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A backend for ZODB that stores pickles in a relational database.

Project description

RelStorage is a storage implementation for ZODB that stores pickles in a relational database. PostgreSQL 8.1 and above (via psycopg2), MySQL 5.0.32+ / 5.1.34+ (via MySQLdb 1.2.2 and above), and Oracle 10g (via cx_Oracle) are currently supported. RelStorage replaces the PGStorage project.


  • It is a drop-in replacement for FileStorage and ZEO.

  • There is a simple way to convert FileStorage to RelStorage and back again. You can also convert a RelStorage instance to a different relational database.

  • Designed for high volume sites: multiple ZODB instances can share the same database. This is similar to ZEO, but RelStorage does not require ZEO.

  • According to some tests, RelStorage handles high concurrency better than the standard combination of ZEO and FileStorage.

  • Whereas FileStorage takes longer to start as the database grows due to an in-memory index of all objects, RelStorage starts quickly regardless of database size.

  • Supports undo and packing.

  • Free, open source (ZPL 2.1)


You can install RelStorage using easy_install:

easy_install RelStorage

If you are not using easy_install (part of the setuptools package), you can get the latest release at PyPI (, then place the relstorage package in the lib/python directory of either the SOFTWARE_HOME or the INSTANCE_HOME. You can do this with the following command:

python2.4 install --install-lib=${INSTANCE_HOME}/lib/python

RelStorage requires a version of ZODB with the invalidation polling patch applied. You can get versions of ZODB with the patch already applied here:

The patches are also included in the source distribution of RelStorage.

You need the Python database adapter that corresponds with your database. Install psycopg2, MySQLdb 1.2.2+, or cx_Oracle 4.3+. Note that Debian Etch ships MySQLdb 1.2.1, but that version has a bug in BLOB handling that manifests itself only with certain character set configurations. MySQLdb 1.2.2 fixes the bug.

Finally, modify etc/zope.conf of your Zope instance. Remove the main mount point and add one of the following blocks. For PostgreSQL:

%import relstorage
<zodb_db main>
  mount-point /
      # The dsn is optional, as are each of the parameters in the dsn.
      dsn dbname='zodb' user='username' host='localhost' password='pass'

For MySQL:

%import relstorage
<zodb_db main>
  mount-point /
      # Most of the options provided by MySQLdb are available.
      # See component.xml.
      db zodb

For Oracle (10g XE in this example):

%import relstorage
<zodb_db main>
  mount-point /
      user username
      password pass
      dsn XE


Migrating from FileStorage

You can convert a FileStorage instance to RelStorage and back using a utility called ZODBConvert. See .

Migrating from PGStorage

The following script migrates your database from PGStorage to RelStorage 1.0 beta:


After you do this, you still need to migrate from 1.0 beta to the latest release.

Migrating to a new version of RelStorage

Sometimes RelStorage needs a schema modification along with a software upgrade. Hopefully, this will not often be necessary.

Version 1.2.0b1 does not require a schema migration from version 1.1.3.

To migrate from version 1.1.1 to version 1.1.2 or 1.1.3, see:


To migrate from version 1.1 to version 1.1.1, see:


To migrate from version 1.0.1 to version 1.1, see:


To migrate from version 1.0 beta to version 1.0c1 through 1.0.1, see:


Optional Features

Specify these options in zope.conf, as parameters for the RelStorage constructor, or as attributes of a relstorage.Options instance. In the latter two cases, use underscores instead of dashes in the parameter names.


Defer polling the database for the specified maximum time interval, in seconds. Set to 0 (the default) to always poll. Fractional seconds are allowed. Use this to lighten the database load on servers with high read volume and low write volume.

The poll-interval option works best in conjunction with the cache-servers option. If both are enabled, RelStorage will poll a single cache key for changes on every request. The database will not be polled unless the cache indicates there have been changes, or the timeout specified by poll-interval has expired. This configuration keeps clients fully up to date, while removing much of the polling burden from the database. A good cluster configuration is to use memcache servers and a high poll-interval (say, 60 seconds).

