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In some circumstances, e.g. while performing a switchover, it is essential
that repmgrd does not take any kind of failover action, as this will put
the cluster into an incorrect state.
Previously it was necessary to stop repmgrd on all nodes (or at least
those nodes which repmgrd would consider as promotion candidates), however
this is a cumbersome and potentially risk-prone operation, particularly if the
replication cluster contains more than a couple of servers.
To prevent this issue from occurring, this patch introduces the ability
to "pause" repmgrd on all nodes wth a single command ("repmgr daemon pause")
which notifies repmgrd not to take any failover action until the node
is "unpaused" ("repmgr daemon unpause").
"repmgr daemon status" provides an overview of each node and whether repmgrd
is running, and if so whether it is paused.
"repmgr standby switchover" has been modified to automatically pause repmgrd
while carrying out the switchover.
See documentation for further details.
426 lines
19 KiB
Plaintext
426 lines
19 KiB
Plaintext
<chapter id="performing-switchover" xreflabel="Performing a switchover with repmgr">
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<indexterm>
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<primary>switchover</primary>
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</indexterm>
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<title>Performing a switchover with repmgr</title>
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<para>
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A typical use-case for replication is a combination of primary and standby
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server, with the standby serving as a backup which can easily be activated
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in case of a problem with the primary. Such an unplanned failover would
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normally be handled by promoting the standby, after which an appropriate
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action must be taken to restore the old primary.
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</para>
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<para>
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In some cases however it's desirable to promote the standby in a planned
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way, e.g. so maintenance can be performed on the primary; this kind of switchover
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is supported by the <xref linkend="repmgr-standby-switchover"> command.
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</para>
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<para>
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<command>repmgr standby switchover</command> differs from other &repmgr;
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actions in that it also performs actions on other servers (the demotion
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candidate, and optionally any other servers which are to follow the new primary),
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which means passwordless SSH access is required to those servers from the one where
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<command>repmgr standby switchover</command> is executed.
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</para>
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<note>
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<simpara>
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<command>repmgr standby switchover</command> performs a relatively complex
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series of operations on two servers, and should therefore be performed after
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careful preparation and with adequate attention. In particular you should
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be confident that your network environment is stable and reliable.
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</simpara>
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<simpara>
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Additionally you should be sure that the current primary can be shut down
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quickly and cleanly. In particular, access from applications should be
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minimalized or preferably blocked completely. Also be aware that if there
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is a backlog of files waiting to be archived, PostgreSQL will not shut
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down until archiving completes.
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</simpara>
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<simpara>
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We recommend running <command>repmgr standby switchover</command> at the
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most verbose logging level (<literal>--log-level=DEBUG --verbose</literal>)
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and capturing all output to assist troubleshooting any problems.
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</simpara>
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<simpara>
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Please also read carefully the sections <xref linkend="preparing-for-switchover"> and
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<xref linkend="switchover-caveats"> below.
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</simpara>
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</note>
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<sect1 id="preparing-for-switchover" xreflabel="Preparing for switchover">
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<indexterm>
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<primary>switchover</primary>
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<secondary>preparation</secondary>
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</indexterm>
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<title>Preparing for switchover</title>
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<para>
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As mentioned in the previous section, success of the switchover operation depends on
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&repmgr; being able to shut down the current primary server quickly and cleanly.
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</para>
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<para>
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Ensure that the promotion candidate has sufficient free walsenders available
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(PostgreSQL configuration item <varname>max_wal_senders</varname>), and if replication
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slots are in use, at least one free slot is available for the demotion candidate (
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PostgreSQL configuration item <varname>max_replication_slots</varname>).
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</para>
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<para>
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Ensure that a passwordless SSH connection is possible from the promotion candidate
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(standby) to the demotion candidate (current primary). If <literal>--siblings-follow</literal>
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will be used, ensure that passwordless SSH connections are possible from the
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promotion candidate to all standbys attached to the demotion candidate.
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</para>
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<note>
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<simpara>
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&repmgr; expects to find the &repmgr; binary in the same path on the remote
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server as on the local server.
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</simpara>
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</note>
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<para>
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Double-check which commands will be used to stop/start/restart the current
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primary; on the current primary execute:
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<programlisting>
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repmgr -f /etc/repmgr.conf node service --list --action=stop
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repmgr -f /etc/repmgr.conf node service --list --action=start
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repmgr -f /etc/repmgr.conf node service --list --action=restart</programlisting>
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</para>
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<para>
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These commands can be defined in <filename>repmgr.conf</filename> with
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<option>service_start_command</option>, <option>service_stop_command</option>
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and <option>service_restart_command</option>.
