Commit 771f1b48 authored by Tim Peters's avatar Tim Peters

Massive rewrite of cache.txt, to match current reality.

parent 2fbda114
ZEO Client Cache
The Client cache provides a disk based cache for each ZEO client.
The client cache allows reads to be done from local disk rather than
by remote access to the storage server.
The client cache provides a disk based cache for each ZEO client. The
client cache allows reads to be done from local disk rather than by remote
access to the storage server.
The cache may be persistent or transient. If the cache is
persistent, then the cache files are retained for use after process
restarts. A non-persistent cache uses temporary files that are
removed when the client storage is closed.
The cache may be persistent or transient. If the cache is persistent, then
the cache file is retained for use after process restarts. A non-
persistent cache uses a temporary file.
The client cache is managed as two files. The cache manager
endeavors to maintain the two files at sizes less than or equal to
one half the cache size. One of the cache files is designated the
"current" cache file. The other cache file is designated the "old"
cache file, if it exists. All writes are done to the current cache
files. When transactions are committed on the client, transactions
are not split between cache files. Large transactions may cause
cache files to be larger than one half the target cache size.
The client cache is managed in a single file, of the specified size.
The life of the cache is as follows:
- When the cache is created, the first of the two cache files is
created and designated the "current" cache file.
- The cache file is opened (if it already exists), or created and set to
the specified size.
- Cache records are written to the cache file, either as
transactions commit locally, or as data are loaded from the
server.
- Cache records are written to the cache file, as transactions commit
locally, and as data are loaded from the server.
- When the cache file size exceeds one half the cache size, the
second cache file is created and designated the "current" cache
file. The first cache file becomes the "old" cache file.
- Writes are to "the current file position". This is a pointer that
travels around the file, circularly. After a record is written, the
pointer advances to just beyond it. Objects starting at the current
file position are evicted, as needed, to make room for the next record
written.
- Cache records are written to the new current cache file, either as
transactions commit locally, or as data are loaded from the
server.
A distinct index file is not created, although indexing structures are
maintained in memory while a ClientStorage is running. When a persistent
client cache file is reopened, these indexing structures are recreated
by analyzing the file contents.
- When a cache hit is found in the old cache file, it is copied to
the current cache file.
Persistent cache files are created in the directory named in the ``var``
argument to the ClientStorage, or if ``var`` is None, in the current
working directory. Persistent cache files have names of the form::
- When the current cache file size exceeds one half the cache size, the
first cache file is recreated and designated the "current" cache
file. The second cache file becomes the "old" cache file.
and so on.
Persistent cache files are created in the directory named in the
'var' argument to the ClientStorage (see ClientStorage.txt) or in
the 'var' subdirectory of the directory given by the INSTANCE_HOME
builtin (created by Zope), or in the current working directory.
Persistent cache files have names of the form::
cstorage-client-n.zec
client-storage.zec
where:
storage -- the storage name
client -- the client name, as given by the 'ZEO_CLIENT' environment
variable or the 'client' argument provided when creating a client
storage.
n -- '0' for the first cache file and '1' for the second.
client -- the client name, as given by the ClientStorage's ``client``
argument
For example, the second cache file for storage 'spam' and client 8881
would be named 'cspam-8881-1.zec'.
storage -- the storage name, as given by the ClientStorage's ``storage``
argument; this is typically a string denoting a small integer,
"1" by default
For example, the cache file for client '8881' and storage 'spam' is named
"8881-spam.zec".
......@@ -16,7 +16,7 @@
ClientCache exposes an API used by the ZEO client storage. FileCache stores
objects on disk using a 2-tuple of oid and tid as key.
ClientCaches API is similar to a storage API with methods like load(),
ClientCache's API is similar to a storage API, with methods like load(),
store(), and invalidate(). It manages in-memory data structures that allow
it to map this richer API onto the simple key-based API of the lower-level
FileCache.
......
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