The last three are optimized for integer keys, values,
and key-value pairs, respectively. This means that, for example,
an IOBTree is meant to map an integer to an object, and is
optimized for having integers keys.
Using BTrees
------------
Suppose you track the movement of all your employees with
heat-seeking cameras hidden in the ceiling tiles. Since your
employees tend to frequently congregate against you, all of the
tracking information could end up to be a lot of data, possibly
thousands of coordinates per day per employee. Further, you want
to key the coordinate on the time that it was taken, so that you
can only look at where your employees were during certain times::
from BTrees import IOBTree
from time import time
class Employee(Persistent):
def __init__(self):
self.movements = IOBTree()
def fix(self, coords):
"get a fix on the employee"
self.movements[int(time())] = coords
def trackToday(self):
"return all the movements of the
employee in the last 24 hours"
current_time = int(time())
return self.movements.items(current_time - 86400,
current_time)
In this example, the 'fix' method is called every time one of your
cameras sees that employee. This information is then stored in a
BTree, with the current 'time()' as the key and the 'coordinates'
as the value.
Because BTrees store their information is a ordered structure,
they can be quickly searched for a range of key values. The
'trackToday' method uses this feature to return a sequence of
coordinates from 24 hours hence to the present.
This example shows how BTrees can be quickly searched for a range
of values from a minimum to a maximum, and how you can use this
technique to oppress your workforce. BTrees have a very rich API,
including doing unions and intersections of result sets.
Not All Objects are Persistent
------------------------------
You don't have to make all of your objects persistent.
Non-persistent objects are often useful to represent either
"canned" behavior (classes that define methods but no state), or
objects that are useful only as a "cache" that can be thrown away
when your persistent object is deactivated (removed from memory
when not used).
ZODB provides you with the ability to have *volatile* attributes.
Volatile attributes are attributes of persistent objects that are
never saved in the database, even if they are capable of being
persistent. Volatile attributes begin with '_v_' are good for
keeping cached information around for optimization. ZODB also
provides you with access to special pickling hooks that allow you
to set volatile information when an object is activated.
Imagine you had a class that stored a complex image that you
needed to calculate. This calculation is expensive. Instead of
calculating the image every time you called a method, it would be
better to calculate it *once* and then cache the result in a
volatile attribute::
def image(self):
"a large and complex image of the terrain"
if hasattr(self, '_v_image'):
return self._v_image
image=expensive_calculation()
self._v_image=image
return image
Here, calling 'image' the first time the object is activated will
cause the method to do the expensive calculation. After the first
call, the image will be cached in a volatile attribute. If the
object is removed from memory, the '_v_image' attribute is not
saved, so the cached image is thrown away, only to be recalculated
the next time you call 'image'.
ZODB and Concurrency
--------------------
Different, threads, processes, and computers on a network can open
connections to a single ZODB object database. Each of these
different processes keeps its own copy of the objects that it uses
in memory.
The problem with allowing concurrent access is that conflicts can
occur. If different threads try to commit changes to the same
objects at the same time, one of the threads will raise a
ConflictError. If you want, you can write your application to
either resolve or retry conflicts a reasonable number of times.
Zope will retry a conflicting ZODB operation three times. This is
usually pretty reasonable behavior. Because conflicts only happen
when two threads write to the same object, retrying a conflict
means that one thread will win the conflict and write itself, and
the other thread will retry a few seconds later.
Pluggable Storages
------------------
Different processes and computers can connection to the same
database using a special kind of storage called a 'ClientStorage'.
A 'ClientStorage' connects to a 'StorageServer' over a network.
In the very beginning, you created a connection to the database by
first creating a storage. This was of the type 'FileStorage'.
Zope comes with several different back end storage objects, but
one of the most interesting is the 'ClientStorage' from the Zope
Enterprise Objects product (ZEO).
The 'ClientStorage' storage makes a TCP/IP connection to a
'StorageServer' (also provided with ZEO). This allows many
different processes on one or machines to work with the same
object database and, hence, the same objects. Each process gets a
cached "copy" of a particular object for speed. All of the
'ClientStorages' connected to a 'StorageServer' speak a special
object transport and cache invalidation protocol to keep all of
your computers synchronized.
Opening a 'ClientStorage' connection is simple. The following
code creates a database connection and gets the root object for a
'StorageServer' listening on "localhost:12345"::
from ZODB import DB
from ZEO import ClientStorage
storage = ClientStorage.ClientStorage('localhost', 12345)
db = DB( storage )
connection = db.open()
root = connection.root()
In the rare event that two processes (or threads) modify the same
object at the same time, ZODB provides you with the ability to
retry or resolve these conflicts yourself.
Resolving Conflicts
-------------------
If a conflict happens, you have two choices. The first choice is
that you live with the error and you try again. Statistically,
conflicts are going to happen, but only in situations where objects
are "hot-spots". Most problems like this can be "designed away";
if you can redesign your application so that the changes get
spread around to many different objects then you can usually get
rid of the hot spot.
Your second choice is to try and *resolve* the conflict. In many
situations, this can be done. For example, consider the following
persistent object::
class Counter(Persistent):
self.count = 0
def hit(self):
self.count = self.count + 1
This is a simple counter. If you hit this counter with a lot of
requests though, it will cause conflict errors as different threads
try to change the count attribute simultaneously.
But resolving the conflict between conflicting threads in this
case is easy. Both threads want to increment the self.count
attribute by a value, so the resolution is to increment the
attribute by the sum of the two values and make both commits
happy.
To resolve a conflict, a class should define an
'_p_resolveConflict' method. This method takes three arguments:
- 'oldState' -- The state of the object that the changes made by
the current transaction were based on. The method is permitted
to modify this value.
- 'savedState' -- The state of the object that is currently
stored in the database. This state was written after 'oldState'
and reflects changes made by a transaction that committed
before the current transaction. The method is permitted to
modify this value.
- 'newState' -- The state after changes made by the current
transaction. The method is *not* permitted to modify this
value. This method should compute a new state by merging
changes reflected in 'savedState' and 'newState', relative to
'oldState'.
The method should return the state of the object after resolving
the differences.
Here is an example of a '_p_resolveConflict' in the 'Counter'
class::
class Counter(Persistent):
self.count = 0
def hit(self):
self.count = self.count + 1
def _p_resolveConflict(self, oldState, savedState, newState):
# Figure out how each state is different:
savedDiff= savedState['count'] - oldState['count']
newDiff= newState['count']- oldState['count']
# Apply both sets of changes to old state:
return oldState['count'] + savedDiff + newDiff
In the above example, '_p_resolveConflict' resolves the difference
between the two conflicting transactions.
Transactions and Subtransactions
--------------------------------
Transactions are a very powerful concept in databases.
Transactions let you make many changes to your information as if
they were all one big change. Imagine software that did online
banking and allowed you to transfer money from one account to
another. You would do this by deducting the amount of the
transfer from one account, and adding that amount onto the
other.
If an error happened while you were adding the money to the
receiving account (say, the bank's computers were unavailable),
then you would want to abort the transaction so that the state of
the accounts went back to the way they were before you changed
anything.
To abort a transaction, you need to call the 'abort' method of the
transactions object::
>>> import transaction
>>> transaction.abort()
This will throw away all the currently changed objects and start a
new, empty transaction.
Subtransactions, sometimes called "inner transactions", are
transactions that happen inside another transaction.
Subtransactions can be commited and aborted like regular "outer"
transactions. Subtransactions mostly provide you with an
optimization technique.
Subtransactions can be commited and aborted. Commiting or
aborting a subtransaction does not commit or abort its outer
transaction, just the subtransaction. This lets you use many,
fine-grained transactions within one big transaction.
Why is this important? Well, in order for a transaction to be
"rolled back" the changes in the transaction must be stored in
memory until commit time. By commiting a subtransaction, you are
telling Zope that "I'm pretty sure what I've done so far is
permenant, you can store this subtransaction somewhere other than
in memory". For very, very large transactions, this can be a big
memory win for you.
If you abort an outer transaction, then all of its inner
subtransactions will also be aborted and not saved. If you abort
an inner subtransaction, then only the changes made during that
subtransaction are aborted, and the outer transaction is *not*
aborted and more changes can be made and commited, including more
subtransactions.
You can commit or abort a subtransaction by calling either
commit() or abort() with an argument of 1::
transaction.commit(1) # or
transaction.abort(1)
Subtransactions offer you a nice way to "batch" all of your "all
or none" actions into smaller "all or none" actions while still
keeping the outer level "all or none" transaction intact. As a
bonus, they also give you much better memory resource performance.
Conclusion
----------
ZODB offers many advanced features to help you develop simple, but
powerful python programs. In this article, you used some of the
more advanced features of ZODB to handle different application
needs, like storing information in large sets, using the database
concurrently, and maintaining transactional integrity. For more
information on ZODB, join the discussion list at zodb-dev@zope.org
where you can find out more about this powerful component of Zope.
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ZODB articles
=============
Contents
--------
.. toctree::
:maxdepth: 2
ZODB-overview.rst
ZODB1.rst
ZODB2.rst
old-guide/index
multi-zodb-gc.rst
Other ZODB Resources
--------------------
- IBM developerWorks `Example-driven ZODB
<http://www.ibm.com/developerworks/aix/library/au-zodb/>`_
- `How To Love ZODB and Forget RDBMS
<http://zope.org/Members/adytumsolutions/HowToLoveZODB_PartI>`_
- `Very old ZODB wiki <http://www.zope.org/Members/jim/ZODB/FrontPage>`_
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Using zc.zodbdgc (fix PosKeyError's)
====================================
*This article was written by Hanno Schlichting*
The `zc.zodbdgc <http://pypi.python.org/pypi/zc.zodbdgc>`_ library contains two
useful features. On the one hand it supports advanced ZODB packing and garbage
collection approaches and on the other hand it includes the ability to create a
database of all persistent references.
The second feature allows us to debug and repair PosKeyErrors by finding the
persistent object(s) that point to the lost object.
Note: This documentation applies to ZODB 3.9 and later. Earlier versions of the
ZODB are not supported, as they lack the fast storage iteration API's required
by `zc.zodbdgc`.
This documentation does not apply to
`RelStorage <http://pypi.python.org/pypi/RelStorage>`_ which has the same
features built-in, but accessible in different ways. Look at the options for
the `zodbpack` script. The `--prepack` option creates a table containing the
same information as we are creating in the reference database.
Setup
-----
We'll assume you are familiar with a buildout setup. A typical config might
look like this::
[buildout]
parts =
zeo
zeopy
zeo-conf
zodbdgc
refdb-conf
[zeo]
recipe = plone.recipe.zeoserver
zeo-address = 127.0.0.1:8100
blob-storage = ${buildout:directory}/var/blobstorage
pack-gc = false
pack-keep-old = false
[zeopy]
recipe = zc.recipe.egg
eggs =
ZODB3
zc.zodbdgc
interpreter = zeopy
scripts = zeopy
[zeo-conf]
recipe = collective.recipe.template
input = inline:
<zodb main>
<zeoclient>
blob-dir ${buildout:directory}/var/blobstorage
shared-blob-dir yes
server ${zeo:zeo-address}
storage 1
name zeostorage
var ${buildout:directory}/var
</zeoclient>
</zodb>
output = ${buildout:directory}/etc/zeo.conf
[zodbdgc]
recipe = zc.recipe.egg
eggs = zc.zodbdgc
[refdb-conf]
recipe = collective.recipe.template
input = inline:
<zodb main>
<filestorage 1>
path ${buildout:directory}/var/refdb.fs
</filestorage>
</zodb>
output = ${buildout:directory}/etc/refdb.conf
Garbage collection
------------------
We configured the ZEO server to skip garbage collection as part of the normal
pack in the above config (`pack-gc = false`). Instead we use explicit garbage
collection via a different job::
bin/multi-zodb-gc etc/zeo.conf
On larger databases garbage collection can take a couple hours. We can run this
only once a week or even less frequent. All explicitly deleted objects will
still be packed away by the normal pack, so the database doesn't grow
out-of-bound. We can also run the analysis against a database copy, taking away
load from the live database and only write the resulting deletions to the
production database.
Packing
-------
We can do regular packing every day while the ZEO server is running, via::
bin/zeopack
Packing without garbage collection is much faster.
Reference analysis and POSKeyErrors
-----------------------------------
If our database has any POSKeyErrors, we can find and repair those.
Either we already have the oids of lost objects, or we can check the entire
database for any errors. To check everything we run the following command::
$ bin/multi-zodb-check-refs etc/zeo.conf
This can take about 15 to 30 minutes on moderately sized databases of up to
10gb, dependent on disk speed. We'll write down the reported errors, as we'll
need them later on to analyze them.
If there are any lost objects, we can create a reference database to make it
easier to debug and find those lost objects::
$ bin/multi-zodb-check-refs -r var/refdb.fs etc/zeo.conf
This is significantly slower and can take several hours to complete. Once this
is complete we can open the generated database via our interpreter::
$ bin/zeopy
>>> import ZODB.config
>>> db = ZODB.config.databaseFromFile(open('./etc/refdb.conf'))
>>> conn = db.open()
>>> refs = conn.root()['references']
If we've gotten this error report::
!!! main 13184375 ?
POSKeyError: 0xc92d77
We can look up the persistent oid it was referenced from via::
>>> parent = list(refs['main'][13184375])
>>> parent
[13178389]
We can also get the hex representation::
>>> from ZODB.utils import p64
>>> p64(parent[0])
'\x00\x00\x00\x00\x00\xc9\x16\x15'
With this information, we should get back to our actual database and look
up this object. We'll leave the ref db open, as we might need to recursively
look up some more objects, until we get one we can identify and work on.
We could load the parent. In a debug prompt we could do something like::
>>> app._p_jar.get('\x00\x00\x00\x00\x00\xc9\x16\x15')
2010-04-28 14:28:28 ERROR ZODB.Connection Couldn't load state for 0xc91615
Traceback (most recent call last):
...
ZODB.POSException.POSKeyError: 0xc92d77
Gah, this gives us the POSKeyError of course. But we can load the actual data
of the parent, to get an idea of what this is::
>>> app._p_jar.db()._storage.load('\x00\x00\x00\x00\x00\xc9\x16\x15', '')
('cBTrees.IOBTree
IOBucket
q\x01.((J$KT\x02ccopy_reg
_reconstructor
q\x02(cfive.intid.keyreference
KeyReferenceToPersistent
...
Now we can be real evil and create a new fake object in place of the missing
one::
>>> import transaction
>>> transaction.begin()
The persistent oid that was reported missing was ``13184375``::
>>> from ZODB.utils import p64
>>> p64(13184375)
'\x00\x00\x00\x00\x00\xc9-w'
>>> from persistent import Persistent
>>> a = Persistent()
>>> a._p_oid = '\x00\x00\x00\x00\x00\xc9-w'
We cannot use the ``add`` method of the connection, as this would assign the
object a new persistent oid. So we replicate its internals here::
>>> a._p_jar = app._p_jar
>>> app._p_jar._register(a)
>>> app._p_jar._added[a._p_oid] = a
>>> transaction.commit()
Both getting the object as well as its parent will work now::
>>> app._p_jar.get('\x00\x00\x00\x00\x00\xc9-w')
<persistent.Persistent object at 0xa3e348c>
>>> app._p_jar.get('\x00\x00\x00\x00\x00\xc9\x16\x15')
BTrees.IOBTree.IOBucket([(39078692, <five.intid.keyreference...
Once we are finished we should be nice and close all databases::
>>> conn.close()
>>> db.close()
Depending on the class of object that went missing, we might need to use a
different persistent class, like a persistent mapping or a BTree bucket.
In general it's best to remove the parent object and thus our fake object from
the database and rebuild the data structure again via the proper application
level API's.
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This directory contains Andrew Kuchling's programmer's guide to ZODB
and ZEO. The tex source was not being updated in the ZODB docs directory
It was originally taken from Andrew's zodb.sf.net project on
SourceForge. Because the original version is no longer updated, this
version [in the zodb docs dir] is best viewed as an independent fork now.
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Write section on __setstate__
Continue working on it
Suppress the full GFDL text in the PDF/PS versions
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import sys, time, os, random
import transaction
from persistent import Persistent
from ZEO import ClientStorage
import ZODB
from ZODB.POSException import ConflictError
from BTrees import OOBTree
class ChatSession(Persistent):
"""Class for a chat session.
