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Add more repmgrd documentation

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Ian Barwick
2017-10-05 15:59:39 +09:00
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doc/repmgrd-network-split.sgml Normal file
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<chapter id="repmgrd-network-split">
<title>Handling network splits with repmgrd</title>
<para>
A common pattern for replication cluster setups is to spread servers over
more than one datacentre. This can provide benefits such as geographically-
distributed read replicas and DR (disaster recovery capability). However
this also means there is a risk of disconnection at network level between
datacentre locations, which would result in a split-brain scenario if
servers in a secondary data centre were no longer able to see the primary
in the main data centre and promoted a standby among themselves.
</para>
<para>
Previous &repmgr; versions used the concept of a "witness server" to
artificially create a quorum of servers in a particular location, ensuring
that nodes in another location will not elect a new primary if they
are unable to see the majority of nodes. However this approach does not
scale well, particularly with more complex replication setups, e.g.
where the majority of nodes are located outside of the primary datacentre.
It also means the <literal>witness</literal> node needs to be managed as an
extra PostgreSQL instance outside of the main replication cluster, which
adds administrative and programming complexity.
</para>
<para>
<literal>repmgr4</literal> introduces the concept of <literal>location</literal>:
each node is associated with an arbitrary location string (default is
<literal>default</literal>); this is set in <filename>repmgr.conf</filename>, e.g.:
<programlisting>
node_id=1
node_name=node1
conninfo='host=node1 user=repmgr dbname=repmgr connect_timeout=2'
data_directory='/var/lib/postgresql/data'
location='dc1'</programlisting>
</para>
<para>
In a failover situation, <command>repmgrd</command> will check if any servers in the
same location as the current primary node are visible. If not, <command>repmgrd</command>
will assume a network interruption and not promote any node in any
other location (it will however enter <xref linkend="repmgrd-degraded-monitoring"> mode until
a primary becomes visible).
</para>
</chapter>