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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, packing, and filesystem-based ZODB blobs.

  • Both history-preserving and history-free storage are available.

  • Capable of failover to replicated SQL databases.

  • Free, open source (ZPL 2.1)


You can install RelStorage using easy_install:

easy_install RelStorage

RelStorage requires a version of ZODB that is aware of MVCC storages. ZODB 3.9 supports RelStorage without any patches. ZODB 3.7 and 3.8 can support RelStorage if you first apply a patch to ZODB. 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.

Configuring Your Database

You need to configure a database and user account for RelStorage. RelStorage will populate the database with its schema the first time it connects.


If you installed PostgreSQL from a binary package, you probably have a user account named postgres. Since PostgreSQL respects the name of the logged-in user by default, switch to the postgres account to create the RelStorage user and database. Even root does not have the PostgreSQL privileges that the postgres account has. For example:

$ sudo su - postgres
$ createuser --pwprompt zodbuser
$ createdb -O zodbuser zodb

New PostgreSQL accounts often require modifications to pg_hba.conf, which contains host-based access control rules. The location of pg_hba.conf varies, but /etc/postgresql/8.4/main/pg_hba.conf is common. PostgreSQL processes the rules in order, so add new rules before the default rules rather than after. Here is a sample rule that allows only local connections by zodbuser to the zodb database:

local  zodb  zodbuser  md5

PostgreSQL re-reads pg_hba.conf when you ask it to reload its configuration file:

/etc/init.d/postgresql reload


Use the mysql utility to create the database and user account. Note that the -p option is usually required. You must use the -p option if the account you are accessing requires a password, but you should not use the -p option if the account you are accessing does not require a password. If you do not provide the -p option, yet the account requires a password, the mysql utility will not prompt for a password and will fail to authenticate.

Most users can start the mysql utility with the following shell command, using any login account:

$ mysql -u root -p

Here are some sample SQL statements for creating the user and database:

CREATE USER 'zodbuser'@'localhost' IDENTIFIED BY 'mypassword';
GRANT ALL ON zodb.* TO 'zodbuser'@'localhost';


Initial setup will require SYS privileges. Using Oracle 10g XE, you can start a SYS session with the following shell commands:

$ su - oracle
$ sqlplus / as sysdba

The commands below will create a PL/SQL package that provides limited access to the DBMS_LOCK package so that RelStorage can acquire user locks. Using sqlplus with SYS privileges, execute the following:

    FUNCTION request_lock(id IN NUMBER, timeout IN NUMBER)
END relstorage_util;

    FUNCTION request_lock(id IN NUMBER, timeout IN NUMBER)
            id => id,
            lockmode => DBMS_LOCK.X_MODE,
            timeout => timeout,
            release_on_commit => TRUE);
    END request_lock;
END relstorage_util;

Here are some sample SQL statements for creating the database user:

GRANT EXECUTE ON relstorage_util TO zodb;

Configuring Plone

To install RelStorage in Plone, see the instructions in the following article:

Plone uses the plone.recipe.zope2instance Buildout recipe to generate zope.conf, so the easiest way to configure RelStorage in a Plone site is to set the rel-storage parameter in buildout.cfg. The rel-storage parameter contains settings separated by newlines, with these values:

  • type: any database type supported (postgresql, mysql, or oracle)

  • RelStorage options like cache-servers and poll-interval

  • Adapter-specific options

An example:

rel-storage =
    type mysql
    db plone
    user plone
    passwd PASSWORD

Configuring Zope 2

To integrate RelStorage in Zope 2, specify a RelStorage backend in etc/zope.conf. 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

To add ZODB blob support, provide a blob-dir parameter that specifies where to store the blobs. For example:

%import relstorage
<zodb_db main>
  mount-point /
  blob-dir ./blobs
      dsn dbname='zodb' user='username' host='localhost' password='pass'

Configuring repoze.zodbconn

To use RelStorage with repoze.zodbconn, a package that makes ZODB available to WSGI applications, create a configuration file with contents similar to the following:

%import relstorage
<zodb main>
      db zodb
  cache-size 100000

repoze.zodbconn expects a ZODB URI. Use a URI of the form zconfig://path/to/configuration#main.

Migrating Existing Data

The zodbconvert Utility

RelStorage comes with a script named zodbconvert that converts databases to different formats. Use it to convert a FileStorage instance to RelStorage and back, or to convert between different kinds of RelStorage instances, or to convert other kinds of storages that support the storage iterator protocol.