Messages are stored in a B-tree, indexed by the time the message
was created. (Eventually we'd want to throw messages out,
add_message(message) -- add a message to the channel
new_messages() -- return new messages since the last call to
this method
"""
def __init__(self, name):
"""Initialize new chat session.
name -- the channel's name
"""
self.name = name
# Internal attribute: _messages holds all the chat messages.
self._messages = OOBTree.OOBTree()
def new_messages(self):
"Return new messages."
# self._v_last_time is the time of the most recent message
# returned to the user of this class.
if not hasattr(self, '_v_last_time'):
self._v_last_time = 0
new = []
T = self._v_last_time
for T2, message in self._messages.items():
if T2 > T:
new.append( message )
self._v_last_time = T2
return new
def add_message(self, message):
"""Add a message to the channel.
message -- text of the message to be added
"""
while 1:
try:
now = time.time()
self._messages[ now ] = message
transaction.commit()
except ConflictError:
# Conflict occurred; this process should abort,
# wait for a little bit, then try again.
transaction.abort()
time.sleep(.2)
else:
# No ConflictError exception raised, so break
# out of the enclosing while loop.
break
# end while
def get_chat_session(conn, channelname):
"""Return the chat session for a given channel, creating the session
if required."""
# We'll keep a B-tree of sessions, mapping channel names to
# session objects. The B-tree is stored at the ZODB's root under
# the key 'chat_sessions'.
root = conn.root()
if not root.has_key('chat_sessions'):
print 'Creating chat_sessions B-tree'
root['chat_sessions'] = OOBTree.OOBTree()
transaction.commit()
sessions = root['chat_sessions']
# Get a session object corresponding to the channel name, creating
# it if necessary.
if not sessions.has_key( channelname ):
print 'Creating new session:', channelname
sessions[ channelname ] = ChatSession(channelname)
transaction.commit()
session = sessions[ channelname ]
return session
if __name__ == '__main__':
if len(sys.argv) != 2:
print 'Usage: %s <channelname>' % sys.argv[0]
sys.exit(0)
storage = ClientStorage.ClientStorage( ('localhost', 9672) )
db = ZODB.DB( storage )
conn = db.open()
s = session = get_chat_session(conn, sys.argv[1])
messages = ['Hi.', 'Hello', 'Me too', "I'M 3L33T!!!!"]
while 1:
# Send a random message
msg = random.choice(messages)
session.add_message( '%s: pid %i' % (msg,os.getpid() ))
# Display new messages
for msg in session.new_messages():
print msg
# Wait for a few seconds
pause = random.randint( 1, 4 )
time.sleep( pause )
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# Use the python docs converter to convert to rst
# Requires http://svn.python.org/projects/doctools/converter
from converter import restwriter, convert_file
import sys
import os
if __name__ == '__main__':
try:
rootdir = sys.argv[1]
destdir = os.path.abspath(sys.argv[2])
except IndexError:
print "usage: convert.py docrootdir destdir"
sys.exit()
os.chdir(rootdir)
class IncludeRewrite:
def get(self, a, b=None):
if os.path.exists(a + '.tex'):
return a + '.rst'
print "UNKNOWN FILE %s" % a
return a
restwriter.includes_mapping = IncludeRewrite()
for infile in os.listdir('.'):
if infile.endswith('.tex'):
convert_file(infile, os.path.join(destdir, infile[:-3]+'rst'))
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.. % gfdl.tex
.. % This file is a chapter. It must be included in a larger document to work
.. % properly.
GNU Free Documentation License
==============================
Version 1.1, March 2000 ---
Copyright 2000 Free Software Foundation, Inc. --- 59 Temple
Place, Suite 330, Boston, MA 02111-1307 USA --- Everyone is permitted to copy
and distribute verbatim copies of this license document, but changing it is not
allowed.
Preamble
The purpose of this License is to make a manual, textbook, or other written
document "free" in the sense of freedom: to assure everyone the effective
freedom to copy and redistribute it, with or without modifying it, either
commercially or noncommercially. Secondarily, this License preserves for the
author and publisher a way to get credit for their work, while not being
considered responsible for modifications made by others.
This License is a kind of "copyleft", which means that derivative works of the
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doc/articles/old-guide/index.rst 0 → 100644
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===============================
Very old ZODB programming guide
===============================
This guide is based heavily on the work of A. M. Kuchling who wrote the
original guide back in 2002 and which was published under the GNU Free
Documentation License, Version 1.1. See the appendix entitled "GNU Free
Documentation License" for more information.
.. toctree::
:maxdepth: 2
introduction.rst
prog-zodb.rst
zeo.rst
transactions.rst
modules.rst
links.rst
gfdl.rst
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.. % Introduction
.. % What is ZODB?
.. % What is ZEO?
.. % OODBs vs. Relational DBs
.. % Other OODBs
Introduction
============
This guide explains how to write Python programs that use the Z Object Database
(ZODB) and Zope Enterprise Objects (ZEO). The latest version of the guide is
always available at `<http://www.zope.org/Wikis/ZODB/guide/index.html>`_.
What is the ZODB?
-----------------
The ZODB is a persistence system for Python objects. Persistent programming
languages provide facilities that automatically write objects to disk and read
them in again when they're required by a running program. By installing the
ZODB, you add such facilities to Python.
It's certainly possible to build your own system for making Python objects
persistent. The usual starting points are the :mod:`pickle` module, for
converting objects into a string representation, and various database modules,
such as the :mod:`gdbm` or :mod:`bsddb` modules, that provide ways to write
strings to disk and read them back. It's straightforward to combine the
:mod:`pickle` module and a database module to store and retrieve objects, and in
fact the :mod:`shelve` module, included in Python's standard library, does this.
The downside is that the programmer has to explicitly manage objects, reading an
object when it's needed and writing it out to disk when the object is no longer
required. The ZODB manages objects for you, keeping them in a cache, writing
them out to disk when they are modified, and dropping them from the cache if
they haven't been used in a while.
OODBs vs. Relational DBs
------------------------
Another way to look at it is that the ZODB is a Python-specific object-oriented
database (OODB). Commercial object databases for C++ or Java often require that
you jump through some hoops, such as using a special preprocessor or avoiding
certain data types. As we'll see, the ZODB has some hoops of its own to jump
through, but in comparison the naturalness of the ZODB is astonishing.
Relational databases (RDBs) are far more common than OODBs. Relational databases
store information in tables; a table consists of any number of rows, each row
containing several columns of information. (Rows are more formally called
relations, which is where the term "relational database" originates.)
Let's look at a concrete example. The example comes from my day job working for
the MEMS Exchange, in a greatly simplified version. The job is to track process
runs, which are lists of manufacturing steps to be performed in a semiconductor
fab. A run is owned by a particular user, and has a name and assigned ID
number. Runs consist of a number of operations; an operation is a single step
to be performed, such as depositing something on a wafer or etching something
off it.
Operations may have parameters, which are additional information required to
perform an operation. For example, if you're depositing something on a wafer,
you need to know two things: 1) what you're depositing, and 2) how much should
be deposited. You might deposit 100 microns of silicon oxide, or 1 micron of
copper.
Mapping these structures to a relational database is straightforward::
CREATE TABLE runs (
int run_id,
varchar owner,
varchar title,
int acct_num,
primary key(run_id)
);
CREATE TABLE operations (
int run_id,
int step_num,
varchar process_id,
PRIMARY KEY(run_id, step_num),
FOREIGN KEY(run_id) REFERENCES runs(run_id),
);
CREATE TABLE parameters (
int run_id,
int step_num,
varchar param_name,
varchar param_value,
PRIMARY KEY(run_id, step_num, param_name)
FOREIGN KEY(run_id, step_num)
REFERENCES operations(run_id, step_num),
);
In Python, you would write three classes named :class:`Run`, :class:`Operation`,
and :class:`Parameter`. I won't present code for defining these classes, since
that code is uninteresting at this point. Each class would contain a single
method to begin with, an :meth:`__init__` method that assigns default values,
such as 0 or ``None``, to each attribute of the class.
It's not difficult to write Python code that will create a :class:`Run` instance
and populate it with the data from the relational tables; with a little more
effort, you can build a straightforward tool, usually called an object-
relational mapper, to do this automatically. (See
`<http://www.amk.ca/python/unmaintained/ordb.html>`_ for a quick hack at a
Python object-relational mapper, and
`<http://www.python.org/workshops/1997-10/proceedings/shprentz.html>`_ for Joel
Shprentz's more successful implementation of the same idea; Unlike mine,
Shprentz's system has been used for actual work.)
However, it is difficult to make an object-relational mapper reasonably quick; a
simple-minded implementation like mine is quite slow because it has to do
several queries to access all of an object's data. Higher performance object-
relational mappers cache objects to improve performance, only performing SQL
queries when they actually need to.
That helps if you want to access run number 123 all of a sudden. But what if
you want to find all runs where a step has a parameter named 'thickness' with a
value of 2.0? In the relational version, you have two unappealing choices:
#. Write a specialized SQL query for this case: ``SELECT run_id FROM operations
WHERE param_name = 'thickness' AND param_value = 2.0``
If such queries are common, you can end up with lots of specialized queries.
When the database tables get rearranged, all these queries will need to be
modified.
#. An object-relational mapper doesn't help much. Scanning through the runs
means that the the mapper will perform the required SQL queries to read run #1,
and then a simple Python loop can check whether any of its steps have the
parameter you're looking for. Repeat for run #2, 3, and so forth. This does a
vast number of SQL queries, and therefore is incredibly slow.
An object database such as ZODB simply stores internal pointers from object to
object, so reading in a single object is much faster than doing a bunch of SQL
queries and assembling the results. Scanning all runs, therefore, is still
inefficient, but not grossly inefficient.
What is ZEO?
------------
The ZODB comes with a few different classes that implement the :class:`Storage`
interface. Such classes handle the job of writing out Python objects to a
physical storage medium, which can be a disk file (the :class:`FileStorage`
class), a BerkeleyDB file (:class:`BDBFullStorage`), a relational database
(:class:`DCOracleStorage`), or some other medium. ZEO adds
:class:`ClientStorage`, a new :class:`Storage` that doesn't write to physical
media but just forwards all requests across a network to a server. The server,
which is running an instance of the :class:`StorageServer` class, simply acts as
a front-end for some physical :class:`Storage` class. It's a fairly simple
idea, but as we'll see later on in this document, it opens up many
possibilities.
About this guide
----------------
The primary author of this guide works on a project which uses the ZODB and ZEO
as its primary storage technology. We use the ZODB to store process runs and
operations, a catalog of available processes, user information, accounting
information, and other data. Part of the goal of writing this document is to
make our experience more widely available. A few times we've spent hours or
even days trying to figure out a problem, and this guide is an attempt to gather
up the knowledge we've gained so that others don't have to make the same
mistakes we did while learning.
The author's ZODB project is described in a paper available here,
`<http://www.amk.ca/python/writing/mx-architecture/>`_
This document will always be a work in progress. If you wish to suggest
clarifications or additional topics, please send your comments to the
`ZODB-dev mailing list <https://groups.google.com/forum/#!forum/zodb>`_.
Acknowledgements
----------------
Andrew Kuchling wrote the original version of this guide, which provided some of
the first ZODB documentation for Python programmers. His initial version has
been updated over time by Jeremy Hylton and Tim Peters.
I'd like to thank the people who've pointed out inaccuracies and bugs, offered
suggestions on the text, or proposed new topics that should be covered: Jeff
Bauer, Willem Broekema, Thomas Guettler, Chris McDonough, George Runyan.
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.. % links.tex
.. % Collection of relevant links
Resources
=========
Introduction to the Zope Object Database, by Jim Fulton: --- Goes into much
greater detail, explaining advanced uses of the ZODB and how it's actually
implemented. A definitive reference, and highly recommended. ---
`<http://www.python.org/workshops/2000-01/proceedings/papers/fulton/zodb3.html>`_
Persistent Programing with ZODB, by Jeremy Hylton and Barry Warsaw: --- Slides
for a tutorial presented at the 10th Python conference. Covers much of the same
ground as this guide, with more details in some areas and less in others. ---
`<http://www.zope.org/Members/bwarsaw/ipc10-slides>`_
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.. % Related Modules
.. % PersistentMapping
.. % PersistentList
.. % BTrees
.. % Total Ordering and Persistence
.. % Iteration and Mutation
.. % BTree Diagnostic Tools
Related Modules
===============
The ZODB package includes a number of related modules that provide useful data
types such as BTrees.
:mod:`persistent.mapping.PersistentMapping`
-------------------------------------------
The :class:`PersistentMapping` class is a wrapper for mapping objects that will
set the dirty bit when the mapping is modified by setting or deleting a key.
.. function:: PersistentMapping(container = {})
Create a :class:`PersistentMapping` object that wraps the mapping object
*container*. If you don't specify a value for *container*, a regular Python
dictionary is used.
:class:`PersistentMapping` objects support all the same methods as Python
dictionaries do.
:mod:`persistent.list.PersistentList`
-------------------------------------
The :class:`PersistentList` class is a wrapper for mutable sequence objects,
much as :class:`PersistentMapping` is a wrapper for mappings.
.. function:: PersistentList(initlist = [])
Create a :class:`PersistentList` object that wraps the mutable sequence object
*initlist*. If you don't specify a value for *initlist*, a regular Python list
is used.
:class:`PersistentList` objects support all the same methods as Python lists do.
BTrees Package
--------------
When programming with the ZODB, Python dictionaries aren't always what you need.
The most important case is where you want to store a very large mapping. When a
Python dictionary is accessed in a ZODB, the whole dictionary has to be
unpickled and brought into memory. If you're storing something very large, such
as a 100,000-entry user database, unpickling such a large object will be slow.
BTrees are a balanced tree data structure that behave like a mapping but
distribute keys throughout a number of tree nodes. The nodes are stored in
sorted order (this has important consequences -- see below). Nodes are then
only unpickled and brought into memory as they're accessed, so the entire tree
doesn't have to occupy memory (unless you really are touching every single key).
The BTrees package provides a large collection of related data structures.
There are variants of the data structures specialized to integers, which are
faster and use less memory. There are five modules that handle the different
variants. The first two letters of the module name specify the types of the
keys and values in mappings -- O for any object, I for 32-bit signed integer,
and (new in ZODB 3.4) F for 32-bit C float. For example, the
:mod:`BTrees.IOBTree` module provides a mapping with integer keys and arbitrary
objects as values.
The four data structures provide by each module are a BTree, a Bucket, a
TreeSet, and a Set. The BTree and Bucket types are mappings and support all the
usual mapping methods, e.g. :func:`update` and :func:`keys`. The TreeSet and
Set types are similar to mappings but they have no values; they support the
methods that make sense for a mapping with no keys, e.g. :func:`keys` but not
:func:`items`. The Bucket and Set types are the individual building blocks for
BTrees and TreeSets, respectively. A Bucket or Set can be used when you are
sure that it will have few elements. If the data structure will grow large, you
should use a BTree or TreeSet. Like Python lists, Buckets and Sets are allocated
in one contiguous piece, and insertions and deletions can take time proportional
to the number of existing elements. Also like Python lists, a Bucket or Set is
a single object, and is pickled and unpickled in its entirety. BTrees and
TreeSets are multi-level tree structures with much better (logarithmic) worst-
case time bounds, and the tree structure is built out of multiple objects, which
ZODB can load individually as needed.
The five modules are named :mod:`OOBTree`, :mod:`IOBTree`, :mod:`OIBTree`,
:mod:`IIBTree`, and (new in ZODB 3.4) :mod:`IFBTree`. The two letter prefixes
are repeated in the data types names. The :mod:`BTrees.OOBTree` module defines
the following types: :class:`OOBTree`, :class:`OOBucket`, :class:`OOSet`, and
:class:`OOTreeSet`. Similarly, the other four modules each define their own
variants of those four types.
The :func:`keys`, :func:`values`, and :func:`items` methods on BTree and TreeSet
types do not materialize a list with all of the data. Instead, they return lazy
sequences that fetch data from the BTree as needed. They also support optional
arguments to specify the minimum and maximum values to return, often called
"range searching". Because all these types are stored in sorted order, range
searching is very efficient.
The :func:`keys`, :func:`values`, and :func:`items` methods on Bucket and Set
types do return lists with all the data. Starting in ZODB 3.3, there are also
:func:`iterkeys`, :func:`itervalues`, and :func:`iteritems` methods that return
iterators (in the Python 2.2 sense). Those methods also apply to BTree and
TreeSet objects.