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</para>
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<important>
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<para>
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If &repmgr; is installed from a package. you should set these commands
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to use the appropriate service commands defined by the package/operating
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system as these will ensure PostgreSQL is stopped/started properly
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taking into account configuration and log file locations etc.
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</para>
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<para>
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If the <option>service_*_command</option> options aren't defined, &repmgr; will
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fall back to using <application>pg_ctl</application> to stop/start/restart
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PostgreSQL, which may not work properly, particularly when executed on a remote
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server.
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</para>
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<para>
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For more details, see <xref linkend="configuration-file-service-commands">.
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</para>
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</important>
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<note>
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<simpara>
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On <literal>systemd</literal> systems we strongly recommend using the appropriate
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<command>systemctl</command> commands (typically run via <command>sudo</command>) to ensure
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<literal>systemd</literal> is informed about the status of the PostgreSQL service.
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</simpara>
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<simpara>
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If using <command>sudo</command> for the <command>systemctl</command> calls, make sure the
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<command>sudo</command> specification doesn't require a real tty for the user. If not set
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this way, <command>repmgr</command> will fail to stop the primary.
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</simpara>
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</note>
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<para>
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Check that access from applications is minimalized or preferably blocked
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completely, so applications are not unexpectedly interrupted.
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</para>
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<note>
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<para>
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If an exclusive backup is running on the current primary, &repmgr; will not perform the
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switchover.
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</para>
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</note>
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<para>
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Check there is no significant replication lag on standbys attached to the
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current primary.
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</para>
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<para>
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If WAL file archiving is set up, check that there is no backlog of files waiting
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to be archived, as PostgreSQL will not finally shut down until all of these have been
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archived. If there is a backlog exceeding <varname>archive_ready_warning</varname> WAL files,
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&repmgr; will emit a warning before attempting to perform a switchover; you can also check
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manually with <command>repmgr node check --archive-ready</command>.
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</para>
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<note>
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<para>
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From <link linkend="release-4.2">repmgr 4.2</link>, &repmgr; will instruct any running
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<application>repmgrd</application> instances to pause operations while the switchover
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is being carried out, to prevent <application>repmgrd</application> from
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unintentionally promoting a node. For more details, see <xref linkend="repmgrd-pausing">.
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</para>
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<para>
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Users of &repmgr; versions prior to 4.2 should ensure that <application>repmgrd</application>
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is not running on any nodes while a switchover is being executed.
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</para>
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</note>
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<para>
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Finally, consider executing <command>repmgr standby switchover</command> with the
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<literal>--dry-run</literal> option; this will perform any necessary checks and inform you about
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success/failure, and stop before the first actual command is run (which would be the shutdown of the
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current primary). Example output:
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<programlisting>
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$ repmgr standby switchover -f /etc/repmgr.conf --siblings-follow --dry-run
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NOTICE: checking switchover on node "node2" (ID: 2) in --dry-run mode
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INFO: SSH connection to host "node1" succeeded
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INFO: archive mode is "off"
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INFO: replication lag on this standby is 0 seconds
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INFO: all sibling nodes are reachable via SSH
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NOTICE: local node "node2" (ID: 2) will be promoted to primary; current primary "node1" (ID: 1) will be demoted to standby
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INFO: following shutdown command would be run on node "node1":
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"pg_ctl -l /var/log/postgresql/startup.log -D '/var/lib/postgresql/data' -m fast -W stop"
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</programlisting>
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</para>
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<important>
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<para>
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Be aware that <option>--dry-run</option> checks the prerequisites
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for performing the switchover and some basic sanity checks on the
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state of the database which might effect the switchover operation
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(e.g. replication lag); it cannot however guarantee the switchover
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operation will succeed. In particular, if the current primary
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does not shut down cleanly, &repmgr; will not be able to reliably
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execute the switchover (as there would be a danger of divergence
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between the former and new primary nodes).
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</para>
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</important>
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<note>
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<simpara>
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See <xref linkend="repmgr-standby-switchover"> for a full list of available
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command line options and <filename>repmgr.conf</filename> settings relevant
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to performing a switchover.