This option can be used without the cache-servers option, but a large poll-interval without cache-servers increases the probability of basing transactions on stale data, which does not affect database consistency, but does increase the probability of conflict errors, leading to low performance.


If pack-gc is false, pack operations do not perform garbage collection. Garbage collection is enabled by default.

If garbage collection is disabled, pack operations keep at least one revision of every object. With garbage collection disabled, the pack code does not need to follow object references, making packing conceivably much faster. However, some of that benefit may be lost due to an ever increasing number of unused objects.

Disabling garbage collection is also a hack that ensures inter-database references never break.


If pack-dry-run is true, pack operations perform a full analysis of what to pack, but no data is actually removed. After a dry run, the pack_object, pack_state, and pack_state_tid tables are filled with the list of object states and objects that would have been removed.


Packing occurs in batches of transactions; this specifies the timeout in seconds for each batch. Note that some database configurations have unpredictable I/O performance and might stall much longer than the timeout. The default timeout is 5.0 seconds.


After each batch, the pack code pauses for a time to allow concurrent transactions to commit. The pack-duty-cycle specifies what fraction of time should be spent on packing. For example, if the duty cycle is 0.75, then 75% of the time will be spent packing: a 6 second pack batch will be followed by a 2 second delay. The duty cycle should be greater than 0.0 and less than or equal to 1.0. Specify 1.0 for no delay between batches.

The default is 0.5. Raise it to finish packing faster; lower it to reduce the effect of packing on transaction commit performance.


This specifies a maximum delay between pack batches. Sometimes the database takes an extra long time to finish a pack batch; at those times it is useful to cap the delay imposed by the pack-duty-cycle. The default is 20 seconds.


Specifies a list of memcache servers. Enabling memcache integration is useful if the connection to the relational database has high latency and the connection to memcache has significantly lower latency. On the other hand, if the connection to the relational database already has low latency, memcache integration may actually hurt overall performance.

Provide a list of host:port pairs, separated by whitespace. “” is a common setting. The default is to disable memcache integration.


Specifies which Python memcache module to use. The default is “memcache”, a pure Python module. There are several alternative modules available through PyPI. This setting has no effect unless cache-servers is set.


You can checkout from Subversion using the following command:

svn co svn:// RelStorage

You can also browse the code:

The best place to discuss development of RelStorage is on the zodb-dev mailing list.


Q: How can I help improve RelStorage?

A: The best way to help is to test and to provide database-specific expertise. Ask questions about RelStorage on the zodb-dev mailing list.

Q: Can I perform SQL queries on the data in the database?

A: No. Like FileStorage and DirectoryStorage, RelStorage stores the data as pickles, making it hard for anything but ZODB to interpret the data. An earlier project called Ape attempted to store data in a truly relational way, but it turned out that Ape worked too much against ZODB principles and therefore could not be made reliable enough for production use. RelStorage, on the other hand, is much closer to an ordinary ZODB storage, and is therefore much safer for production use.

Q: How does RelStorage performance compare with FileStorage?

A: According to benchmarks, RelStorage with PostgreSQL is often faster than FileStorage, especially under high concurrency.

Q: Why should I choose RelStorage?

A: Because RelStorage is a fairly small layer that builds on world-class databases. These databases have proven reliability and scalability, along with numerous support options.

Q: Can RelStorage replace ZRS (Zope Replication Services)?

A: In theory, yes. With RelStorage, you can use the replication features native to your database. However, this capability has not yet been tested.

Project URLs

Change History

Version 1.2.0b2 (2009-05-05)

  • RelStorage now implements IMVCCStorage, making it compatible with ZODB 3.9.0b1 and above.

  • Removed two-phase commit support from the PostgreSQL adapter. The feature turned out to be unnecessary.

  • Added MySQL 5.1.34 and above to the list of supportable databases.

  • Fixed minor test failures under Windows. Windows is now a supportable platform.