A BTree object supports all the methods you would expect of a mapping, with a
few extensions that exploit the fact that the keys are sorted. The example below
demonstrates how some of the methods work. The extra methods are :func:`minKey`
and :func:`maxKey`, which find the minimum and maximum key value subject to an
optional bound argument, and :func:`byValue`, which should probably be ignored
(it's hard to explain exactly what it does, and as a result it's almost never
used -- best to consider it deprecated). The various methods for enumerating
keys, values and items also accept minimum and maximum key arguments ("range
search"), and (new in ZODB 3.3) optional Boolean arguments to control whether a
range search is inclusive or exclusive of the range's endpoints. ::
>>> from BTrees.OOBTree import OOBTree
>>> t = OOBTree()
>>> t.update({1: "red", 2: "green", 3: "blue", 4: "spades"})
>>> len(t)
4
>>> t[2]
'green'
>>> s = t.keys() # this is a "lazy" sequence object
>>> s
<OOBTreeItems object at 0x0088AD20>
>>> len(s) # it acts like a Python list
4
>>> s[-2]
3
>>> list(s) # materialize the full list
[1, 2, 3, 4]
>>> list(t.values())
['red', 'green', 'blue', 'spades']
>>> list(t.values(1, 2)) # values at keys in 1 to 2 inclusive
['red', 'green']
>>> list(t.values(2)) # values at keys >= 2
['green', 'blue', 'spades']
>>> list(t.values(min=1, max=4)) # keyword args new in ZODB 3.3
['red', 'green', 'blue', 'spades']
>>> list(t.values(min=1, max=4, excludemin=True, excludemax=True))
['green', 'blue']
>>> t.minKey() # smallest key
1
>>> t.minKey(1.5) # smallest key >= 1.5
2
>>> for k in t.keys():
... print k,
1 2 3 4
>>> for k in t: # new in ZODB 3.3
... print k,
1 2 3 4
>>> for pair in t.iteritems(): # new in ZODB 3.3
... print pair,
...
(1, 'red') (2, 'green') (3, 'blue') (4, 'spades')
>>> t.has_key(4) # returns a true value, but exactly what undefined
2
>>> t.has_key(5)
0
>>> 4 in t # new in ZODB 3.3
True
>>> 5 in t # new in ZODB 3.3
False
>>>
Each of the modules also defines some functions that operate on BTrees --
:func:`difference`, :func:`union`, and :func:`intersection`. The
:func:`difference` function returns a Bucket, while the other two methods return
a Set. If the keys are integers, then the module also defines
:func:`multiunion`. If the values are integers or floats, then the module also
defines :func:`weightedIntersection` and :func:`weightedUnion`. The function
doc strings describe each function briefly.
.. % XXX I'm not sure all of the following is actually correct. The
.. % XXX set functions have complicated behavior.
``BTrees/Interfaces.py`` defines the operations, and is the official
documentation. Note that the interfaces don't define the concrete types
returned by most operations, and you shouldn't rely on the concrete types that
happen to be returned: stick to operations guaranteed by the interface. In
particular, note that the interfaces don't specify anything about comparison
behavior, and so nothing about it is guaranteed. In ZODB 3.3, for example, two
BTrees happen to use Python's default object comparison, which amounts to
comparing the (arbitrary but fixed) memory addresses of the BTrees. This may or
may not be true in future releases. If the interfaces don't specify a behavior,
then whether that behavior appears to work, and exactly happens if it does
appear to work, are undefined and should not be relied on.
Total Ordering and Persistence
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The BTree-based data structures differ from Python dicts in several fundamental
ways. One of the most important is that while dicts require that keys support
hash codes and equality comparison, the BTree-based structures don't use hash
codes and require a total ordering on keys.
Total ordering means three things:
#. Reflexive. For each *x*, ``x == x`` is true.
#. Trichotomy. For each *x* and *y*, exactly one of ``x < y``, ``x == y``, and
``x > y`` is true.
#. Transitivity. Whenever ``x <= y`` and ``y <= z``, it's also true that ``x <=
z``.
The default comparison functions for most objects that come with Python satisfy
these rules, with some crucial cautions explained later. Complex numbers are an
example of an object whose default comparison function does not satisfy these
rules: complex numbers only support ``==`` and ``!=`` comparisons, and raise an
exception if you try to compare them in any other way. They don't satisfy the
trichotomy rule, and must not be used as keys in BTree-based data structures
(although note that complex numbers can be used as keys in Python dicts, which
do not require a total ordering).
Examples of objects that are wholly safe to use as keys in BTree-based
structures include ints, longs, floats, 8-bit strings, Unicode strings, and
tuples composed (possibly recursively) of objects of wholly safe types.
It's important to realize that even if two types satisfy the rules on their own,
mixing objects of those types may not. For example, 8-bit strings and Unicode
strings both supply total orderings, but mixing the two loses trichotomy; e.g.,
``'x' < chr(255)`` and ``u'x' == 'x'``, but trying to compare ``chr(255)`` to
``u'x'`` raises an exception. Partly for this reason (another is given later),
it can be dangerous to use keys with multiple types in a single BTree-based
structure. Don't try to do that, and you don't have to worry about it.
Another potential problem is mutability: when a key is inserted in a BTree-
based structure, it must retain the same order relative to the other keys over
time. This is easy to run afoul of if you use mutable objects as keys. For
example, lists supply a total ordering, and then ::
>>> L1, L2, L3 = [1], [2], [3]
>>> from BTrees.OOBTree import OOSet
>>> s = OOSet((L2, L3, L1)) # this is fine, so far
>>> list(s.keys()) # note that the lists are in sorted order
[[1], [2], [3]]
>>> s.has_key([3]) # and [3] is in the set
1
>>> L2[0] = 5 # horrible -- the set is insane now
>>> s.has_key([3]) # for example, it's insane this way
0
>>> s
OOSet([[1], [5], [3]])
>>>
Key lookup relies on that the keys remain in sorted order (an efficient form of
binary search is used). By mutating key L2 after inserting it, we destroyed the
invariant that the OOSet is sorted. As a result, all future operations on this
set are unpredictable.
A subtler variant of this problem arises due to persistence: by default, Python
does several kinds of comparison by comparing the memory addresses of two
objects. Because Python never moves an object in memory, this does supply a
usable (albeit arbitrary) total ordering across the life of a program run (an
object's memory address doesn't change). But if objects compared in this way
are used as keys of a BTree-based structure that's stored in a database, when
the objects are loaded from the database again they will almost certainly wind
up at different memory addresses. There's no guarantee then that if key K1 had
a memory address smaller than the memory address of key K2 at the time K1 and K2
were inserted in a BTree, K1's address will also be smaller than K2's when that
BTree is loaded from a database later. The result will be an insane BTree,
where various operations do and don't work as expected, seemingly at random.
Now each of the types identified above as "wholly safe to use" never compares
two instances of that type by memory address, so there's nothing to worry about
here if you use keys of those types. The most common mistake is to use keys
that are instances of a user-defined class that doesn't supply its own
:meth:`__cmp__` method. Python compares such instances by memory address. This
is fine if such instances are used as keys in temporary BTree-based structures
used only in a single program run. It can be disastrous if that BTree-based
structure is stored to a database, though.
::
>>> class C:
... pass
...
>>> a, b = C(), C()
>>> print a < b # this may print 0 if you try it
1
>>> del a, b
>>> a, b = C(), C()
>>> print a < b # and this may print 0 or 1
0
>>>
That example illustrates that comparison of instances of classes that don't
define :meth:`__cmp__` yields arbitrary results (but consistent results within a
single program run).
Another problem occurs with instances of classes that do define :meth:`__cmp__`,
but define it incorrectly. It's possible but rare for a custom :meth:`__cmp__`
implementation to violate one of the three required formal properties directly.
It's more common for it to "fall back" to address-based comparison by mistake.
For example::
class Mine:
def __cmp__(self, other):
if other.__class__ is Mine:
return cmp(self.data, other.data)
else:
return cmp(self.data, other)
It's quite possible there that the ``else`` clause allows a result to be
computed based on memory address. The bug won't show up until a BTree-based
structure uses objects of class :class:`Mine` as keys, and also objects of other
types as keys, and the structure is loaded from a database, and a sequence of
comparisons happens to execute the ``else`` clause in a case where the
relative order of object memory addresses happened to change.
This is as difficult to track down as it sounds, so best to stay far away from
the possibility.
You'll stay out of trouble by follwing these rules, violating them only with
great care:
#. Use objects of simple immutable types as keys in BTree-based data structures.
#. Within a single BTree-based data structure, use objects of a single type as
keys. Don't use multiple key types in a single structure.
#. If you want to use class instances as keys, and there's any possibility that
the structure may be stored in a database, it's crucial that the class define a
:meth:`__cmp__` method, and that the method is carefully implemented.
Any part of a comparison implementation that relies (explicitly or implicitly)
on an address-based comparison result will eventually cause serious failure.
#. Do not use :class:`Persistent` objects as keys, or objects of a subclass of
:class:`Persistent`.
That last item may be surprising. It stems from details of how conflict
resolution is implemented: the states passed to conflict resolution do not
materialize persistent subobjects (if a persistent object P is a key in a BTree,
then P is a subobject of the bucket containing P). Instead, if an object O
references a persistent subobject P directly, and O is involved in a conflict,
the states passed to conflict resolution contain an instance of an internal
:class:`PersistentReference` stub class everywhere O references P. Two
:class:`PersistentReference` instances compare equal if and only if they
"represent" the same persistent object; when they're not equal, they compare by
memory address, and, as explained before, memory-based comparison must never
happen in a sane persistent BTree. Note that it doesn't help in this case if
your :class:`Persistent` subclass defines a sane :meth:`__cmp__` method:
conflict resolution doesn't know about your class, and so also doesn't know
about its :meth:`__cmp__` method. It only sees instances of the internal
:class:`PersistentReference` stub class.
Iteration and Mutation
^^^^^^^^^^^^^^^^^^^^^^
As with a Python dictionary or list, you should not mutate a BTree-based data
structure while iterating over it, except that it's fine to replace the value
associated with an existing key while iterating. You won't create internal
damage in the structure if you try to remove, or add new keys, while iterating,
but the results are undefined and unpredictable. A weak attempt is made to
raise :exc:`RuntimeError` if the size of a BTree-based structure changes while
iterating, but it doesn't catch most such cases, and is also unreliable.
Example::
>>> from BTrees.IIBTree import *
>>> s = IISet(range(10))
>>> list(s)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> for i in s: # the output is undefined
... print i,
... s.remove(i)
0 2 4 6 8
Traceback (most recent call last):
File "<stdin>", line 1, in ?
RuntimeError: the bucket being iterated changed size
>>> list(s) # this output is also undefined
[1, 3, 5, 7, 9]
>>>
Also as with Python dictionaries and lists, the safe and predictable way to
mutate a BTree-based structure while iterating over it is to iterate over a copy
of the keys. Example::
>>> from BTrees.IIBTree import *
>>> s = IISet(range(10))
>>> for i in list(s.keys()): # this is well defined
... print i,
... s.remove(i)
0 1 2 3 4 5 6 7 8 9
>>> list(s)
[]
>>>
BTree Diagnostic Tools
^^^^^^^^^^^^^^^^^^^^^^
A BTree (or TreeSet) is a complex data structure, really a graph of variable-
size nodes, connected in multiple ways via three distinct kinds of C pointers.
There are some tools available to help check internal consistency of a BTree as
a whole.
Most generally useful is the :mod:`BTrees.check` module. The
:func:`check.check` function examines a BTree (or Bucket, Set, or TreeSet) for
value-based consistency, such as that the keys are in strictly increasing order.
See the function docstring for details. The :func:`check.display` function
displays the internal structure of a BTree.
BTrees and TreeSets also have a :meth:`_check` method. This verifies that the
(possibly many) internal pointers in a BTree or TreeSet are mutually consistent,
and raises :exc:`AssertionError` if they're not.
If a :func:`check.check` or :meth:`_check` call fails, it may point to a bug in
the implementation of BTrees or conflict resolution, or may point to database
corruption.
Repairing a damaged BTree is usually best done by making a copy of it. For
example, if *self.data* is bound to a corrupted IOBTree, ::
self.data = IOBTree(self.data)
usually suffices. If object identity needs to be preserved, ::
acopy = IOBTree(self.data)
self.data.clear()
self.data.update(acopy)
does the same, but leaves *self.data* bound to the same object.
doc/articles/old-guide/prog-zodb.rst 0 → 100644
View file @ de6dba53
.. % ZODB Programming
.. % How ZODB works (ExtensionClass, dirty bits)
.. % Installing ZODB
.. % Rules for Writing Persistent Classes
ZODB Programming
================
Installing ZODB
---------------
ZODB is packaged using the standard distutils tools.
Requirements
^^^^^^^^^^^^
You will need Python 2.3 or higher. Since the code is packaged using distutils,
it is simply a matter of untarring or unzipping the release package, and then
running ``python setup.py install``.
You'll need a C compiler to build the packages, because there are various C
extension modules. Binary installers are provided for Windows users.
Installing the Packages
^^^^^^^^^^^^^^^^^^^^^^^
Download the ZODB tarball containing all the packages for both ZODB and ZEO from
`<http://www.zope.org/Products/ZODB3.3>`_. See the :file:`README.txt` file in
the top level of the release directory for details on building, testing, and
installing.
You can find information about ZODB and the most current releases in the ZODB
Wiki at `<http://www.zope.org/Wikis/ZODB>`_.
How ZODB Works
--------------
The ZODB is conceptually simple. Python classes subclass a
:class:`persistent.Persistent` class to become ZODB-aware. Instances of
persistent objects are brought in from a permanent storage medium, such as a
disk file, when the program needs them, and remain cached in RAM. The ZODB
traps modifications to objects, so that when a statement such as ``obj.size =
1`` is executed, the modified object is marked as "dirty." On request, any
dirty objects are written out to permanent storage; this is called committing a
transaction. Transactions can also be aborted or rolled back, which results in
any changes being discarded, dirty objects reverting to their initial state
before the transaction began.
The term "transaction" has a specific technical meaning in computer science.
It's extremely important that the contents of a database don't get corrupted by
software or hardware crashes, and most database software offers protection
against such corruption by supporting four useful properties, Atomicity,
Consistency, Isolation, and Durability. In computer science jargon these four
terms are collectively dubbed the ACID properties, forming an acronym from their
names.
The ZODB provides all of the ACID properties. Definitions of the ACID
properties are:
Atomicity
means that any changes to data made during a transaction are all-or-nothing.
Either all the changes are applied, or none of them are. If a program makes a
bunch of modifications and then crashes, the database won't be partially
modified, potentially leaving the data in an inconsistent state; instead all the
changes will be forgotten. That's bad, but it's better than having a partially-
applied modification put the database into an inconsistent state.
Consistency
means that each transaction executes a valid transformation of the database
state. Some databases, but not ZODB, provide a variety of consistency checks in
the database or language; for example, a relational database constraint columns
to be of particular types and can enforce relations across tables. Viewed more
generally, atomicity and isolation make it possible for applications to provide
consistency.
Isolation
means that two programs or threads running in two different transactions cannot
see each other's changes until they commit their transactions.
Durability
means that once a transaction has been committed, a subsequent crash will not
cause any data to be lost or corrupted.
Opening a ZODB
--------------
There are 3 main interfaces supplied by the ZODB: :class:`Storage`, :class:`DB`,
and :class:`Connection` classes. The :class:`DB` and :class:`Connection`
interfaces both have single implementations, but there are several different
classes that implement the :class:`Storage` interface.
* :class:`Storage` classes are the lowest layer, and handle storing and
retrieving objects from some form of long-term storage. A few different types of
Storage have been written, such as :class:`FileStorage`, which uses regular disk
files, and :class:`BDBFullStorage`, which uses Sleepycat Software's BerkeleyDB
database. You could write a new Storage that stored objects in a relational
database, for example, if that would better suit your application. Two example
storages, :class:`DemoStorage` and :class:`MappingStorage`, are available to use
as models if you want to write a new Storage.
* The :class:`DB` class sits on top of a storage, and mediates the interaction
between several connections. One :class:`DB` instance is created per process.
