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ZODB Replicated Storage

Project description

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::


- Primary/secondary replication

- Support for read-only secondary storages

- Service registration and discovery with ZooKeeper


- 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


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

%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``

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.


Primaries and secondaries can register with ZooKeeper, so Secondaries
can find primaries to replicate from without needing to configure a
specific address. See ``zk.test`` and ``zkconfig.test`` in the source
directory for more details.

Code and contributions


Include src/**/*.rst files in sdist.

2.5.0 2015-01-25

Added nagios plugins for monitoring replication.

2.4.4 2013-08-17

Fixed packaging bug: component.xml was left out.

2.4.3 2013-08-15

Packaging update: allow installation without setuptools.

2.4.2 2013-05-27

Initial open-source release for ZODB 3.9 or later.

2.0.5 2013-05-27

Initial open-source release for ZODB 3.8.

Initial open-source release. Requires ZODB 3.9 or later.

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