When converting between two history-preserving databases (note that FileStorage uses a history-preserving format), zodbconvert utility preserves all objects and transactions, meaning you can still use the ZODB undo feature after the conversion, and you can convert back using the same process in reverse. When converting from a history-free database to either a history-free database or a history-preserving database, zodbconvert retains all data, but the converted transactions will not be undoable. When converting from a history-preserving storage to a history-free storage, zodbconvert drops all historical information during the conversion.

How to use zodbconvert

Create a ZConfig style configuration file that specifies two storages, one named “source”, the other “destination”. The configuration file format is very much like zope.conf. Then run zodbconvert, providing the name of the configuration file as a parameter.

The utility does not modify the source storage. Before copying the data, the utility verifies the destination storage is completely empty. If the destination storage is not empty, the utility aborts without making any changes to the destination. (Adding transactions to an existing database is complex and out of scope for zodbconvert.)

Here is a sample zodbconvert configuration file:

<filestorage source>
  path /zope/var/Data.fs

<relstorage destination>
    db zodb

This configuration file specifies that the utility should copy all of the transactions from Data.fs to a MySQL database called “zodb”. If you want to reverse the conversion, exchange the names “source” and “destination”. All storage types and storage parameters available in zope.conf are also available in this configuration file.

Options for zodbconvert


Clears all data from the destination storage before copying. Use this only if you are certain the destination has no useful data. Currently only works when the destination is a RelStorage instance.


Opens both storages and analyzes what would be copied, but does not actually copy.

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.

No schema migration is required if you are using PostgreSQL or MySQL and upgrading from version 1.1.2 or later. See the notes subdirectory if you are upgrading from an older version.

To migrate Oracle to version 1.4, see migrate-to-1.4.txt.

RelStorage Options

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


The name of the storage. Defaults to a descriptive name that includes the adapter connection parameters, except the database password.


If true, only reads may be executed against the storage.


If supplied, the storage will provide blob support; this parameter specifies the name of the directory to hold blob data. The directory will be created if it does not exist. If no value (or an empty value) is provided, then no blob support will be provided.


If this parameter is set to true (the default), the adapter will create and use a history-preserving database schema (like FileStorage). A history-preserving schema supports ZODB-level undo, but also grows more quickly and requires extensive packing on a regular basis.

If this parameter is set to false, the adapter will create and use a history-free database schema. Undo will not be supported, but the database will not grow as quickly. The database will still require regular garbage collection (which is accessible through the database pack mechanism.)

This parameter must not change once the database schema has been installed, because the schemas for history-preserving and history-free storage are different. If you want to convert between a history-preserving and a history-free database, use the zodbconvert utility to copy to a new database.


If this parameter is provided, it specifies a text file that contains a list of database replicas the adapter can choose from. For MySQL and PostgreSQL, put in the replica file a list of host:port or host values, one per line. For Oracle, put in a list of DSN values. Blank lines and lines starting with # are ignored.

The adapter prefers the first replica specified in the file. If the first is not available, the adapter automatically tries the rest of the replicas, in order. If the file changes, the adapter will drop existing SQL database connections and make new connections when ZODB starts a new transaction.


If this parameter has a nonzero value, when the adapter selects a replica other than the primary replica, the adapter will try to revert to the primary replica after the specified timeout (in seconds). The default is 600, meaning 10 minutes.


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 memcached servers. Using memcached with RelStorage improves the speed of frequent object accesses while slightly reducing the speed of other operations.

Provide a list of host:port pairs, separated by whitespace. “” is a common setting. Some memcached modules, such as pylibmc, allow you to specify a path to a Unix socket instead of a host:port pair.

The default is to disable memcached integration.


Specifies which Python memcache module to use. The default is “memcache”, a pure Python module. An alternative module is “relstorage.pylibmc_wrapper”. This setting has no effect unless cache-servers is set.


The prefix for all keys in the cache. All clients using a database should use the same cache-prefix. Use this if you use a single cache for multiple databases.


RelStorage caches pickled objects in memory, similar to a ZEO cache. This cache is shared between threads. This parameter configures the approximate maximum amount of memory the cache should consume, in megabytes. It defaults to 10.


This is an advanced option. RelStorage uses a system of checkpoints to improve the cache hit rate. This parameter configures how many objects should be stored before creating a new checkpoint. The default is 10000.


During commit, RelStorage acquires a database-wide lock. This parameter specifies how long to wait for the lock before failing the attempt to commit. The default is 30 seconds.

The MySQL and Oracle adapters support this parameter. The PostgreSQL adapter currently does not.


During commit, RelStorage acquires a database-wide lock. This parameter specifies the lock ID. This parameter currently applies only to the Oracle adapter.