* Finally, the :class:`Connection` class caches objects, and moves them into and
out of object storage. A multi-threaded program should open a separate
:class:`Connection` instance for each thread. Different threads can then modify
objects and commit their modifications independently.
Preparing to use a ZODB requires 3 steps: you have to open the :class:`Storage`,
then create a :class:`DB` instance that uses the :class:`Storage`, and then get
a :class:`Connection` from the :class:`DB instance`. All this is only a few
lines of code::
from ZODB import FileStorage, DB
storage = FileStorage.FileStorage('/tmp/test-filestorage.fs')
db = DB(storage)
conn = db.open()
Note that you can use a completely different data storage mechanism by changing
the first line that opens a :class:`Storage`; the above example uses a
:class:`FileStorage`. In section :ref:`zeo`, "How ZEO Works", you'll see how
ZEO uses this flexibility to good effect.
Using a ZODB Configuration File
-------------------------------
ZODB also supports configuration files written in the ZConfig format. A
configuration file can be used to separate the configuration logic from the
application logic. The storages classes and the :class:`DB` class support a
variety of keyword arguments; all these options can be specified in a config
file.
The configuration file is simple. The example in the previous section could use
the following example::
<zodb>
<filestorage>
path /tmp/test-filestorage.fs
</filestorage>
</zodb>
The :mod:`ZODB.config` module includes several functions for opening database
and storages from configuration files. ::
import ZODB.config
db = ZODB.config.databaseFromURL('/tmp/test.conf')
conn = db.open()
The ZConfig documentation, included in the ZODB3 release, explains the format in
detail. Each configuration file is described by a schema, by convention stored
in a :file:`component.xml` file. ZODB, ZEO, zLOG, and zdaemon all have schemas.
Writing a Persistent Class
--------------------------
Making a Python class persistent is quite simple; it simply needs to subclass
from the :class:`Persistent` class, as shown in this example::
from persistent import Persistent
class User(Persistent):
pass
The :class:`Persistent` base class is a new-style class implemented in C.
For simplicity, in the examples the :class:`User` class will simply be used as a
holder for a bunch of attributes. Normally the class would define various
methods that add functionality, but that has no impact on the ZODB's treatment
of the class.
The ZODB uses persistence by reachability; starting from a set of root objects,
all the attributes of those objects are made persistent, whether they're simple
Python data types or class instances. There's no method to explicitly store
objects in a ZODB database; simply assign them as an attribute of an object, or
store them in a mapping, that's already in the database. This chain of
containment must eventually reach back to the root object of the database.
As an example, we'll create a simple database of users that allows retrieving a
:class:`User` object given the user's ID. First, we retrieve the primary root
object of the ZODB using the :meth:`root` method of the :class:`Connection`
instance. The root object behaves like a Python dictionary, so you can just add
a new key/value pair for your application's root object. We'll insert an
:class:`OOBTree` object that will contain all the :class:`User` objects. (The
:class:`BTree` module is also included as part of Zope.) ::
dbroot = conn.root()
# Ensure that a 'userdb' key is present
# in the root
if not dbroot.has_key('userdb'):
from BTrees.OOBTree import OOBTree
dbroot['userdb'] = OOBTree()
userdb = dbroot['userdb']
Inserting a new user is simple: create the :class:`User` object, fill it with
data, insert it into the :class:`BTree` instance, and commit this transaction.
::
# Create new User instance
import transaction
newuser = User()
# Add whatever attributes you want to track
newuser.id = 'amk'
newuser.first_name = 'Andrew' ; newuser.last_name = 'Kuchling'
...
# Add object to the BTree, keyed on the ID
userdb[newuser.id] = newuser
# Commit the change
transaction.commit()
The :mod:`transaction` module defines a few top-level functions for working with
transactions. :func:`commit` writes any modified objects to disk, making the
changes permanent. :func:`abort` rolls back any changes that have been made,
restoring the original state of the objects. If you're familiar with database
transactional semantics, this is all what you'd expect. :func:`get` returns a
:class:`Transaction` object that has additional methods like :meth:`note`, to
add a note to the transaction metadata.
More precisely, the :mod:`transaction` module exposes an instance of the
:class:`ThreadTransactionManager` transaction manager class as
``transaction.manager``, and the :mod:`transaction` functions :func:`get` and
:func:`begin` redirect to the same-named methods of ``transaction.manager``.
The :func:`commit` and :func:`abort` functions apply the methods of the same
names to the :class:`Transaction` object returned by
``transaction.manager.get()``. This is for convenience. It's also possible to
create your own transaction manager instances, and to tell ``DB.open()`` to use
your transaction manager instead.
Because the integration with Python is so complete, it's a lot like having
transactional semantics for your program's variables, and you can experiment
with transactions at the Python interpreter's prompt::
>>> newuser
<User instance at 81b1f40>
>>> newuser.first_name # Print initial value
'Andrew'
>>> newuser.first_name = 'Bob' # Change first name
>>> newuser.first_name # Verify the change
'Bob'
>>> transaction.abort() # Abort transaction
>>> newuser.first_name # The value has changed back
'Andrew'
Rules for Writing Persistent Classes
------------------------------------
Practically all persistent languages impose some restrictions on programming
style, warning against constructs they can't handle or adding subtle semantic
changes, and the ZODB is no exception. Happily, the ZODB's restrictions are
fairly simple to understand, and in practice it isn't too painful to work around
them.
The summary of rules is as follows:
* If you modify a mutable object that's the value of an object's attribute, the
ZODB can't catch that, and won't mark the object as dirty. The solution is to
either set the dirty bit yourself when you modify mutable objects, or use a
wrapper for Python's lists and dictionaries (:class:`PersistentList`,
:class:`PersistentMapping`) that will set the dirty bit properly.
* Recent versions of the ZODB allow writing a class with :meth:`__setattr__` ,
:meth:`__getattr__`, or :meth:`__delattr__` methods. (Older versions didn't
support this at all.) If you write such a :meth:`__setattr__` or
:meth:`__delattr__` method, its code has to set the dirty bit manually.
* A persistent class should not have a :meth:`__del__` method. The database
moves objects freely between memory and storage. If an object has not been used
in a while, it may be released and its contents loaded from storage the next
time it is used. Since the Python interpreter is unaware of persistence, it
would call :meth:`__del__` each time the object was freed.
Let's look at each of these rules in detail.
Modifying Mutable Objects
^^^^^^^^^^^^^^^^^^^^^^^^^
The ZODB uses various Python hooks to catch attribute accesses, and can trap
most of the ways of modifying an object, but not all of them. If you modify a
:class:`User` object by assigning to one of its attributes, as in
``userobj.first_name = 'Andrew'``, the ZODB will mark the object as having been
changed, and it'll be written out on the following :meth:`commit`.
The most common idiom that *isn't* caught by the ZODB is mutating a list or
dictionary. If :class:`User` objects have a attribute named ``friends``
containing a list, calling ``userobj.friends.append(otherUser)`` doesn't mark
``userobj`` as modified; from the ZODB's point of view, ``userobj.friends`` was
only read, and its value, which happened to be an ordinary Python list, was
returned. The ZODB isn't aware that the object returned was subsequently
modified.
This is one of the few quirks you'll have to remember when using the ZODB; if
you modify a mutable attribute of an object in place, you have to manually mark
the object as having been modified by setting its dirty bit to true. This is
done by setting the :attr:`_p_changed` attribute of the object to true::
userobj.friends.append(otherUser)
userobj._p_changed = True
You can hide the implementation detail of having to mark objects as dirty by
designing your class's API to not use direct attribute access; instead, you can
use the Java-style approach of accessor methods for everything, and then set the
dirty bit within the accessor method. For example, you might forbid accessing
the ``friends`` attribute directly, and add a :meth:`get_friend_list` accessor
and an :meth:`add_friend` modifier method to the class. :meth:`add_friend`
would then look like this::
def add_friend(self, friend):
self.friends.append(otherUser)
self._p_changed = True
Alternatively, you could use a ZODB-aware list or mapping type that handles the
dirty bit for you. The ZODB comes with a :class:`PersistentMapping` class, and
I've contributed a :class:`PersistentList` class that's included in my ZODB
distribution, and may make it into a future upstream release of Zope.
.. % XXX It'd be nice to discuss what happens when an object is ``ghosted'' (e.g.
.. % you set an object's _p_changed = None). The __p_deactivate__ method should
.. % not be used (it's also obsolete).
:meth:`__getattr__`, :meth:`__delattr__`, and :meth:`__setattr__`
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ZODB allows persistent classes to have hook methods like :meth:`__getattr__` and
:meth:`__setattr__`. There are four special methods that control attribute
access; the rules for each are a little different.
The :meth:`__getattr__` method works pretty much the same for persistent classes
as it does for other classes. No special handling is needed. If an object is a
ghost, then it will be activated before :meth:`__getattr__` is called.
The other methods are more delicate. They will override the hooks provided by
:class:`Persistent`, so user code must call special methods to invoke those
hooks anyway.
The :meth:`__getattribute__` method will be called for all attribute access; it
overrides the attribute access support inherited from :class:`Persistent`. A
user-defined :meth:`__getattribute__` must always give the :class:`Persistent`
base class a chance to handle special attribute, as well as :attr:`__dict__` or
:attr:`__class__`. The user code should call :meth:`_p_getattr`, passing the
name of the attribute as the only argument. If it returns True, the user code
should call :class:`Persistent`'s :meth:`__getattribute__` to get the value. If
not, the custom user code can run.
A :meth:`__setattr__` hook will also override the :class:`Persistent`
:meth:`__setattr__` hook. User code must treat it much like
:meth:`__getattribute__`. The user-defined code must call :meth:`_p_setattr`
first to all :class:`Persistent` to handle special attributes;
:meth:`_p_setattr` takes the attribute name and value. If it returns True,
:class:`Persistent` handled the attribute. If not, the user code can run. If
the user code modifies the object's state, it must assigned to
:attr:`_p_changed`.
A :meth:`__delattr__` hooks must be implemented the same was as a the last two
hooks. The user code must call :meth:`_p_delattr`, passing the name of the
attribute as an argument. If the call returns True, :class:`Persistent` handled
the attribute; if not, the user code can run.
:meth:`__del__` methods
^^^^^^^^^^^^^^^^^^^^^^^
A :meth:`__del__` method is invoked just before the memory occupied by an
unreferenced Python object is freed. Because ZODB may materialize, and
dematerialize, a given persistent object in memory any number of times, there
isn't a meaningful relationship between when a persistent object's
:meth:`__del__` method gets invoked and any natural aspect of a persistent
object's life cycle. For example, it is emphatically not the case that a
persistent object's :meth:`__del__` method gets invoked only when the object is
no longer referenced by other objects in the database. :meth:`__del__` is only
concerned with reachability from objects in memory.
Worse, a :meth:`__del__` method can interfere with the persistence machinery's
goals. For example, some number of persistent objects reside in a
:class:`Connection`'s memory cache. At various times, to reduce memory burden,
objects that haven't been referenced recently are removed from the cache. If a
persistent object with a :meth:`__del___` method is so removed, and the cache
was holding the last memory reference to the object, the object's
:meth:`__del__` method will be invoked. If the :meth:`__del__` method then
references any attribute of the object, ZODB needs to load the object from the
database again, in order to satisfy the attribute reference. This puts the
object back into the cache again: such an object is effectively immortal,
occupying space in the memory cache forever, as every attempt to remove it from
cache puts it back into the cache. In ZODB versions prior to 3.2.2, this could
even cause the cache reduction code to fall into an infinite loop. The infinite
loop no longer occurs, but such objects continue to live in the memory cache
forever.
Because :meth:`__del__` methods don't make good sense for persistent objects,
and can create problems, persistent classes should not define :meth:`__del__`
methods.
Writing Persistent Classes
--------------------------
Now that we've looked at the basics of programming using the ZODB, we'll turn to
some more subtle tasks that are likely to come up for anyone using the ZODB in a
production system.
Changing Instance Attributes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Ideally, before making a class persistent you would get its interface right the
first time, so that no attributes would ever need to be added, removed, or have
their interpretation change over time. It's a worthy goal, but also an
impractical one unless you're gifted with perfect knowledge of the future. Such
unnatural foresight can't be required of any person, so you therefore have to be
prepared to handle such structural changes gracefully. In object-oriented
database terminology, this is a schema update. The ZODB doesn't have an actual
schema specification, but you're changing the software's expectations of the
data contained by an object, so you're implicitly changing the schema.
One way to handle such a change is to write a one-time conversion program that
will loop over every single object in the database and update them to match the
new schema. This can be easy if your network of object references is quite
structured, making it easy to find all the instances of the class being
modified. For example, if all :class:`User` objects can be found inside a
single dictionary or BTree, then it would be a simple matter to loop over every
:class:`User` instance with a :keyword:`for` statement. This is more difficult
if your object graph is less structured; if :class:`User` objects can be found
as attributes of any number of different class instances, then there's no longer
any easy way to find them all, short of writing a generalized object traversal
function that would walk over every single object in a ZODB, checking each one
to see if it's an instance of :class:`User`.
Some OODBs support a feature called extents, which allow quickly finding all the
instances of a given class, no matter where they are in the object graph;
unfortunately the ZODB doesn't offer extents as a feature.
.. % XXX Rest of section not written yet: __getstate__/__setstate__
doc/articles/old-guide/transactions.rst 0 → 100644
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.. % Transactions and Versioning
.. % Committing and Aborting
.. % Subtransactions
.. % Undoing
.. % Versions
.. % Multithreaded ZODB Programs
Transactions and Versioning
===========================
Committing and Aborting
-----------------------
Changes made during a transaction don't appear in the database until the
transaction commits. This is done by calling the :meth:`commit` method of the
current :class:`Transaction` object, where the latter is obtained from the
:meth:`get` method of the current transaction manager. If the default thread
transaction manager is being used, then ``transaction.commit()`` suffices.
Similarly, a transaction can be explicitly aborted (all changes within the
transaction thrown away) by invoking the :meth:`abort` method of the current
:class:`Transaction` object, or simply ``transaction.abort()`` if using the
default thread transaction manager.
Prior to ZODB 3.3, if a commit failed (meaning the ``commit()`` call raised an
exception), the transaction was implicitly aborted and a new transaction was
implicitly started. This could be very surprising if the exception was
suppressed, and especially if the failing commit was one in a sequence of
subtransaction commits.
So, starting with ZODB 3.3, if a commit fails, all further attempts to commit,
join, or register with the transaction raise
:exc:`ZODB.POSException.TransactionFailedError`. You must explicitly start a
new transaction then, either by calling the :meth:`abort` method of the current
transaction, or by calling the :meth:`begin` method of the current transaction's
transaction manager.
Subtransactions
---------------
Subtransactions can be created within a transaction. Each subtransaction can be
individually committed and aborted, but the changes within a subtransaction are
not truly committed until the containing transaction is committed.
The primary purpose of subtransactions is to decrease the memory usage of
transactions that touch a very large number of objects. Consider a transaction
during which 200,000 objects are modified. All the objects that are modified in
a single transaction have to remain in memory until the transaction is
committed, because the ZODB can't discard them from the object cache. This can
potentially make the memory usage quite large. With subtransactions, a commit
can be be performed at intervals, say, every 10,000 objects. Those 10,000
objects are then written to permanent storage and can be purged from the cache
to free more space.
To commit a subtransaction instead of a full transaction, pass a true value to
the :meth:`commit` or :meth:`abort` method of the :class:`Transaction` object.
::
# Commit a subtransaction
transaction.commit(True)
# Abort a subtransaction
transaction.abort(True)
A new subtransaction is automatically started upon successful committing or
aborting the previous subtransaction.
Undoing Changes
---------------
Some types of :class:`Storage` support undoing a transaction even after it's
been committed. You can tell if this is the case by calling the
:meth:`supportsUndo` method of the :class:`DB` instance, which returns true if
the underlying storage supports undo. Alternatively you can call the
:meth:`supportsUndo` method on the underlying storage instance.
If a database supports undo, then the :meth:`undoLog(start, end[, func])` method
on the :class:`DB` instance returns the log of past transactions, returning
transactions between the times *start* and *end*, measured in seconds from the
epoch. If present, *func* is a function that acts as a filter on the
transactions to be returned; it's passed a dictionary representing each
transaction, and only transactions for which *func* returns true will be
included in the list of transactions returned to the caller of :meth:`undoLog`.