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</simpara>
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</note>
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<sect2 id="switchover-pg-rewind" xreflabel="Switchover and pg_rewind">
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<indexterm>
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<primary>pg_rewind</primary>
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<secondary>using with "repmgr standby switchover"</secondary>
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</indexterm>
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<title>Switchover and pg_rewind</title>
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<para>
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If the demotion candidate does not shut down smoothly or cleanly, there's a risk it
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will have a slightly divergent timeline and will not be able to attach to the new
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primary. To fix this situation without needing to reclone the old primary, it's
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possible to use the <application>pg_rewind</application> utility, which will usually be
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able to resync the two servers.
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</para>
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<para>
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To have &repmgr; execute <application>pg_rewind</application> if it detects this
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situation after promoting the new primary, add the <option>--force-rewind</option>
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option.
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</para>
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<note>
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<simpara>
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If &repmgr; detects a situation where it needs to execute <application>pg_rewind</application>,
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it will execute a <literal>CHECKPOINT</literal> on the new primary before executing
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<application>pg_rewind</application>.
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</simpara>
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</note>
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<para>
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For more details on <application>pg_rewind</application>, see:
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<ulink url="https://www.postgresql.org/docs/current/static/app-pgrewind.html">https://www.postgresql.org/docs/current/static/app-pgrewind.html</ulink>.
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</para>
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<para>
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<application>pg_rewind</application> has been part of the core PostgreSQL distribution since
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version 9.5. Users of versions 9.3 and 9.4 will need to manually install it; the source code is available here:
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<ulink url="https://github.com/vmware/pg_rewind">https://github.com/vmware/pg_rewind</ulink>.
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If the <application>pg_rewind</application>
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binary is not installed in the PostgreSQL <filename>bin</filename> directory, provide
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its full path on the demotion candidate with <option>--force-rewind</option>.
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</para>
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<para>
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Note that building the 9.3/9.4 version of <application>pg_rewind</application> requires the PostgreSQL
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source code. Also, PostgreSQL 9.3 does not provide <varname>wal_log_hints</varname>,
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meaning data checksums must have been enabled when the database was initialized.
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</para>
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</sect2>
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</sect1>
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<sect1 id="switchover-execution" xreflabel="Executing the switchover command">
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<indexterm>
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<primary>switchover</primary>
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<secondary>execution</secondary>
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</indexterm>
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<title>Executing the switchover command</title>
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<para>
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To demonstrate switchover, we will assume a replication cluster with a
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primary (<literal>node1</literal>) and one standby (<literal>node2</literal>);
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after the switchover <literal>node2</literal> should become the primary with
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<literal>node1</literal> following it.
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</para>
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<para>
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The switchover command must be run from the standby which is to be promoted,
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and in its simplest form looks like this:
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<programlisting>
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$ repmgr -f /etc/repmgr.conf standby switchover
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NOTICE: executing switchover on node "node2" (ID: 2)
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INFO: searching for primary node
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INFO: checking if node 1 is primary
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INFO: current primary node is 1
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INFO: SSH connection to host "node1" succeeded
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INFO: archive mode is "off"
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INFO: replication lag on this standby is 0 seconds
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NOTICE: local node "node2" (ID: 2) will be promoted to primary; current primary "node1" (ID: 1) will be demoted to standby
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NOTICE: stopping current primary node "node1" (ID: 1)
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NOTICE: issuing CHECKPOINT
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DETAIL: executing server command "pg_ctl -l /var/log/postgres/startup.log -D '/var/lib/pgsql/data' -m fast -W stop"
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INFO: checking primary status; 1 of 6 attempts
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NOTICE: current primary has been cleanly shut down at location 0/3001460
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NOTICE: promoting standby to primary
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DETAIL: promoting server "node2" (ID: 2) using "pg_ctl -l /var/log/postgres/startup.log -w -D '/var/lib/pgsql/data' promote"
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server promoting
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NOTICE: STANDBY PROMOTE successful
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DETAIL: server "node2" (ID: 2) was successfully promoted to primary
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INFO: setting node 1's primary to node 2
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NOTICE: starting server using "pg_ctl -l /var/log/postgres/startup.log -w -D '/var/lib/pgsql/data' restart"
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NOTICE: NODE REJOIN successful
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DETAIL: node 1 is now attached to node 2
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NOTICE: switchover was successful
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DETAIL: node "node2" is now primary
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NOTICE: STANDBY SWITCHOVER is complete
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</programlisting>
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</para>
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<para>
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The old primary is now replicating as a standby from the new primary, and the
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cluster status will now look like this:
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<programlisting>
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$ repmgr -f /etc/repmgr.conf cluster show
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ID | Name | Role | Status | Upstream | Location | Connection string
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----+-------+---------+-----------+----------+----------+--------------------------------------
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1 | node1 | standby | running | node2 | default | host=node1 dbname=repmgr user=repmgr
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2 | node2 | primary | * running | | default | host=node2 dbname=repmgr user=repmgr
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</programlisting>
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</para>
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<para>
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If <application>repmgrd</application> is in use, it's worth double-checking that
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all nodes are unpaused by executing <command><link linkend="repmgr-daemon-status">repmgr-daemon-status</link></command>.
