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Replication Slave
==================
This page contains the configuration and implementation details for Drizzle replication.
See these pages for user-level information:
.. toctree::
:maxdepth: 2
user_example
admin
Description
-----------
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
-------------
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
----------------------
The replication slave plugin creates two worker threads, each accessing a
work queue (implemented as an InnoDB table) that contains the replication
events. This is a producer/consumer paradigm where one thread populates the
queue (the producer), and the other thread (the consumer) reads events from
the queue.
The producer thread (or I/O thread) is in charge of connecting to the master
server and pulling down replication events from the master's transaction
log and storing them locally in the slave queue. It is required that the
master use the InnoDB replication log (:option:`--innodb.replication-log <drizzled --innodb.replication-log>`).
The consumer thread (or applier thread) reads the replication events from
the local slave queue, applies them locally, and then deletes successfully
applied events from the queue.
Schemas and Tables
------------------
The slave plugin creates its own schema and set of tables to store its
metadata. It stores everything in the **sys_replication** schema. The
following are the tables that it will create:
.. dbtable:: sys_replication.io_state
Stores metadata about the IO/producer thread.
.. dbtable:: sys_replication.applier_state
Stores metadata about the applier/consumer thread.
.. dbtable:: sys_replication.queue
The replication event queue.
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