The dictionary contains keys for various properties of the transaction. The
most important keys are ``id``, for the transaction ID, and ``time``, for the
time at which the transaction was committed. ::
>>> print storage.undoLog(0, sys.maxint)
[{'description': '',
'id': 'AzpGEGqU/0QAAAAAAAAGMA',
'time': 981126744.98,
'user_name': ''},
{'description': '',
'id': 'AzpGC/hUOKoAAAAAAAAFDQ',
'time': 981126478.202,
'user_name': ''}
...
To store a description and a user name on a commit, get the current transaction
and call the :meth:`note(text)` method to store a description, and the
:meth:`setUser(user_name)` method to store the user name. While :meth:`setUser`
overwrites the current user name and replaces it with the new value, the
:meth:`note` method always adds the text to the transaction's description, so it
can be called several times to log several different changes made in the course
of a single transaction. ::
transaction.get().setUser('amk')
transaction.get().note('Change ownership')
To undo a transaction, call the :meth:`DB.undo(id)` method, passing it the ID of
the transaction to undo. If the transaction can't be undone, a
:exc:`ZODB.POSException.UndoError` exception will be raised, with the message
"non-undoable transaction". Usually this will happen because later transactions
modified the objects affected by the transaction you're trying to undo.
After you call :meth:`undo` you must commit the transaction for the undo to
actually be applied. [#]_ There is one glitch in the undo process. The thread
that calls undo may not see the changes to the object until it calls
:meth:`Connection.sync` or commits another transaction.
Versions
--------
.. warning::
Versions should be avoided. They're going to be deprecated, replaced by better
approaches to long-running transactions.
While many subtransactions can be contained within a single regular transaction,
it's also possible to contain many regular transactions within a long-running
transaction, called a version in ZODB terminology. Inside a version, any number
of transactions can be created and committed or rolled back, but the changes
within a version are not made visible to other connections to the same ZODB.
Not all storages support versions, but you can test for versioning ability by
calling :meth:`supportsVersions` method of the :class:`DB` instance, which
returns true if the underlying storage supports versioning.
A version can be selected when creating the :class:`Connection` instance using
the :meth:`DB.open([*version*])` method. The *version* argument must be a string
that will be used as the name of the version. ::
vers_conn = db.open(version='Working version')
Transactions can then be committed and aborted using this versioned connection.
Other connections that don't specify a version, or provide a different version
name, will not see changes committed within the version named ``Working
version``. To commit or abort a version, which will either make the changes
visible to all clients or roll them back, call the :meth:`DB.commitVersion` or
:meth:`DB.abortVersion` methods. XXX what are the source and dest arguments for?
The ZODB makes no attempt to reconcile changes between different versions.
Instead, the first version which modifies an object will gain a lock on that
object. Attempting to modify the object from a different version or from an
unversioned connection will cause a :exc:`ZODB.POSException.VersionLockError` to
be raised::
from ZODB.POSException import VersionLockError
try:
transaction.commit()
except VersionLockError, (obj_id, version):
print ('Cannot commit; object %s '
'locked by version %s' % (obj_id, version))
The exception provides the ID of the locked object, and the name of the version
having a lock on it.
Multithreaded ZODB Programs
---------------------------
ZODB databases can be accessed from multithreaded Python programs. The
:class:`Storage` and :class:`DB` instances can be shared among several threads,
as long as individual :class:`Connection` instances are created for each thread.
.. rubric:: Footnotes
.. [#] There are actually two different ways a storage can implement the undo feature.
Most of the storages that ship with ZODB use the transactional form of undo
described in the main text. Some storages may use a non-transactional undo
makes changes visible immediately.
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.. % ZEO
.. % Installing ZEO
.. % How ZEO works (ClientStorage)
.. % Configuring ZEO
.. _zeo:
ZEO
===
How ZEO Works
-------------
The ZODB, as I've described it so far, can only be used within a single Python
process (though perhaps with multiple threads). ZEO, Zope Enterprise Objects,
extends the ZODB machinery to provide access to objects over a network. The
name "Zope Enterprise Objects" is a bit misleading; ZEO can be used to store
Python objects and access them in a distributed fashion without Zope ever
entering the picture. The combination of ZEO and ZODB is essentially a Python-
specific object database.
ZEO consists of about 12,000 lines of Python code, excluding tests. The code is
relatively small because it contains only code for a TCP/IP server, and for a
new type of Storage, :class:`ClientStorage`. :class:`ClientStorage` simply makes
remote procedure calls to the server, which then passes them on a regular
:class:`Storage` class such as :class:`FileStorage`. The following diagram lays
out the system:
XXX insert diagram here later
Any number of processes can create a :class:`ClientStorage` instance, and any
number of threads in each process can be using that instance.
:class:`ClientStorage` aggressively caches objects locally, so in order to avoid
using stale data the ZEO server sends an invalidation message to all the
connected :class:`ClientStorage` instances on every write operation. The
invalidation message contains the object ID for each object that's been
modified, letting the :class:`ClientStorage` instances delete the old data for
the given object from their caches.
This design decision has some consequences you should be aware of. First, while
ZEO isn't tied to Zope, it was first written for use with Zope, which stores
HTML, images, and program code in the database. As a result, reads from the
database are *far* more frequent than writes, and ZEO is therefore better suited
for read-intensive applications. If every :class:`ClientStorage` is writing to
the database all the time, this will result in a storm of invalidate messages
being sent, and this might take up more processing time than the actual database
operations themselves. These messages are small and sent in batches, so there
would need to be a lot of writes before it became a problem.
On the other hand, for applications that have few writes in comparison to the
number of read accesses, this aggressive caching can be a major win. Consider a
Slashdot-like discussion forum that divides the load among several Web servers.
If news items and postings are represented by objects and accessed through ZEO,
then the most heavily accessed objects -- the most recent or most popular
postings -- will very quickly wind up in the caches of the
:class:`ClientStorage` instances on the front-end servers. The back-end ZEO
server will do relatively little work, only being called upon to return the
occasional older posting that's requested, and to send the occasional invalidate
message when a new posting is added. The ZEO server isn't going to be contacted
for every single request, so its workload will remain manageable.
Installing ZEO
--------------
This section covers how to install the ZEO package, and how to configure and
run a ZEO Storage Server on a machine.
Requirements
^^^^^^^^^^^^
The ZEO server software is included in ZODB3. As with the rest of ZODB3, you'll
need Python 2.3 or higher.
Running a server
^^^^^^^^^^^^^^^^
The runzeo.py script in the ZEO directory can be used to start a server. Run it
with the -h option to see the various values. If you're just experimenting, a
good choise is to use ``python ZEO/runzeo.py -a /tmp/zeosocket -f
/tmp/test.fs`` to run ZEO with a Unix domain socket and a :class:`FileStorage`.
Testing the ZEO Installation
----------------------------
Once a ZEO server is up and running, using it is just like using ZODB with a
more conventional disk-based storage; no new programming details are introduced
by using a remote server. The only difference is that programs must create a
:class:`ClientStorage` instance instead of a :class:`FileStorage` instance.
From that point onward, ZODB-based code is happily unaware that objects are
being retrieved from a ZEO server, and not from the local disk.
As an example, and to test whether ZEO is working correctly, try running the
following lines of code, which will connect to the server, add some bits of data
to the root of the ZODB, and commits the transaction::
from ZEO import ClientStorage
from ZODB import DB
import transaction
# Change next line to connect to your ZEO server
addr = 'kronos.example.com', 1975
storage = ClientStorage.ClientStorage(addr)
db = DB(storage)
conn = db.open()
root = conn.root()
# Store some things in the root
root['list'] = ['a', 'b', 1.0, 3]
root['dict'] = {'a':1, 'b':4}
# Commit the transaction
transaction.commit()
If this code runs properly, then your ZEO server is working correctly.
You can also use a configuration file. ::
<zodb>
<zeoclient>
server localhost:9100
</zeoclient>
</zodb>
One nice feature of the configuration file is that you don't need to specify
imports for a specific storage. That makes the code a little shorter and allows
you to change storages without changing the code. ::
import ZODB.config
db = ZODB.config.databaseFromURL('/tmp/zeo.conf')
ZEO Programming Notes
---------------------
ZEO is written using :mod:`asyncore`, from the Python standard library. It
assumes that some part of the user application is running an :mod:`asyncore`
mainloop. For example, Zope run the loop in a separate thread and ZEO uses
that. If your application does not have a mainloop, ZEO will not process
incoming invalidation messages until you make some call into ZEO. The
:meth:`Connection.sync` method can be used to process pending invalidation
messages. You can call it when you want to make sure the :class:`Connection`
has the most recent version of every object, but you don't have any other work
for ZEO to do.
Sample Application: chatter.py
------------------------------
For an example application, we'll build a little chat application. What's
interesting is that none of the application's code deals with network
programming at all; instead, an object will hold chat messages, and be
magically shared between all the clients through ZEO. I won't present the
complete script here; you can download it from :download:`chatter.py
<chatter.py>`. Only the interesting portions of the code will be covered here.
The basic data structure is the :class:`ChatSession` object, which provides an
:meth:`add_message` method that adds a message, and a :meth:`new_messages`
method that returns a list of new messages that have accumulated since the last
call to :meth:`new_messages`. Internally, :class:`ChatSession` maintains a
B-tree that uses the time as the key, and stores the message as the
corresponding value.
The constructor for :class:`ChatSession` is pretty simple; it simply creates an
attribute containing a B-tree::
class ChatSession(Persistent):
def __init__(self, name):
self.name = name
# Internal attribute: _messages holds all the chat messages.
self._messages = BTrees.OOBTree.OOBTree()
:meth:`add_message` has to add a message to the ``_messages`` B-tree. A
complication is that it's possible that some other client is trying to add a
message at the same time; when this happens, the client that commits first wins,
and the second client will get a :exc:`ConflictError` exception when it tries to
commit. For this application, :exc:`ConflictError` isn't serious but simply
means that the operation has to be retried; other applications might treat it as
a fatal error. The code uses ``try...except...else`` inside a ``while`` loop,
breaking out of the loop when the commit works without raising an exception. ::
def add_message(self, message):
"""Add a message to the channel.
message -- text of the message to be added
"""
while 1:
try:
now = time.time()
self._messages[now] = message
get_transaction().commit()
except ConflictError:
# Conflict occurred; this process should abort,
# wait for a little bit, then try again.
transaction.abort()
time.sleep(.2)
else:
# No ConflictError exception raised, so break
# out of the enclosing while loop.
break
# end while
:meth:`new_messages` introduces the use of *volatile* attributes. Attributes of
a persistent object that begin with ``_v_`` are considered volatile and are
never stored in the database. :meth:`new_messages` needs to store the last time
the method was called, but if the time was stored as a regular attribute, its
value would be committed to the database and shared with all the other clients.
:meth:`new_messages` would then return the new messages accumulated since any
other client called :meth:`new_messages`, which isn't what we want. ::
def new_messages(self):
"Return new messages."
# self._v_last_time is the time of the most recent message
# returned to the user of this class.
if not hasattr(self, '_v_last_time'):
self._v_last_time = 0
new = []
T = self._v_last_time
for T2, message in self._messages.items():
if T2 > T:
new.append(message)
self._v_last_time = T2
return new
This application is interesting because it uses ZEO to easily share a data
structure; ZEO and ZODB are being used for their networking ability, not
primarily for their data storage ability. I can foresee many interesting
applications using ZEO in this way:
* With a Tkinter front-end, and a cleverer, more scalable data structure, you
could build a shared whiteboard using the same technique.
* A shared chessboard object would make writing a networked chess game easy.
* You could create a Python class containing a CD's title and track information.
To make a CD database, a read-only ZEO server could be opened to the world, or
an HTTP or XML-RPC interface could be written on top of the ZODB.
* A program like Quicken could use a ZODB on the local disk to store its data.
This avoids the need to write and maintain specialized I/O code that reads in
your objects and writes them out; instead you can concentrate on the problem
domain, writing objects that represent cheques, stock portfolios, or whatever.
doc/conf.py 0 → 100644
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# -*- coding: utf-8 -*-
#
# ZODB documentation and articles documentation build configuration file, created by
# sphinx-quickstart on Sat Feb 21 09:17:33 2009.
#
# This file is execfile()d with the current directory set to its containing dir.
#
# The contents of this file are pickled, so don't put values in the namespace
# that aren't pickleable (module imports are okay, they're removed automatically).
#
# Note that not all possible configuration values are present in this
# autogenerated file.
#
# All configuration values have a default; values that are commented out
# serve to show the default.
# If your extensions are in another directory, add it here. If the directory
# is relative to the documentation root, use os.path.abspath to make it
# absolute, like shown here.
#sys.path.append(os.path.abspath('.'))
# General configuration
# ---------------------
# Add any Sphinx extension module names here, as strings. They can be extensions
# coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
extensions = ['sphinx.ext.autodoc',
'j1m.sphinxautointerface',
'j1m.sphinxautozconfig']
# Add any paths that contain templates here, relative to this directory.
templates_path = ['.templates']
# The suffix of source filenames.
source_suffix = '.rst'
# The encoding of source files.
#source_encoding = 'utf-8'
# The master toctree document.
master_doc = 'index'
# General information about the project.
project = u'ZODB'
copyright = u'2009-2016, Zope Foundation'
# The version info for the project you're documenting, acts as replacement for
# |version| and |release|, also used in various other places throughout the
# built documents.
#
# The short X.Y version.
version = '5.0'
# The full version, including alpha/beta/rc tags.
#release = '3.10.3'
# The language for content autogenerated by Sphinx. Refer to documentation
# for a list of supported languages.
#language = None
# There are two options for replacing |today|: either, you set today to some
# non-false value, then it is used:
#today = ''
# Else, today_fmt is used as the format for a strftime call.
#today_fmt = '%B %d, %Y'
exclude_patterns = [
'README.rst',
'eggs/**',
]
# The reST default role (used for this markup: `text`) to use for all documents.
#default_role = None
# If true, '()' will be appended to :func: etc. cross-reference text.
#add_function_parentheses = True
# If true, the current module name will be prepended to all description
# unit titles (such as .. function::).
#add_module_names = True
# If true, sectionauthor and moduleauthor directives will be shown in the
# output. They are ignored by default.
#show_authors = False
# The name of the Pygments (syntax highlighting) style to use.
pygments_style = 'sphinx'
# Options for HTML output
# -----------------------
# The style sheet to use for HTML and HTML Help pages. A file of that name
# must exist either in Sphinx' static/ path, or in one of the custom paths
# given in html_static_path.
#html_style = 'default.css'
# The name for this set of Sphinx documents. If None, it defaults to
# "<project> v<release> documentation".
#html_title = None
# A shorter title for the navigation bar. Default is the same as html_title.
#html_short_title = None
# The name of an image file (relative to this directory) to place at the top
# of the sidebar.
html_logo = 'zodb.png'
# The name of an image file (within the static path) to use as favicon of the
# docs. This file should be a Windows icon file (.ico) being 16x16 or 32x32
# pixels large.
html_favicon = 'zodb.ico'
# Add any paths that contain custom static files (such as style sheets) here,
# relative to this directory. They are copied after the builtin static files,
# so a file named "default.css" will overwrite the builtin "default.css".
html_static_path = ['.static']
# If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
# using the given strftime format.
#html_last_updated_fmt = '%b %d, %Y'
# If true, SmartyPants will be used to convert quotes and dashes to
# typographically correct entities.
#html_use_smartypants = True
# Custom sidebar templates, maps document names to template names.
#html_sidebars = {}
# Additional templates that should be rendered to pages, maps page names to
# template names.
#html_additional_pages = {}
# If false, no module index is generated.
#html_use_modindex = True
# If false, no index is generated.
#html_use_index = True
# If true, the index is split into individual pages for each letter.
#html_split_index = False
# If true, the reST sources are included in the HTML build as _sources/<name>.
#html_copy_source = True
# If true, an OpenSearch description file will be output, and all pages will
# contain a <link> tag referring to it. The value of this option must be the
# base URL from which the finished HTML is served.
#html_use_opensearch = ''
# If nonempty, this is the file name suffix for HTML files (e.g. ".xhtml").
#html_file_suffix = ''
# Output file base name for HTML help builder.
htmlhelp_basename = 'ZODBdocumentationandarticlesdoc'
# Options for LaTeX output
# ------------------------
# The paper size ('letter' or 'a4').
#latex_paper_size = 'letter'
# The font size ('10pt', '11pt' or '12pt').
#latex_font_size = '10pt'
# Grouping the document tree into LaTeX files. List of tuples
# (source start file, target name, title, author, document class [howto/manual]).
latex_documents = [
('index', 'ZODBdocumentationandarticles.tex', ur'ZODB documentation and articles',
ur'Zope Developer Community', 'manual'),
]
# The name of an image file (relative to this directory) to place at the top of
# the title page.
#latex_logo = None
# For "manual" documents, if this is true, then toplevel headings are parts,
# not chapters.
#latex_use_parts = False
# Additional stuff for the LaTeX preamble.
#latex_preamble = ''
# Documents to append as an appendix to all manuals.
#latex_appendices = []
# If false, no module index is generated.
#latex_use_modindex = True
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======================
ZODB programming guide
======================
This guide consists of a collection of topics that should be of
interest to most developers. They're provuded in order of importance,
which is also an order from least to most advanced, but they can be
read in any order.
