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