Commit c0fd9ff7 authored by Tim Peters's avatar Tim Peters

Add the ZEO (in)compatibility story.

parent b4c38fd2
......@@ -52,6 +52,17 @@ DB
file ZODB/tests/multidb.txt for a tutorial doctest. Thanks to Christian
Theune for his work on this during the PyCon 2005 ZODB sprint.
ZEO compatibility
-----------------
There are severe restrictions on using ZEO servers and clients at or after
ZODB 3.3 with ZEO servers and clients from ZODB versions before 3.3. See the
reworked ``Compatibility`` section in ``README.txt`` for details. If
possible, it will be easiest to move clients and servers to 3.3+
simultaneously. With care, it's possible to use a 3.3+ ZEO server with
pre-3.3 ZEO clients, but not possible to use a pre-3.3 ZEO server with 3.3+
ZEO clients.
BTrees
------
......
......@@ -31,14 +31,36 @@ Compatibility
ZODB 3.4 requires Python 2.3.4 or later. For best results, we recommend
Python 2.3.5.
The Zope 2.8 and X3 releases should be compatible with this version of ZODB.
Note that Zope 2.7 and higher includes ZEO, so this package should only be
needed to run a ZEO server.
The ZEO server in ZODB 3.4 is currently incompatible with earlier
versions of ZODB. If you want to test the software, you must be
running this release for both client and server. A backwards
compatibility mechanism will be provided in a later release.
The Zope 2.8 release, and Zope3 releases, should be compatible with this
version of ZODB. Note that Zope 2.7 and higher includes ZEO, so this package
should only be needed to run a ZEO server.
ZEO servers and clients are wholly compatible among 3.3, 3.3.1 and 3.4: a ZEO
client from any of those versions can talk with a ZEO server from any.
Trying to mix ZEO clients and servers from 3.3 or later from ZODB releases
before 3.3 is much harder. ZODB 3.3 introduced multiversion concurrency
control (MVCC), and earlier ZEO servers do not support MVCC: a 3.3+ ZEO
client cannot talk with an older ZEO server as a result.
In the other direction, a 3.3+ ZEO server can talk with older ZEO clients,
but because the names of some basic classes have changed, if any 3.3+ clients
commit modifications to the database it's likely that the database will
contain instances of classes that don't exist in (can't be loaded by) older
ZEO clients. For example, the database root object was an instance of
``ZODB.PersistentMapping.PersistentMapping`` before ZODB 3.3, but is an
instance of ``persistent.mapping.PersistentMapping`` in ZODB 3.3. A 3.3.1+
client can still load a ``ZODB.PersistentMapping.PersistentMapping`` object,
but this is just an alias for ``persistent.mapping.PersistentMapping``, and
an object of the latter type will be stored if a 3.3 client commits a change
to the root object. An older ZEO client cannot load the root object so
changed.
This limits migration possibilities: a 3.3+ ZEO server can be used with
older (pre-3.3) ZEO clients and serve an older database, so long as no 3.3+
ZEO clients commit changes to the database. The most practical upgrade path
is to bring up both servers and clients using 3.3+, not trying to mix pre-3.3
and post-3.3 ZEO clients and servers.
Prerequisites
-------------
......
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