If you haven't yet, you should read the :ref:`Tutorial <tutorial-label>`.
.. toctree::
:maxdepth: 2
install-and-run
writing-persistent-objects.rst
.. todo:
transaction.rst
storages.rst
configuration.rst
threading.rst
packing-and-garbage-collection.rst
blobs.rst
multi-databases.rst
doc/guide/install-and-run.rst 0 → 100644
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==========================
Installing and running ZODB
===========================
This topic discusses some boring nitty-gritty details needed to
actually run ZODB.
Installation
============
Installation of ZODB is pretty straightforward using Python's
packaging system. For example, using pip::
pip install ZODB
You may need additional optional packages, such as `ZEO
<https://pypi.python.org/pypi/ZEO>`_ or `RelStorage
<https://pypi.python.org/pypi/RelStorage>`_, depending your deployment
choices.
Configuration
=============
You can set up ZODB in your application using either Python, or
ZODB's configuration language. For simple database setup, and
especially for exploration, the Python APIs are sufficient.
For more complex configurations, you'll probably find ZODB's
configuration language easier to use.
To understand database setup, it's important to understand ZODB's
architecture. ZODB separates database functionality
from storage concerns. When you create a database object,
you specify a storage object for it to use, as in::
import ZODB, ZODB.FileStorage
storage = ZODB.FileStorage.FileStorage('mydata.fs')
db = ZODB.DB(storage)
So when you define a database, you'll also define a storage. In the
example above, we define a :class:`file storage
<ZODB.FileStorage.FileStorage.FileStorage>` and then use it to define
a database.
Sometimes, storages are created through composition. For example, if
we want to save space, we could layer a ``ZlibStorage``
[#zlibstoragefn]_ over the file storage::
import ZODB, ZODB.FileStorage, zc.zlibstorage
storage = ZODB.FileStorage.FileStorage('mydata.fs')
compressed_storage = zc.zlibstorage.ZlibStorage(storage)
db = ZODB.DB(compressed_storage)
`ZlibStorage <https://pypi.python.org/pypi/zc.zlibstorage>`_
compresses database records [#zlib]_.
Python configuration
--------------------
To set up a database with Python, you'll construct a storage using the
:ref:`storage APIs <included-storages-label>`, and then pass the
storage to the :class:`~ZODB.DB` class to create a database, as shown
in the examples in the previous section.
The :class:`~ZODB.DB` class also accepts a string path name as its
storage argument to automatically create a file storage. You can also
pass ``None`` as the storage to automatically use a
:class:`~ZODB.MappingStorage.MappingStorage`, which is convenient when
exploring ZODB::
db = ZODB.DB(None) # Create an in-memory database.
Text configuration
------------------
ZODB supports a text-based configuration language. It uses a syntax
similar to Apache configuration files. The syntax was chosen to be
familiar to site administrators.
ZODB's text configuration uses `ZConfig
<https://pypi.python.org/pypi/ZConfig>`_. You can use ZConfig to
create your application's configuration, but it's more common to
include ZODB configuration strings in their own files or embedded in
simpler configuration files, such as `configarser
<https://docs.python.org/3/library/configparser.html#module-configparser>`_
files.
A database configuration string has a ``zodb`` section wrapping a
storage section, as in::
<zodb>
cache-size-bytes 100MB
<mappingstorage>
</mappingstorage>
</zodb>
.. -> snippet
In the example above, the :ref:`mappingstorage
<mappingstorage-text-configuration>` section defines the storage used
by the database.
To create a database from a string, use
:func:`ZODB.config.databaseFromString`::
>>> import ZODB.config
>>> db = ZODB.config.databaseFromString(snippet)
To load databases from file names or URLs, use
:func:`ZODB.config.databaseFromURL`.
URI-based configuration
-----------------------
Another database configuration option is provided by the `zodburi
<https://pypi.python.org/pypi/zodburi>`_ package. See:
http://docs.pylonsproject.org/projects/zodburi. It's less powerful
than the Python or text configuration options, but allows
configuration to be reduced to a single URI and handles most cases.
Using databases: connections
============================
Once you have a database, you need to get a database connection to to
much of anything. Connections take care of loading and saving objects
and manage object caches. Each connection has it's own cache
[#caches-are-expensive]_.
Getting connections
-------------------
Amongst [#amongst]_ the common ways of getting a connection:
db.open()
The database :meth:`~ZODB.DB.open` method opens a
connection, returning a connection object::
>>> conn = db.open()
It's up to the application to call
:meth:`~ZODB.Connection.Connection.close` when the application is
done using the connection.
If changes are made, the application :ref:`commits transactions
<commit-transactions>` to make them permanent.
db.transaction()
The database :meth:`~ZODB.DB.transaction` method
returns a context manager that can be used with the `python with
statement
<https://docs.python.org/3/reference/compound_stmts.html#grammar-token-with_stmt>`_
to execute a block of code in a transaction::
with db.transaction() as connection:
connection.root.foo = 1
.. -> src
>>> exec(src)
>>> with db.transaction() as connection:
... print(connection.root.foo)
1
>>> _ = conn.transaction_manager.begin() # get updates on conn
In the example above, we used ``as connection`` to get the database
connection used in the variable ``connection``.
some_object._p_jar
For code that's already running in the context of an open
connection, you can get the current connection as the ``_p_jar``
attribute of some persistent object that was accessed via the
connection.
Getting objects
---------------
Once you have a connection, you access objects by traversing the
object graph from the root object.
The database root object is a mapping object that holds the top level
objects in the database. There should only be a small number of
top-level objects (often only one). You can get the root object by calling a
connection's ``root`` attribute::
>>> root = conn.root()
>>> root
{'foo': 1}
>>> root['foo']
1
For convenience [#root-convenience]_, you can also get top-level
objects by accessing attributes of the connection root object:
>>> conn.root.foo
1
Once you have a top-level object, you use its methods, attributes, or
operations to access other objects and so on to get the objects you
need. Often indexing data structures like BTrees_ are used to
make it possible to search objects in large collections.
.. [#zlibstoragefn] `zc.zlibstorage
<https://pypi.python.org/pypi/zc.zlibstorage>`_ is an optional
package that you need to install separately.
.. [#zlib] ZlibStorage uses the :mod:`zlib` standard module, which
uses the `zlib library <http://www.zlib.net/>`_.
.. [#caches-are-expensive] ZODB can be very efficient at caching data
in memory, especially if your `working set
<https://en.wikipedia.org/wiki/Working_set>`_ is small enough to
fit in memory, because the cache is simply an object tree and
accessing a cached object typically requires no database
interaction. Because each connection has its own cache,
connections can be expensive, depending on their cache sizes. For
this reason, you'll generally want to limit the number of open
connections you have at any one time. Connections are pooled, so
opening a connection is inexpensive.
.. [#amongst] https://www.youtube.com/watch?v=7WJXHY2OXGE
.. [#root-convenience] The ability to access top-level objects of the
database as root attributes is a recent convenience. Originally,
the ``root()`` method was used to access the root object which was
then accessed as a mapping. It's still potentially useful to
access top-level objects using the mapping interface if their names
aren't valid attribute names.
.. _BTrees: https://pythonhosted.org/BTrees/
doc/guide/writing-persistent-objects.rst 0 → 100644
View file @ de6dba53
==========================
Writing persistent objects
==========================
In the :ref:`Tutorial <tutorial-label>`, we discussed the basics of
implementing persistent objects by subclassing
``persistent.Persistent``. This is probably enough for 80% of
persistent-object classes you write, but there are some other aspects
of writing persistent classes you should be aware of.
Access and modification
=======================
Two of the main jobs of the ``Persistent`` base class is to detect
when an object has been accessed and when it has been modified. When
an object is accessed, its state may need to be loaded from the
database. When an object is modified, the modification needs to be
saved if a transaction is committed.
``Persistent`` detects object accesses by hooking into object
attribute access and update. In the case of object update, there
may be other ways of modifying state that we need to make provision for.
Rules of persistence
====================
When implementing persistent objects, be aware that an object's
attributes should be :
- immutable (such as strings or integers),
- persistent (subclass Persistent), or
- You need to take special precautions.
If you modify a non-persistent mutable value of a persistent-object
attribute, you need to mark the persistent object as changed yourself
by setting ``_p_changed`` to True::
import persistent
class Book(persistent.Persistent):
def __init__(self, title):
self.title = title
self.authors = []
def add_author(self, author):
self.authors.append(author)
self._p_changed = True
.. -> src
>>> exec(src)
>>> db = ZODB.DB(None)
>>> with db.transaction() as conn:
... conn.root.book = Book("ZODB")
>>> conn = db.open()
>>> book = conn.root.book
>>> bool(book._p_changed)
False
>>> book.authors.append('Jim')
>>> bool(book._p_changed)
False
>>> book.add_author('Carlos')
>>> bool(book._p_changed)
True
>>> db.close()
In this example, ``Book`` objects have an ``authors`` object that's a
regular Python list, so it's mutable and non-persistent. When we add
an author, we append it to the ``authors`` attribute's value. Because
we didn't set an attribute on the book, it's not marked as changed, so
we set ``_p_changed`` ourselves.
Using standard Python lists, dicts, or sets is a common thing to do,
so this pattern of setting ``_p_changed`` is common.
Let's look at some alternatives.
Using tuples for small sequences instead of lists
-------------------------------------------------
If objects contain sequences that are small or that don't change
often, you can use tuples instead of lists::
import persistent
class Book(persistent.Persistent):
def __init__(self, title):
self.title = title
self.authors = ()
def add_author(self, author):
self.authors += (author, )
.. -> src
>>> exec(src)
>>> db = ZODB.DB(None)
>>> with db.transaction() as conn:
... conn.root.book = Book("ZODB")
>>> conn = db.open()
>>> book = conn.root.book
>>> bool(book._p_changed)
False
>>> book.add_author('Carlos')
>>> bool(book._p_changed)
True
>>> db.close()
Because tuples are immutable, they satisfy the rules of persistence
without any special handling.
Using persistent data structures
--------------------------------
The ``persistent`` package provides persistent versions of ``list``
and ``dict``, namely ``persistent.list.PersistentList`` and
``persistent.mapping.PersistentMapping``. We can update our example to
use ``PersistentList``::
import persistent
import persistent.list
class Book(persistent.Persistent):
def __init__(self, title):
self.title = title
self.authors = persistent.list.PersistentList()
def add_author(self, author):
self.authors.append(author)
.. -> src
>>> exec(src)
>>> db = ZODB.DB(None)
>>> with db.transaction() as conn:
... conn.root.book = Book("ZODB")
>>> conn = db.open()
>>> book = conn.root.book
>>> bool(book._p_changed)
False
>>> book.add_author('Carlos')
>>> bool(book._p_changed)
False
>>> bool(book.authors._p_changed)
True
>>> db.close()
Note that in this example, when we added an author, the book itself
didn't change, but the ``authors`` attribute value did. Because
``authors`` is a persistent object, it's stored in a separate database
record from the book record and is managed by ZODB independent of the
management of the book.
In addition to ``PersistentList`` and ``PersistentMapping``, general
persistent data structures are provided by the BTrees_ package,
most notably ``BTree`` and ``TreeSet`` objects. Unlike
``PersistentList`` and ``PersistentMapping``, ``BTree`` and
``TreeSet`` objects are scalable and can easily hold millions of
objects, because their data are spread over many subobjects.
It's generally better to use ``BTree`` objects than
``PersistentMapping`` objects, because they're scalable and because
they handle :ref:`conflicts <conflicts-label>` better. ``TreeSet``
objects are the only ZODB-provided persistent set implementation.
``BTree`` and ``TreeSets`` come in a number of families provided via
different modules and differ in their internal implementations:
=============== =============== ================
Module Key type Value Type
=============== =============== ================
BTrees.OOBTree object object
BTrees.IOBTree integer Object
BTrees.OIBTree object integer
BTrees.IIBTree integer integer
BTrees.IFBTree integer float
BTrees.LOBTree 64-bit integer Object
BTrees.OLBTree object 64-bit integer
BTrees.LLBTree 64-bit integer 64-bit integer
BTrees.LFBTree 64-bit integer float
=============== =============== ================
Here's a version of the example that uses a ``TreeSet``::
import persistent
from BTrees.OOBTree import TreeSet
class Book(persistent.Persistent):
def __init__(self, title):
self.title = title
self.authors = TreeSet()
def add_author(self, author):
self.authors.add(author)
.. -> src
>>> exec(src)
>>> db = ZODB.DB(None)
>>> with db.transaction() as conn:
... conn.root.book = Book("ZODB")
>>> conn = db.open()
>>> book = conn.root.book
>>> bool(book._p_changed)
False
>>> book.add_author('Carlos')
>>> bool(book._p_changed)
False
>>> bool(book.authors._p_changed)
True
>>> db.close()
Scalable sequences are a bit more challenging. The `zc.blist
<https://pypi.python.org/pypi/zc.blist/>`_ package provides a scalable
list implementation that works well for some sequence use cases.
Properties
==========
If you implement some attributes using Python properties (or other
types of descriptors), they are treated just like any other attributes
by the persistence machinery. When you set an attribute through a
property, the object is considered changed, even if the property
didn't actually modify the object state.
Special attributes
==================
There are some attributes that are treated specially.
Attributes with names starting with ``_p_`` are reserved for use by
the persistence machinery and by ZODB. These include (but aren't
limited to):
_p_changed
The ``_p_changed`` attribute has the value ``None`` if the
object is a :ref:`ghost <ghost-label>`, True if it's changed, and
False if it's not a ghost and not changed.
_p_oid
The object's unique id in the database.
_p_serial
The object's revision identifier also know as the object serial
number, also known as the object transaction id. It's a timestamp
and if not set has the value 0 encoded as string of 8 zero bytes.
_p_jar
The database connection the object was accessed through. This is
commonly used by database-aware application code to get hold of an
object's database connection.
Attributes with names starting with ``_v_`` are treated as volatile.
They aren't saved to the database. They're useful for caching data
that can be computed from saved data and shouldn't be saved [#cache]_.
They should be treated as though they can disappear between
transactions. Setting a volatile attribute doesn't cause an object to
be considered to be modified.
An object's ``__dict__`` attribute is treated specially in that
getting it doesn't cause an object's state to be loaded. It may have
the value ``None`` rather than a dictionary for :ref:`ghosts
<ghost-label>`.
Object storage and management
=============================
Every persistent object is stored in its own database record. Some
storages maintain multiple object revisions, in which case each
persistent object is stored in its own set of records. Data for
different persistent objects are stored separately.
The database manages each object separately, according to a :ref:`life
cycle <object-life-cycle-label>`.
This is important when considering how to distribute data across your
objects. If you use lots of small persistent objects, then more
objects may need to be loaded or saved and you may incur more memory
overhead. On the other hand, if objects are too big, you may load or
save more data than would otherwise be needed.
You can't change your mind in subclassing persistent
====================================================
Currently, you can't change your mind about whether a class is
persistent (subclasses ``persistent.Persistent``) or not. If you save
objects in a database who's classes subclass ``persistent.Persistent``,
you can't change your mind later and make them non-persistent, and the
other way around. This may be a `bug or misfeature
<https://github.com/zopefoundation/ZODB/issues/99>`_.
.. _schema-migration-label:
Schema migration
================
Object requirements and implementations tend to evolve over time.