Version 1.1.3 (2009-02-04)

  • In rare circumstances, ZODB can legitimately commit an object twice in a single transaction. Fixed RelStorage to accept that.

  • Auto reconnect to Oracle sometimes did not work because cx_Oracle was raising a different kind of exception than expected. Fixed.

  • Included LICENSE.txt in the source distribution.

Version 1.1.2 (2009-01-27)

  • When both cache-servers and poll-interval are set, we now poll the cache for changes on every request. This makes it possible to use a high poll-interval to reduce the database polling burden, yet every client can see changes immediately.

  • Added the pack-dry-run option, which causes pack operations to only populate the pack tables with the list of objects and states to pack, but not actually pack.

  • Refined the pack algorithm. It was not removing as many object states as it should have. As a bonus, there is now a method of adapters called fill_object_refs(), which could be useful for debugging. It ensures the object_ref table is fully populated.

  • Began using zc.buildout for development.

  • Increased automated test coverage.

  • Fixed KeyError reporting to not trip over a related KeyError while logging.

Version 1.1.1 (2008-12-27)

  • Worked around MySQL performance bugs in packing. Used temporary tables and another column in the pack_object table. The other databases may benefit from the optimization as well.

  • Applied an optimization using setinputsizes() to the Oracle code, bringing write speed back up to where it was in version 1.0.

Version 1.1 (2008-12-19)

  • Normalized poll-invalidation patches as Solaris’ patch command would not accept the current format. The patches now apply with: patch -d lib/python/ZODB -p0 < poll-invalidation-1-zodb-3-X-X.patch

  • In MySQL, Use DROP TABLE IF EXISTS instead of TRUNCATE to clear ‘temp_store’ because:

    • TRUNCATE has one page of caveats in the MySQL documentation.

    • TEMPORARY TABLEs have half a page of caveats when it comes to replication.

    • The end result is that ‘temp_store’ may not exist on the replication slave at the exact same time(s) it exists on the master.

  • Implemented the database size query in MySQL, based on a patch from Kazuhiko Shiozaki. Thanks!

  • Optimized Oracle object retrieval by causing BLOBs to be sent inline when possible, based on a patch by Helge Tesdal. By default, the optimization is activated automatically when cx_Oracle 5 is used.

  • Updated the storage iterator code to be compatible with ZODB 3.9. The RelStorage tests now pass with the shane-poll-invalidations branch of ZODB 3.9.

  • Added a translation of README.txt to Brazilian Portuguese by Rogerio Ferreira. Thanks!

Version 1.1c1

  • Added optional memcache integration. This is useful when the connection to the relational database has high latency.

  • Made it possible to set the pack and memcache options in zope.conf.

  • Log more info when a KeyError occurs within RelStorage.

Version 1.1b2

  • Made the MySQL locks database-specific rather than server-wide. This is important for multi-database configurations.

  • In the PostgreSQL adapter, made the pack lock fall back to table locking rather than advisory locks for PostgreSQL 8.1.

  • Changed a query for following object references (used during packing) to work around a MySQL performance bug. Thanks to Anton Stonor for discovering this.

Version 1.1b1

  • Fixed the use of without setuptools. Thanks to Chris Withers.

  • Fixed type coercion of the transaction extension field. This fixes an issue with converting databases. Thanks to Kevin Smith for discovering this.

  • Added logging to the pack code to help diagnose performance issues.

  • Additions to the object_ref table are now periodically committed during pre_pack so that the work is not lost if pre_pack fails.

  • Modified the pack code to pack one transaction at a time and release the commit lock frequently. This should help large pack operations.

  • Fixed buildout-based installation of the zodbconvert script. Thanks to Jim Fulton.

Version 1.0.1

  • The speedtest script failed if run on a test database that has no tables. Now the script creates the tables if needed. Thanks to Flavio Coelho for discovering this.

  • Reworked the auto-reconnect logic so that applications never see temporary database disconnects if possible. Thanks to Rigel Di Scala for pointing out this issue.

