ZODB Replicated Storage

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.

Contents

Features
Requirements
Installation
Using ZRS
- Configuration with ZConfig
Code and contributions

Features

Primary/secondary replication
Support for read-only secondary storages

Requirements

ZODB 3.9 or later.

Installation

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

<zrs>
 replicate-to 5000

 <filestorage>
     path /path/to/data/file
 </filestorage>
</zrs>

Here is a line-by-line walk through:

%import zc.zrs

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

<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.

<filestorage>
  path /path/to/data/file
</filestorage>

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

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

 <filestorage>
     path /path/to/secondary/data/file
 </filestorage>
</zrs>

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.