This isn't a problem for objects that are short lived, but persistent
objects may have lifetimes that extend for years. There needs to be
some way of making sure that state for an older object schema can
still be loaded into an object with the new schema.
Adding attributes
-----------------
Perhaps the commonest schema change is to add attributes. This is
usually accomplished easily by adding a default value in a class
definition::
class Book(persistent.Persistent):
publisher = 'UNKNOWN'
def __init__(self, title, publisher):
self.title = title
self.publisher = publisher
self.authors = TreeSet()
def add_author(self, author):
self.authors.add(author)
Removing attributes
-------------------
Removing attributes generally doesn't require any action, assuming
that their presence in older objects doesn't do any harm.
Renaming/moving classes
-----------------------
The easiest way to handle renaming or moving classes is to leave
aliases for the old name. For example, if we have a class,
``library.Book``, and want to move it to ``catalog.Publication``, we
can keep a ``library`` module that contains::
from catalog import Publication as Book # XXX deprecated name
A downside of this approach is that it clutters code and may even
cause us to keep modules solely to hold aliases. (`zope.deferredimport
<http://zopedeferredimport.readthedocs.io/en/latest/narrative.html>`_
can help with this by making these aliases a little more efficient and
by generating deprecation warnings.)
Migration scripts
-----------------
If the simple approaches above aren't enough, then migration scripts
can be used. How these scripts are written is usually application
dependent, as the application usually determines where objects of a
given type reside in the database. (There are also some low-level
interfaces for iterating over all of the objects of a database, but
these are usually impractical for large databases.)
An improvement to running migration scripts manually is to use a
generational framework like `zope.generations
<https://pypi.python.org/pypi/zope.generations>`_. With a generational
framework, each migration is assigned a migration number and the
number is recorded in the database as each migration is run. This is
useful because remembering what migrations are needed is automated.
Upgrading multiple clients without down time
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Production applications typically have multiple clients for
availability and load balancing. This means an active application may
be committing transactions using multiple software and schema
versions. In this situation, you may need to plan schema migrations
in multiple steps:
#. Upgrade software on all clients to a version that works with the old and new
version of the schema and that writes data using the old schema.
#. Upgrade software on all clients to a version that works with the old and new
version of the schema and that writes data using the new schema.
#. Migrate objects written with the old schema to the new schema.
#. Remove support for the old schema from the software.
.. _object-life-cycle-label:
Object life cycle states and special attributes (advanced)
==========================================================
Persistent objects typically transition through a collection of
states. Most of the time, you don't need to think too much about this.
Unsaved
When an object is created, it's said to be in an *unsaved* state
until it's associated with a database.
Added
When an unsaved object is added to a database, but hasn't been
saved by committing a transaction, it's in the *added* state.
Note that most objects are added implicitly by being set as
subobjects (attribute values or items) of objects already in the
database.
Saved
When an object is added and saved through a transaction commit, the
object is in the *saved* state.
Changed
When a saved object is updated, it enters the *changed* state to
indicate that there are changes that need to be committed. It
remains in this state until either:
- The current transaction is committed, and the object transitions to
the saved state, or
- The current transaction is aborted, and the object transitions to
the ghost state.
.. _ghost-label:
Ghost
An object in the *ghost* state is an empty shell. It has no
state. When it's accessed, its state will be loaded automatically,
and it will enter the saved state. A saved object can become a
ghost if it hasn't been accessed in a while and the database
releases its state to make room for other objects. A changed
object can also become a ghost if the transaction it's modified in is
aborted.
An object that's loaded from the database is loaded as a
ghost. This typically happens when the object is a subobject of
another object who's state is loaded.
We can interrogate and control an object's state, although somewhat
indirectly. To do this, we'll look at some special persistent-object
attributes, described in `Special attributes`_, above.
Let's look at some state transitions with an example. First, we create
an unsaved book::
>>> book = Book("ZODB")
>>> from ZODB.utils import z64
>>> book._p_changed, bool(book._p_oid)
(False, False)
We can tell that it's unsaved because it doesn't have an object id, ``_p_oid``.
If we add it to a database::
>>> import ZODB
>>> connection = ZODB.connection(None)
>>> connection.add(book)
>>> book._p_changed, bool(book._p_oid), book._p_serial == z64
(False, True, True)
We know it's added because it has an oid, but its serial (object
revision timestamp), ``_p_serial``, is the special zero value, and it's
value for ``_p_changed`` is False.
If we commit the transaction that added it::
>>> import transaction
>>> transaction.commit()
>>> book._p_changed, bool(book._p_oid), book._p_serial == z64
(False, True, False)
We see that the object is in the saved state because it has an object
id and serial, and is unchanged.
Now if we modify the object, it enters the changed state:
>>> book.title = "ZODB Explained"
>>> book._p_changed, bool(book._p_oid), book._p_serial == z64
(True, True, False)
If we abort the transaction, the object becomes a ghost:
>>> transaction.abort()
>>> book._p_changed, bool(book._p_oid)
(None, True)
We can see it's a ghost because ``_p_changed`` is None.
(``_p_serial`` isn't meaningful for ghosts.)
If we access the object, it will be loaded into the saved state, which
is indicated by a false ``_p_changed`` and an object id and non-zero serial.
>>> book.title
'ZODB'
>>> book._p_changed, bool(book._p_oid), book._p_serial == z64
(False, True, False)
Note that accessing ``_p_`` attributes didn't cause the object's state
to be loaded.
We've already seen how modifying ``_p_changed`` can cause an object to
be marked as modified. We can also use it to make an object into a
ghost:
>>> book._p_changed = None
>>> book._p_changed, bool(book._p_oid)
(None, True)
Other things you can do, but shouldn't (advanced)
=================================================
The first rule here is don't be clever!!! It's very tempting to be
clever, but it's almost never worth it.
Overriding ``__getstate__`` and ``__setstate__``
------------------------------------------------
When an object is saved in a database, its ``__getstate__`` method is
called without arguments to get the object's state. The default
implementation simply returns a copy of an object's instance
dictionary. (It's a little more complicated for objects with slots.)
An object's state is loaded by loading the state from the database and
passing it to the object's ``__setstate__`` method. The default
implementation expects a dictionary, which it uses to populate the
object's instance dictionary.
Early on, we thought that overriding these methods would be useful for
tasks like providing more efficient state representations or for
:ref:`schema migration <schema-migration-label>`, but we found that
the result was to make object implementations brittle and/or complex
and the benefit usually wasn't worth it.
Overriding ``__getattr__``, ``__getattribute__``, or ``__setattribute__``
-------------------------------------------------------------------------
This is something extremely clever people might attempt, but it's
probably never worth the bother. It's possible, but it requires such
deep understanding of persistence and internals that we're not even
going to document it. :)
Links
=====
`persistent.Persistent
<http://persistent.readthedocs.io/en/latest/index.html>`_ provides
additional documentation on the ``Persistent`` base class.
The `zc.blist <https://pypi.python.org/pypi/zc.blist/>`_ package provides
a scalable sequence implementation for many use cases.
The `zope.cachedescriptors
<https://pypi.python.org/pypi/zope.cachedescriptors>`_ package
provides descriptor implementations that facilitate implementing
caching attributes, especially ``_v_`` volatile attributes.
The `zope.deferredimport
<http://zopedeferredimport.readthedocs.io/en/latest/narrative.html>`_
package provides lazy import and support for deprecating import
location, which is helpful when moving classes, especially persistent
classes.
The `zope.generations
<https://pypi.python.org/pypi/zope.generations>`_ package provides a
framework for managing schema-migration scripts.
.. [#cache] The `zope.cachedescriptors
<https://pypi.python.org/pypi/zope.cachedescriptors>`_ package
provides some descriptors that help implement attributes that cache
data.
.. _BTrees: https://pythonhosted.org/BTrees/
doc/index.rst 0 → 100644
View file @ de6dba53
==========================================
ZODB - a native object database for Python
==========================================
Because ZODB is an object database:
- no separate language for database operations
- very little impact on your code to make objects persistent
- no database mapper that partially hides the database.
Using an object-relational mapping **is not** like using an object database.
- almost no seam between code and database.
Check out the :doc:`tutorial`!
Transactions
============
Make programs easier to reason about.
Transactions are atomic
Changes made in a transaction are either saved in their entirety or
not at all.
This makes error handling a lot easier. If you have an error, you
just abort the current transaction. You don't have to worry about
undoing previous database changes.
Transactions provide isolation
Transactions allow multiple logical threads (threads or processes)
to access databases and the database prevents the threads from
making conflicting changes.
This allows you to scale your application across multiple threads,
processes or machines without having to use low-level locking
primitives.
You still have to deal with concurrency on some level. For
timestamp-based systems like ZODB, you may have to retry conflicting
transactions. With locking-based systems, you have to deal with
possible deadlocks.
Transactions affect multiple objects
Most NoSQL databases don't have transactions. Their notions of
consistency are much weaker, typically applying to single documents.
There can be good reasons to use NoSQL databases for their extreme
scalability, but otherwise, think hard about giving up the benefits
of transactions.
ZODB transaction support:
- `ACID <https://en.wikipedia.org/wiki/ACID>`_ transactions with
`snapshot isolation
<https://en.wikipedia.org/wiki/Snapshot_isolation>`_
- Distributed transaction support using two-phase commit
This allows transactions to span multiple ZODB databases and to span
ZODB and non-ZODB databases.
Other notable ZODB features
===========================
Pluggable layered storage
ZODB has a pluggable storage architecture. This allows a variety of
storage schemes including memory-based, file-based and distributed
(client-server) storage. Through storage layering, storage
components provide compression, encryption, replication and more.
Database caching with invalidation
Every database connection has a cache that is a consistent partial database
replica. When accessing database objects, data already in the cache
is accessed without any database interactions. When data are
modified, invalidations are sent to clients causing cached objects
to be invalidated. The next time invalidated objects are accessed
they'll be loaded from the database.
This makes caching extremely efficient, but provides some limit to
the number of clients. The server has to send an invalidation
message to each client for each write.
Easy testing
ZODB provides in-memory storage implementations as well as
copy-on-write layered "demo storage" implementations that make testing
database-related code very easy.
Time travel
ZODB storages typically add new records on write and remove old
records on "pack" operations. This allows limited time travel, back
to the last pack time. This can be very useful for forensic
analysis.
Binary large objects, Blobs
Many databases have these, but so does ZODB.
In applications, Blobs are files, so they can be treated as files in
many ways. This can be especially useful when serving media. If you
use AWS, there's a Blob implementation that stores blobs in S3 and
caches them on disk.
When should you use ZODB?
=========================
You want to focus on your application without writing a lot of database code.
Even if find you need to incorporate or switch to another database
later, you can use ZODB in the early part of your project to make
initial discovery and learning much quicker.
Your application has complex relationships and data structures.
In relational databases you have to join tables to model complex
data structures and these joins can be tedious and expensive. You
can mitigate this to some extent in databases like Postgres by using
more powerful data types like arrays and JSON columns, but when
relationships extend across rows, you still have to do joins.
In NoSQL databases, you can model complex data structures with
documents, but if you have relationships across documents, then you
have to do joins and join capabilities in NoSQL databases are
typically far less powerful and transactional semantics typically don't
cross documents, if they exist at all.
In ZODB, you can make objects as complex as you want and cross
object relationships are handled with Python object references.
You access data through object attributes and methods.
If your primary object access is search, then other database
technologies might be a better fit.
ZODB has no query language other than Python. It's primary support
for search is through mapping objects called BTrees. People have
build higher-level search APIs on top of ZODB. These work well
enough to support some search.
You read data a lot more than you write it.
ZODB caches aggressively, and if you're working set fits (or mostly
fits) in memory, performance is very good because it rarely has to
touch the database server.
If your application is very write heavy (e.g. logging), then you're
better off using something else. Sometimes, you can use a database
suitable for heavy writes in combination with ZODB.
Need to test logic that uses your database.
ZODB has a number of storage implementations, including layered
in-memory implementations that make testing very easy.
A database without an in-memory storage option can make testing very
complicated.
When should you *not* use ZODB?
===============================
- Search is a dominant data access path
- You have high write volume
- Caching is unlikely to benefit you
This can be the case when write volume is high, or when you tend to
access small amounts of data from a working set way too large to fit in
memory and when there's no good mechanism for dividing the working
set across application servers.
- You need to use non-Python tools to access your database.
especially tools designed to work with relational databases
How does ZODB scale?
====================
Not as well as many technologies, but some fairly large applications
have been built on ZODB.
At Zope Corporation, several hundred newspaper content-management
systems and web sites were hosted using a multi-database configuration
with most data in a main database and a catalog database. The
databases had several hundred gigabytes of ordinary database records
plus multiple terabytes of blob data.
ZODB is mature
==============
ZODB is very mature. Development started in 1996 and it has been used
in production in thousands of applications for many years.
ZODB is in heavy use in the `Pyramid <http://www.pylonsproject.org/>`_
and `Plone <https://plone.org/>`_ communities and in many other
applications.
Learning more
=============
.. toctree::
:maxdepth: 1
tutorial
guide/index
reference/index
articles/index
* `The ZODB Book (in progress) <http://zodb.readthedocs.org/en/latest/>`_
Downloads
=========
ZODB is distributed through the `Python Package Index
<http://pypi.python.org/pypi/ZODB>`_.
You can install the ZODB using pip command::
$ pip install ZODB
Community and contributing
==========================
Discussion occurs on the `ZODB mailing list
<https://groups.google.com/forum/#!forum/zodb>`_. (And for the
transaction system on the `transaction list
<https://groups.google.com/forum/#!forum/python-transaction>`_)
Bug reporting and feature requests are submitted through github issue
trackers for various ZODB components:
- `ZODB <https://github.com/zopefoundation/zodb>`_
- `persistent <https://github.com/zopefoundation/persistent>`_
- `transaction <https://github.com/zopefoundation/transaction>`_
- `BTrees <https://github.com/zopefoundation/BTrees>`_
- `ZEO (client-server framework) <https://github.com/zopefoundation/ZEO>`_
If you'd like to contribute then we'll gladly accept work on documentation,
helping out other developers and users at the mailing list, submitting bugs,
creating proposals and writing code.
ZODB is a project managed by the Zope Foundation so you can get write access
for contributing directly - check out the foundation's `Zope Developer Information <http://docs.zope.org/developer>`_.
doc/reference/index.rst 0 → 100644
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=======================
Reference Documentation
=======================
.. toctree::
:maxdepth: 2
zodb.rst
storages.rst
doc/reference/storages.rst 0 → 100644
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=============
Storage APIs
=============
.. contents::
Storage interfaces
==================
There are various storage implementations that implement standard
storage interfaces. Thet differ primarily in their constructors.
Application code rarely calls storage methods, and those it calls are
generally called indirectly through databases. There are
interface-defined methods that are called internally by ZODB. These
aren't shown below.
IStorage
--------
.. autointerface:: ZODB.interfaces.IStorage
:members: close, getName, getSize, history, isReadOnly, lastTransaction,
__len__, pack, sortKey
IStorageIteration
-----------------
.. autointerface:: ZODB.interfaces.IStorageIteration
IStorageUndoable
----------------
.. autointerface:: ZODB.interfaces.IStorageUndoable
:members: undoLog, undoInfo
IStorageCurrentRecordIteration
------------------------------
.. autointerface:: ZODB.interfaces.IStorageCurrentRecordIteration
IBlobStorage
------------
.. autointerface:: ZODB.interfaces.IBlobStorage
:members: temporaryDirectory
IStorageRecordInformation
-------------------------
.. autointerface:: ZODB.interfaces.IStorageRecordInformation
IStorageTransactionInformation
------------------------------
.. autointerface:: ZODB.interfaces.IStorageTransactionInformation
.. _included-storages-label:
Included storages
=================
FileStorage
-----------
.. autoclass:: ZODB.FileStorage.FileStorage.FileStorage
:members: __init__
.. autointerface:: ZODB.FileStorage.interfaces.IFileStoragePacker
FileStorage text configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
File storages are configured using the ``filestorage`` section::
<filestorage>
path Data.fs
</filestorage>
which accepts the following options:
.. zconfigsectionkeys:: ZODB component.xml filestorage
MappingStorage
--------------
.. autoclass:: ZODB.MappingStorage.MappingStorage
:members: __init__
.. _mappingstorage-text-configuration:
MappingStorage text configuration
---------------------------------
File storages are configured using the ``mappingstorage`` section::
<mappingstorage>
</mappingstorage>
Options:
.. zconfigsectionkeys:: ZODB component.xml mappingstorage
DemoStorage
-----------
.. autoclass:: ZODB.DemoStorage.DemoStorage
:members: __init__, push, pop
DemoStorage text configuration
------------------------------
Demo storages are configured using the ``demostorage`` section::
<demostorage>
<filestorage base>
path base.fs
</filestorage>
<mappingstorage changes>
name Changes
</mappingstorage>
</demostorage>
.. -> src
>>> import ZODB.config
>>> storage = ZODB.config.storageFromString(src)
>>> storage.base.getName()
'base.fs'
>>> storage.changes.getName()
'Changes'
``demostorage`` sections can contain up to 2 storage subsections,
named ``base`` and ``changes``, specifying the demo storage's base and
changes storages. See :meth:`ZODB.DemoStorage.DemoStorage.__init__`
for more on the base anc changes storages.