Adapter Options

PostgreSQL Adapter Options

The PostgreSQL adapter accepts:


Specifies the data source name for connecting to PostgreSQL. A PostgreSQL DSN is a list of parameters separated with whitespace. A typical DSN looks like:

dbname='zodb' user='username' host='localhost' password='pass'

If dsn is omitted, the adapter will connect to a local database with no password. Both the user and database name will match the name of the owner of the current process.

MySQL Adapter Options

The MySQL adapter accepts most parameters supported by the MySQL-python library, including:


string, host to connect


string, user to connect as


string, password to use


string, database to use


integer, TCP/IP port to connect to


string, location of unix_socket (UNIX-ish only)


mapping, maps MySQL FIELD_TYPE.* to Python functions which convert a string to the appropriate Python type


number of seconds to wait before the connection attempt fails.


if set, gzip compression is enabled


if set, connect to server via named pipe (Windows only)


command which is run once the connection is created


see the MySQL documentation for mysql_options()


see the MySQL documentation for mysql_options()


client flags from MySQLdb.constants.CLIENT


int, non-zero enables LOAD LOCAL INFILE, zero disables

Oracle Adapter Options

The Oracle adapter accepts:


The Oracle account name


The Oracle account password


The Oracle data source name. The Oracle client library will normally expect to find the DSN in /etc/oratab.

The zodbpack Script

RelStorage also comes with a script named zodbpack that packs any ZODB storage that allows concurrent connections (including RelStorage and ZEO, but not including FileStorage). Use zodbpack in cron scripts. Pass the script the name of a configuration file that lists the storages to pack, in ZConfig format. An example configuration file:

  pack-gc true
  pack-duty-cycle 0.9
    db zodb

Options for zodbpack

--days or -d

Specifies how many days of historical data to keep. Defaults to 0, meaning no history is kept. This is meaningful even for history-free storages, since unreferenced objects are not removed from the database until the specified number of days have passed.


You can check out 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 more appropriate 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: Yes, RelStorage inherits the asynchronous master/slave replication capability of MySQL and other databases. The author is currently looking for funding opportunities to support master/master replication.

Q: How do I set up an environment to run the RelStorage tests?

A: See README.txt in the relstorage/tests directory.

Project URLs

Change History

1.4.0b1 (2009-11-17)

  • Added the keep-history option. Set it to false to keep no history. (Packing is still required for garbage collection and blob deletion.)

  • Added the replica-conf and replica-timeout options. Set replica-conf to a filename containing the location of database replicas. Changes to the file take effect at transaction boundaries.

  • Expanded the option documentation in README.txt.

  • Revised the way RelStorage uses memcached. Minimized the number of trips to both the cache server and the database.

  • Added an in-process pickle cache that serves a function similar to the ZEO cache.

  • Added a wrapper module for pylibmc.

  • Store operations now use multi-insert and multi-delete SQL statements to reduce the effect of network latency.

  • Renamed to to overcome import issues. Also moved the Options class to

  • Updated the patch for ZODB 3.7 and 3.8 to fix an issue with blobs and subtransactions.

  • Divided the implementation of database adapters into many small objects, making the adapter code more modular. Added interfaces that describe the duties of each part.

  • Oracle: Sped up restore operations by sending short blobs inline.

  • Oracle: Use a timeout on commit locks. This requires installation of a small PL/SQL package that can access DBMS_LOCK. See README.txt.

  • Oracle: Used PL/SQL bulk insert operations to improve write performance.

  • PostgreSQL: use the documented ALTER SEQUENCE RESTART WITH statement instead of ALTER SEQUENCE START WITH.

  • Moved MD5 sum computation to the adapters so they can choose not to use MD5.

  • Changed loadSerial to load from the store connection only if the load connection can not provide the object requested.

  • Stopped wrapping database disconnect exceptions. Now the code catches and handles them directly.

  • Use the store connection rather than the load connection for OID allocation.

  • Detect and handle backward time travel, which can happen after failover to an out-of-date asynchronous slave database. For simplicity, invalidate the whole ZODB cache when this happens.

  • Replaced the speed test script with a separately distributed package, zodbshootout.

  • Added the zodbpack script.

1.3.0b1 (2009-09-04)

  • Added support for a blob directory. No BlobStorage wrapper is needed. Cluster nodes will need to use a shared filesystem such as NFS or SMB/CIFS.

  • Added the blob-dir parameter to the ZConfig schema and README.txt.

1.2.0 (2009-09-04)

  • In Oracle, trim transaction descriptions longer than 2000 bytes.

  • When opening the database for the first time, don’t issue a warning about the inevitable POSKeyError on the root OID.

  • If RelStorage tries to unpickle a corrupt object state during packing, it will now report the oid and tid in the log.

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.

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