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ZODB Replicated Storage

Project description

ZODB replicated storage (ZRS) provides database replication for ZODB. For each database, a primary storage and one or more secondary storages may be defined. The secondary storages will automatically replicate data from the primary storage.

Replication is superior to back-ups because as long as secondaries are running, secondary data is kept updated. In the event of a failure of a primary storage, just reconfigure a secondary to be the primary, and it can begin handling application requests.


  • Primary/secondary replication

  • Support for read-only secondary storages


  • ZODB 3.9 or later.


ZRS is installed like any other Python package, by installing the zc.zrs package with easy_install, zc.buildout, pip, etc.

Using ZRS

ZRS provides two ZODB storage implementations: a primary storage and a secondary storage. Both storages are used with file storages.

  • The primary storage is a readable and writable storage.

  • The secondary storage is read-only.

    A secondary storage can be used by read-only application clients to reduce server load.

    Secondary storages replicate from primary storages, or other secondary storages.

Theoretically, ZRS storages can be used wherever a ZODB storage would be used. In practice, however, they’re used in ZEO servers.

Configuration with ZConfig

If using an application, like a ZEO server or Zope, that uses ZConfig to configure ZODB storages, the configuration of a ZRS primary or secondary storage may be included in the configuration file as with any other storage e.g, to configure a primary storage, use something like:

%import zc.zrs

 replicate-to 5000

     path /path/to/data/file

Here is a line-by-line walk through:

%import zc.zrs

The import statement is necessary to load the ZConfig schema definitions for ZRS.


The zrs section defines a ZRS storage. A ZRS storage may be a primary storage or a secondary storage. A ZRS storage without a replicate-from option (as in the example above) is a primary storage.

replicate-to 5000

The replicate-to option specifies the replication address. Secondary storages will connect to this address to download replication data. This address can be a port number or a host name (interface name) and port number separated by a colon.

  path /path/to/data/file

A ZRS storage section must include a filestorage section specifying a file storage to contain the data.

Configuring a secondary storage is similar to configuring a primary storage:

%import zc.zrs

 replicate-from primary-host:5000
 replicate-to 5000
 keep-alive-delay 60

     path /path/to/secondary/data/file

For a secondary storage, a replicate-from option is used to specify the address to replicate data from.

Because primary and secondary storages are generally on separate machines, the host is usually specified in a replicate-from option.

A secondary storage can also specify a replicate-to option. If this option is used, other secondary storages can then replicate from the secondary, rather than replicating from the primary.

Secondary storages also support the following optional option:

keep-alive-delay SECONDS

In some network configurations, TCP connections are broken after extended periods of inactivity. This may even be done in a way that a client doesn’t detect the disconnection. To prevent this, you can use the keep-alive-delay option to cause the secondary storage to send periodic no-operation messages to the server.

Code and contributions

Recent Changes

For earlier changes, see the HISTORY.rst.

3.1.0 (2017-04-07)

  • Python 3 (3.4 amd 3.5) support.

    3.6 support will be added when Twisted supports Python 3.6. (There are worrying test failures under Python 3.6.)

  • Minor convenience change to API: When instantiating primary or secondary servers, you can pass a file-name instead of a storage instance and a file storage will be created automatically.

3.0.0 (2017-04-04)

  • Add support for ZODB 5

  • Drop ZooKeeper support.

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