Options:
.. zconfigsectionkeys:: ZODB component.xml demostorage
Noteworthy non-included storages
================================
A number of important ZODB storages are distriubuted separately, including:
RelStorage
`RelStorage <http://relstorage.readthedocs.io/en/latest/>`_
stores data in relational databases. This is especially
useful when you have requirements or existing infrastructure for
storing data in relational databases. Unlike the included storages,
multiple processes can share the same database.
For more imformation, see http://relstorage.readthedocs.io/en/latest/.
ZEO
`ZEO <https://github.com/zopefoundation/ZEO>`_ is a client-server
database implementation for ZODB. To use ZEO, you run a ZEO server,
and use ZEO clients in your application. Unlike the included
storages, multiple processes can share the same database.
For more imformation, see https://github.com/zopefoundation/ZEO.
ZRS
`ZRS <https://github.com/zc/zrs>`_
provides replication from one database to another. It's most
commonly used with ZEO. With ZRS, you create a ZRS primary database
around a :class:`~ZODB.FileStorage.FileStorage.FileStorage` and in a
separate process, you creatre a ZRS secondary storage around any
:interface:`storage <ZODB.interfaces.IStorage>`. As transactions are
committed on the primary, they're copied asynchronously to
secondaries.
For more imformation, see https://github.com/zc/zrs.
zlibstorage
`zlibstorage <https://pypi.python.org/pypi/zc.zlibstorage>`_
compresses database records using the compression
algorithm used by `gzip <http://www.gzip.org/>`_.
For more imformation, see https://pypi.python.org/pypi/zc.zlibstorage.
beforestorage
`beforestorage <https://pypi.python.org/pypi/zc.beforestorage>`_
provides a point-in-time view of a database that might
be changing. This can be useful to provide a non-changing view of a
production database for use with a :class:`~ZODB.DemoStorage.DemoStorage`.
For more imformation, see https://pypi.python.org/pypi/zc.beforestorage.
cipher.encryptingstorage
`cipher.encryptingstorage
<https://pypi.python.org/pypi/cipher.encryptingstorage/>`_ provided
compression and encryption of database records.
For more informayion see,
https://pypi.python.org/pypi/cipher.encryptingstorage/.
doc/reference/zodb.rst 0 → 100644
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=========
ZODB APIs
=========
.. contents::
ZODB module functions
=====================
.. method:: DB(storage, *args, **kw)
Create a databse. See :py:class:`ZODB.DB`.
.. autofunction:: ZODB.connection
Databases
=========
.. autoclass:: ZODB.DB
:members: __init__, open, close, pack,
cacheDetail, cacheExtremeDetail, cacheMinimize,
cacheSize, cacheDetailSize, getCacheSize, getCacheSizeBytes,
lastTransaction, getName, getPoolSize, getSize,
getHistoricalCacheSize, getHistoricalCacheSizeBytes,
getHistoricalPoolSize, getHistoricalTimeout,
objectCount, connectionDebugInfo,
setCacheSize, setCacheSizeBytes,
setHistoricalCacheSize, setHistoricalCacheSizeBytes,
setPoolSize, setHistoricalPoolSize, setHistoricalTimeout,
history,
supportsUndo, undoLog, undoInfo, undoMultiple, undo,
transaction, storage
.. _database-text-configuration:
Database text configuration
---------------------------
Databases are configured with ``zodb`` sections::
<zodb>
cache-size-bytes 100MB
<mappingstorage>
</mappingstorage>
</zodb>
A ``zodb`` section must have a storage sub-section specifying a
storage and any of the following options:
.. zconfigsectionkeys:: ZODB component.xml zodb
.. _multidatabase-text-configuration:
For a multi-database configuration, use multiple ``zodb`` sections and
give the sections names::
<zodb first>
cache-size-bytes 100MB
<mappingstorage>
</mappingstorage>
</zodb>
<zodb second>
<mappingstorage>
</mappingstorage>
</zodb>
.. -> src
>>> import ZODB.config
>>> db = ZODB.config.databaseFromString(src)
>>> sorted(db.databases)
['first', 'second']
>>> db._cache_size_bytes
104857600
When the configuration is loaded, a single database will be returned,
but all of the databases will be available through the returned
database's ``databases`` attribute.
Connections
===========
.. autoclass:: ZODB.Connection.Connection
:members: add, cacheGC, cacheMinimize, close, db, get,
getDebugInfo, get_connection, isReadOnly, oldstate,
onCloseCallback, root, setDebugInfo, sync
TimeStamp (transaction ids)
===========================
.. class:: ZODB.TimeStamp.TimeStamp(year, month, day, hour, minute, seconds)
Create a time-stamp object. Time stamps facilitate the computation
of transaction ids, which are based on times. The arguments are
integers, except for seconds, which may be a floating-point
number. Time stamps have microsecond precision. Time stamps are
implicitly UTC based.
Time stamps are orderable and hashable.
.. method:: day()
Return the time stamp's day.
.. method:: hour()
Return the time stamp's hour.
.. method:: laterThan(other)
Return a timestamp instance which is later than 'other'.
If self already qualifies, return self.
Otherwise, return a new instance one moment later than 'other'.
.. method:: minute()
Return the time stamp's minute.
.. method:: month()
Return the time stamp's month.
.. method:: raw()
Get an 8-byte representatin of the time stamp for use in APIs
that require a time stamp.
.. method:: second()
Return the time stamp's second.
.. method:: timeTime()
Return the time stamp as seconds since the epoc, as used by the
``time`` module.
.. method:: year()
Return the time stamp's year.
Loading configuration
=====================
.. automodule:: ZODB.config
:members: databaseFromString, databaseFromFile, databaseFromURL,
storageFromString, storageFromFile, storageFromURL
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Sphinx
docutils
ZODB
j1m.sphinxautointerface
j1m.sphinxautozconfig
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.. _tutorial-label:
========
Tutorial
========
This tutorial is intended to guide developers with a step-by-step introduction
of how to develop an application which stores its data in the ZODB.
Introduction
============
To save application data in ZODB, you'll generally define classes that
subclass ``persistent.Persistent``::
# account.py
import persistent
class Account(persistent.Persistent):
def __init__(self):
self.balance = 0.0
def deposit(self, amount):
self.balance += amount
def cash(self, amount):
assert amount < self.balance
self.balance -= amount
This code defines a simple class that holds the balance of a bank
account and provides two methods to manipulate the balance: deposit
and cash.
Subclassing ``Persistent`` provides a number of features:
- The database will automatically track object changes made by setting
attributes [#changed]_.
- Data will be saved in its own database record.
You can save data that doesn't subclass ``Persistent``, but it will be
stored in the database record of whatever persistent object
references it.
- Objects will have unique persistent identity.
Multiple objects can refer to the same persistent object and they'll
continue to refer to the same object even after being saved
and loaded from the database.
Non-persistent objects are essentially owned by their containing
persistent object and if multiple persistent objects refer to the
same non-persistent subobject, they'll (eventually) get their own
copies.
Note that we put the class in a named module. Classes aren't stored
in the ZODB [#persistentclasses]_. They exist on the file system and
their names, consisting of their class and module names, are stored in
the database. It's sometimes tempting to create persistent classes in
scripts or in interactive sessions, but if you do, then their module
name will be ``'__main__'`` and you'll always have to define them that
way.
Installation
============
Before being able to use ZODB we have to install it. A common way to
do this is with pip::
$ pip install ZODB
Creating Databases
==================
When a program wants to use the ZODB it has to establish a connection,
like any other database. For the ZODB we need 3 different parts: a
storage, a database and finally a connection::
import ZODB, ZODB.FileStorage
storage = ZODB.FileStorage.FileStorage('mydata.fs')
db = ZODB.DB(storage)
connection = db.open()
root = connection.root
ZODB has a pluggable storage framework. This means there are a
variety of storage implementations to meet different needs, from
in-memory databases, to databases stored in local files, to databases
on remote database servers, and specialized databases for compression,
encryption, and so on. In the example above, we created a database
that stores its data in a local file, using the ``FileStorage``
class.
Having a storage, we then use it to instantiate a database, which we
then connect to by calling ``open()``. A process with multiple
threads will often have multiple connections to the same database,
with different threads having different connections.
There are a number of convenient shortcuts you can use for some of the
commonly used storages:
- You can pass a file name to the ``DB`` constructor to have it construct
a FileStorage for you::
db = ZODB.DB('mydata.fs')
You can pass None to create an in-memory database::
memory_db = ZODB.DB(None)
- If you're only going to use one connection, you can call the
``connection`` function::
connection = ZODB.connection('mydata.fs')
memory_connection = ZODB.connection(None)
Storing objects
===============
To store an object in the ZODB we simply attach it to any other object
that already lives in the database. Hence, the root object functions
as a boot-strapping point. The root object is meant to serve as a
namespace for top-level objects in your database. We could store
account objects directly on the root object::
import account
# Probably a bad idea:
root.account1 = account.Account()
But if you're going to store many objects, you'll want to use a
collection object [#root]_::
import account, BTrees.OOBTree
root.accounts = BTrees.OOBTree.BTree()
root.accounts['account-1'] = Account()
Another common practice is to store a persistent object in the root of
the database that provides an application-specific root::
root.accounts = AccountManagementApplication()
That can facilitate encapsulation of an application that shares a
database with other applications. This is a little bit like using
modules to avoid namespace colisions in Python programs.
Containers and search
=====================
BTrees provide the core scalable containers and indexing facility for
ZODB. There are different families of BTrees. The most general are
OOBTrees, which have object keys and values. There are specialized
BTrees that support integer keys and values. Integers can be stored
more efficiently, and compared more quickly than objects and they're
often used as application-level object identifiers. It's critical,
when using BTrees, to make sure that its keys have a stable ordering.
ZODB doesn't provide a query engine. The primary way to access
objects in ZODB is by traversing (accessing attributes or items, or
calling methods) other objects. Object traversal is typically much
faster than search.
You can use BTrees to build indexes for efficient search, when
necessary. If your application is search centric, or if you prefer to
approach data access that way, then ZODB might not be the best
technology for you.
Transactions
============
You now have objects in your root object and in your database.
However, they are not permanently stored yet. The ZODB uses
transactions and to make your changes permanent, you have to commit
the transaction::
import transaction
transaction.commit()
Now you can stop and start your application and look at the root object again,
and you will find the data you saved.
If your application makes changes during a transaction and finds that it does
not want to commit those changes, then you can abort the transaction and have
the changes rolled back [#rollback]_ for you::
transaction.abort()
Transactions are a very powerful way to protect the integrity of a
database. Transactions have the property that all of the changes made
in a transaction are saved, or none of them are. If in the midst of a
program, there's an error after making changes, you can simply abort
the transaction (or not commit it) and all of the intermediate changes
you make are automatically discarded.
Memory Management
=================
ZODB manages moving objects in and out of memory for you. The unit of
storage is the persistent object. When you access attributes of a
persistent objects, it's loaded from the database automatically, if
necessary. If too many objects are in memory, then objects used least
recently are evicted [#eviction]_. The maximum number of objects or
bytes in memory is configurable.
Summary
=======
You have seen how to install ZODB and how to open a database in your
application and to start storing objects in it. We also touched the
two simple transaction commands: ``commit`` and ``abort``. The
reference documentation contains sections with more information on the
individual topics.
.. [#changed]
You can manually mark an object as changed by setting its
``_p_changed__`` attribute to ``True``. You might do this if you
update a subobject, such as a standard Python ``list`` or ``set``,
that doesn't subclass ``Persistent``.
.. [#persistentclasses]
Actually, there is semi-experimental support for storing classes in
the database, but applications rarely do this.
.. [#root]
The root object is a fairy simple persistent object that's stored
in a single database record. If you stored many objects in it,
its database record would become very large, causing updates to be
inefficient and causing memory to be used ineffeciently.
Another reason not to store items directly in the root object is
that doing so would make adding a second collection of objects
later awkward.
.. [#rollback]
A caveat is that ZODB can only roll back changes to objects that
have been stored and committed to the database. Objects not
previously committed can't be rolled back because there's no
previous state to roll back to.
.. [#eviction]
Objects aren't actually evicted, but their state is released, so
they take up much less memory and any objects they referenced can
be removed from memory.
doc/zodb-guide.txt deleted 100644 → 0
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The ZODB/ZEO Programming Guide has been moved into it's own package
(zodbdocs) and published at http://docs.zope.org/zodb.
doc/zodb.svg 0 → 100644
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg width="89px" height="87px" viewBox="0 0 89 87" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:sketch="http://www.bohemiancoding.com/sketch/ns">
<!-- Generator: Sketch 3.3.3 (12072) - http://www.bohemiancoding.com/sketch -->
<title>ZODB_logo</title>
<desc>Created with Sketch.</desc>
<defs></defs>
<g id="Page-1" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd" sketch:type="MSPage">
<g id="ZODB_logo" sketch:type="MSLayerGroup" transform="translate(1.000000, 1.000000)" stroke="#1F5913" stroke-width="2">
<path d="M43.5,85 C67.5243866,85 87,65.9721019 87,42.5 C87,19.0278981 67.5243866,0 43.5,0 C19.4756134,0 0,19.0278981 0,42.5 C0,65.9721019 19.4756134,85 43.5,85 Z M22.567182,62.9328646 C11.1442877,51.5100624 11.1442877,32.9900388 22.567092,21.5671466 C33.9898964,10.1443445 52.5100136,10.1442545 63.932818,21.5670566 C75.3557123,32.9899488 75.3557123,51.5099724 63.932908,62.9328646 C52.5101036,74.3556668 33.9899864,74.3557568 22.567182,62.9328646 Z" id="Oval-1" fill="#42982F" sketch:type="MSShapeGroup"></path>
<path d="M9.83640265,14.5898623 L11.8733061,24.310227 L16.8264662,25.5598626 L22.8161699,18.1609813" id="shapePath4-Copy-2" sketch:type="MSShapeGroup" transform="translate(16.326286, 20.074862) rotate(27.000000) translate(-16.326286, -20.074862) "></path>
</g>
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</svg>
\ No newline at end of file
src/ZODB/tests/testdocumentation.py 0 → 100644
View file @ de6dba53
##############################################################################
#
# Copyright (c) Zope Foundation and Contributors.
# All Rights Reserved.
#
# This software is subject to the provisions of the Zope Public License,
# Version 2.0 (ZPL). A copy of the ZPL should accompany this distribution.
# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
# FOR A PARTICULAR PURPOSE.
#
##############################################################################
from os.path import join
import os
import doctest
import unittest
import manuel.capture
import manuel.doctest
import manuel.testing
import zope.testing.module
import ZODB
def setUp(test):
test.globs.update(
ZODB=ZODB,
)
zope.testing.module.setUp(test)
def tearDown(test):
zope.testing.module.tearDown(test)
def test_suite():
base, src = os.path.split(os.path.dirname(os.path.dirname(ZODB.__file__)))
assert src == 'src'
base = join(base, 'doc')
guide = join(base, 'guide')
reference = join(base, 'reference')
return unittest.TestSuite((
manuel.testing.TestSuite(
manuel.doctest.Manuel() + manuel.capture.Manuel(),
join(guide, 'writing-persistent-objects.rst'),
join(guide, 'install-and-run.rst'),
join(reference, 'zodb.rst'),
join(reference, 'storages.rst'),
setUp=setUp, tearDown=tearDown,
),
))
if __name__ == '__main__':
unittest.main(defaultTest='test_suite')
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