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</para>
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<note>
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<para>
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Users of &repmgr; versions prior to 4.2 will need to manually restart <application>repmgrd</application>
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on all nodes after the switchover is completed.
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</para>
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</note>
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</sect1>
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<sect1 id="switchover-caveats" xreflabel="Caveats">
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<indexterm>
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<primary>switchover</primary>
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<secondary>caveats</secondary>
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</indexterm>
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<title>Caveats</title>
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<para>
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<itemizedlist spacing="compact" mark="bullet">
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<listitem>
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<simpara>
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If using PostgreSQL 9.3 or 9.4, you should ensure that the shutdown command
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is configured to use PostgreSQL's <varname>fast</varname> shutdown mode (the default in 9.5
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and later). If relying on <command>pg_ctl</command> to perform database server operations,
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you should include <literal>-m fast</literal> in <varname>pg_ctl_options</varname>
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in <filename>repmgr.conf</filename>.
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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<command>pg_rewind</command> *requires* that either <varname>wal_log_hints</varname> is enabled, or that
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data checksums were enabled when the cluster was initialized. See the
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<ulink url="https://www.postgresql.org/docs/current/static/app-pgrewind.html">pg_rewind documentation</ulink>
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for details.
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</simpara>
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</listitem>
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</itemizedlist>
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</para>
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</sect1>
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<sect1 id="switchover-troubleshooting" xreflabel="Troubleshooting">
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<indexterm>
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<primary>switchover</primary>
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<secondary>troubleshooting</secondary>
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</indexterm>
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<title>Troubleshooting switchover issues</title>
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<para>
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As <link linkend="performing-switchover">emphasised previously</link>, performing a switchover
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is a non-trivial operation and there are a number of potential issues which can occur.
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While &repmgr; attempts to perform sanity checks, there's no guaranteed way of determining the success of
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a switchover without actually carrying it out.
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</para>
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<sect2 id="switchover-troubleshooting-primary-shutdown">
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<title>Demotion candidate (old primary) does not shut down</title>
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<para>
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&repmgr; may abort a switchover with a message like:
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<programlisting>
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ERROR: shutdown of the primary server could not be confirmed
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HINT: check the primary server status before performing any further actions</programlisting>
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</para>
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<para>
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This means the shutdown of the old primary has taken longer than &repmgr; expected,
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and it has given up waiting.
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</para>
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<para>
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In this case, check the PostgreSQL log on the primary server to see what is going
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on. It's entirely possible the shutdown process is just taking longer than the
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timeout set by the configuration parameter <varname>shutdown_check_timeout</varname>
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(default: 60 seconds), in which case you may need to adjust this parameter.
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</para>
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<note>
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<para>
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Note that <varname>shutdown_check_timeout</varname>is set on the node where
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<command>repmgr standby switchover</command> is executed (promotion candidate); setting it on the
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demotion candidate (former primary) will have no effect.
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</para>
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</note>
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<para>
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If the primary server has shut down cleanly, and no other node has been promoted,
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it is safe to restart it, in which case the replication cluster will be restored
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to its original configuration.
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</para>
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</sect2>
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<sect2 id="switchover-troubleshooting-exclusive-backup">
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<title>Switchover aborts with an "exclusive backup" error</title>
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<para>
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&repmgr; may abort a switchover with a message like:
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<programlisting>
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ERROR: unable to perform a switchover while primary server is in exclusive backup mode
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HINT: stop backup before attempting the switchover</programlisting>
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</para>
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<para>
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This means an exclusive backup is running on the current primary; interrupting this
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will not only abort the backup, but potentially leave the primary with an ambiguous
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backup state.
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</para>
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<para>
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To proceed, either wait until the backup has finished, or cancel it with the command
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<command>SELECT pg_stop_backup()</command>. For more details see the PostgreSQL
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documentation section
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<ulink url="https://www.postgresql.org/docs/current/static/continuous-archiving.html#BACKUP-LOWLEVEL-BASE-BACKUP-EXCLUSIVE">Making an exclusive low level backup</ulink>.
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</para>
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</sect2>
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</sect1>
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</chapter>
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