  • Improved the log messages explaining database connection failures.

  • Moved poll_invalidations to the common adapter base class, reducing the amount of code to maintain.

Version 1.0

  • Added a utility for converting between storages called zodbconvert.

Version 1.0c1

  • The previous fix for non-ASCII characters was incorrect. Now transaction metadata is stored as raw bytes. A schema migration is required; see notes/migrate-1.0-beta.txt.

  • Integrated setuptools and made an egg.

Version 1.0 beta

  • Renamed to reflect expanding database support.

  • Added support for Oracle 10g.

  • Major overhaul with many scalability and reliability improvements, particularly in the area of packing.

  • Moved to and switched to ZPL 2.1.

  • Made two-phase commit optional in both Oracle and PostgreSQL. They both use commit_lock in such a way that the commit is not likely to fail in the second phase.

  • Switched most database transaction isolation levels from serializable to read committed. It turns out that commit_lock already provides the serializability guarantees we need, so it is safe to take advantage of the potential speed gains. The one major exception is the load connection, which requires an unchanging view of the database.

  • Stored objects are now buffered in a database table rather than a file.

  • Stopped using the LISTEN and NOTIFY statements in PostgreSQL since they are not strictly transactional in the sense we require.

  • Started using a prepared statement in PostgreSQL for getting the newest transaction ID quickly.

  • Removed the code in the Oracle adapter for retrying connection attempts. (It is better to just reconfigure Oracle.)

  • Added support for MySQL 5.0.

  • Added the poll_interval option. It reduces the frequency of database polls, but it also increases the potential for conflict errors on servers with high write volume.

  • Implemented the storage iterator protocol, making it possible to copy transactions to and from FileStorage and other RelStorage instances.

  • Fixed a bug that caused OIDs to be reused after importing transactions. Added a corresponding test.

  • Made it possible to disable garbage collection during packing. Exposed the option in zope.conf.

  • Valery Suhomlinov discovered a problem with non-ASCII data in transaction metadata. The problem has been fixed for all supported databases.

PGStorage history

Version 0.4

  • Began using the PostgreSQL LISTEN and NOTIFY statements as a shortcut for invalidation polling.

  • Removed the commit_order code. The commit_order idea was intended to allow concurrent commits, but that idea is a little too ambitious while other more important ideas are being tested. Something like it may come later.

  • Improved connection management: only one database connection is held continuously open per storage instance.

  • Reconnect to the database automatically.

  • Removed test mode.

  • Switched from using a ZODB.Connection subclass to a ZODB patch. The Connection class changes in subtle ways too often to subclass reliably; a patch is much safer.

  • PostgreSQL 8.1 is now a dependency because PGStorage uses two phase commit.

  • Fixed an undo bug. Symptom: attempting to examine the undo log revealed broken pickles. Cause: the extension field was not being wrapped in psycopg2.Binary upon insert. Solution: used psycopg2.Binary. Unfortunately, this doesn’t fix existing transactions people have committed. If anyone has any data to keep, fixing the old transactions should be easy.

  • Moved from a private CVS repository to Sourceforge. See . Also switched to the MIT license.

  • David Pratt added a basic getSize() implementation so that the Zope management interface displays an estimate of the size of the database.

  • Turned PGStorage into a top-level package. Python generally makes top-level packages easier to install.

Version 0.3

  • Made compatible with Zope 3, although an undo bug apparently remains.

Version 0.2

  • Fixed concurrent commits, which were generating deadlocks. Fixed by adding a special table, “commit_lock”, which is used for synchronizing increments of commit_seq (but only at final commit.) If you are upgrading from version 0.1, you need to change your database using the ‘psql’ prompt:

    create table commit_lock ();

  • Added speed tests and an OpenDocument spreadsheet comparing FileStorage / ZEO with PGStorage. PGStorage wins at reading objects and writing a lot of small transactions, while FileStorage / ZEO wins at writing big transactions. Interestingly, they tie when writing a RAM disk.

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