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Replication enables data from one Drizzle database server (the master) to be replicated to one or more Drizzle database servers (the slaves). It provides a native implementation of replication between two Drizzle processes. Depending on your configuration, you can replicate all databases, selected databases, or selected tables within a database.

Drizzle’s replication system is entirely new and different from MySQL. Replication events are stored using Google Protocol Buffer messages in an InnoDB table. These events are read by the slave, stored locally, and applied. The advantage of the Google Protocol Buffer messages is a script or program can be put together in pretty much any language in minutes, and read the replication log. In other words, replication plugins are easy to implement, which enable developers to entirely customize their replication system.

This plugin requires a master that is running with the InnoDB replication log enabled. The slave plugin allows a server to replicate from another server that is using the innodb-trx log. It is a very simple setup: 

master options: –innodb.replication-log=true slave options: –plugin-add=slave –slave.config-file=XYZ

The config file may contain the following options in option=value format: 

master-host – hostname/ip of the master host master-port – port used by the master server master-user – username master-pass – password max-reconnects – max # of reconnect attempts if the master disappears seconds-between-reconnects – how long to wait between reconnect attempts

Configuration

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Most of the options that can be used to control the replication slave plugin

can only be given in a configuration file. The only exceptions are the

:option:`--slave.config-file` and :option:`--slave.max-commit-id` options.

.. program:: drizzled

.. option:: --slave.config-file=arg

Path to the replication slave configuration file. By default, the

plugin will look for a file named `slave.cfg` in the `etc` directory

of the Drizzle installation. If you want to specify a different path or

configuration file name, it is best to specify a full path to the

file. The relative path used by plugins is within the :option:`--datadir`

directory, so a full path is recommended.

.. option:: --slave.max-commit-id=arg

Manually set the maximum commit ID the slave is assumed to have retrieved

from the master. This value will be used by the slave to determine where

to begin retrieving replication messages from the master transaction log.

This option can be used to provision a new slave machine by setting it to

the value output from the drizzledump client when used with the

--single-transaction option.

This value is not allowed to be set via the configuration file since

you would normally only set it once on initial slave startup. This

eliminates the possibility of forgetting to delete it from the configuration

file for subsequent slave restarts.

The options below are read from the configuration file.

.. confval:: master-host

Hostname/IP address of the master server.

.. confval:: master-port

Drizzle port used by the master server. Default is 3306.

.. confval:: master-user

Username to use for connecting to the master server.

.. confval:: master-pass

Password associated with the username given by :confval:`master-user`.

.. confval:: max-reconnects

The number of reconnection attempts the slave plugin will try if the

master server becomes unreachable. Default is 10.

.. confval:: seconds-between-reconnects

The number of seconds to wait between reconnect attempts when the master

server becomes unreachable. Default is 30.

.. confval:: io-thread-sleep

The number of seconds the IO (producer) thread sleeps between queries to the

master for more replication events. Default is 5.

.. confval:: applier-thread-sleep

The number of seconds the applier (consumer) thread sleeps between applying

replication events from the local queue. Default is 5.

Implementation Details

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The replication slave plugin creates two worker threads, each accessing a

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work queue (implemented as an InnoDB table) that contains the replication

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events. This is a producer/consumer paradigm where one thread populates the

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queue (the producer), and the other thread (the consumer) reads events from

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the queue.

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The producer thread (or I/O thread) is in charge of connecting to the master

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server and pulling down replication events from the master's transaction

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log and storing them locally in the slave queue. It is required that the

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master use the InnoDB replication log (:option:`--innodb.replication-log <drizzled --innodb.replication-log>`).

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The consumer thread (or applier thread) reads the replication events from

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the local slave queue, applies them locally, and then deletes successfully

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applied events from the queue.

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Schemas and Tables

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The slave plugin creates its own schema and set of tables to store its

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metadata. It stores everything in the **sys_replication** schema. The

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following are the tables that it will create:

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.. dbtable:: sys_replication.io_state

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Stores metadata about the IO/producer thread.

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.. dbtable:: sys_replication.applier_state

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Stores metadata about the applier/consumer thread.

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.. dbtable:: sys_replication.queue

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The replication event queue.

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See :ref:`slave_applier`.

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