mirror of
https://github.com/EnterpriseDB/repmgr.git
synced 2026-03-22 22:56:29 +00:00
doc: document "primary_visibility_consensus"
This commit is contained in:
Ian Barwick
@@ -168,6 +168,225 @@
|
|||||||
|
|
||||||
</sect1>
|
</sect1>
|
||||||
|
|
||||||
|
|
||||||
|
<sect1 id="repmgrd-primary-visibility-consensue" xreflabel="Primary visibility consensus">
|
||||||
|
<title>Primary visibility consensus</title>
|
||||||
|
|
||||||
|
<indexterm>
|
||||||
|
<primary>repmgrd</primary>
|
||||||
|
<secondary>primary visibility consensus</secondary>
|
||||||
|
</indexterm>
|
||||||
|
|
||||||
|
<indexterm>
|
||||||
|
<primary>primary_visibility_consensus</primary>
|
||||||
|
</indexterm>
|
||||||
|
|
||||||
|
<para>
|
||||||
|
In more complex replication setups, particularly where replication occurs between
|
||||||
|
multiple datacentres, it's possible that some but not all standbys get cut off from the
|
||||||
|
primary (but not from the other standbys).
|
||||||
|
</para>
|
||||||
|
<para>
|
||||||
|
In this situation, normally it's not desirable for any of the standbys which have been
|
||||||
|
cut off to initiate a failover, as the primary is still functioning and standbys are
|
||||||
|
connected. Beginning with <link linkend="release-4.4">&repmgr; 4.4</link>
|
||||||
|
it is now possible for the affected standbys to build a consensus about whether
|
||||||
|
the primary is still available to some standbys ("primary visibility consensus").
|
||||||
|
This is done by polling each standby for the time it last saw the primary;
|
||||||
|
if any have seen the primary very recently, it's reasonable
|
||||||
|
to infer that the primary is still available and a failover should not be started.
|
||||||
|
</para>
|
||||||
|
|
||||||
|
<para>
|
||||||
|
The time the primary was last seen by each node can be checked by executing
|
||||||
|
<link linkend="repmgr-daemon-status"><command>repmgr daemon status</command></link>,
|
||||||
|
which includes this in its output, e.g.:
|
||||||
|
<programlisting>$ repmgr -f /etc/repmgr.conf daemon status
|
||||||
|
ID | Name | Role | Status | Upstream | repmgrd | PID | Paused? | Upstream last seen
|
||||||
|
----+-------+---------+-----------+----------+---------+-------+---------+--------------------
|
||||||
|
1 | node1 | primary | * running | | running | 96563 | no | n/a
|
||||||
|
2 | node2 | standby | running | node1 | running | 96572 | no | 1 second(s) ago
|
||||||
|
3 | node3 | standby | running | node1 | running | 96584 | no | 0 second(s) ago</programlisting>
|
||||||
|
|
||||||
|
</para>
|
||||||
|
|
||||||
|
<para>
|
||||||
|
To enable this functionality, in <filename>repmgr.conf</filename> set:
|
||||||
|
<programlisting>
|
||||||
|
primary_visibility_consensus=true</programlisting>
|
||||||
|
</para>
|
||||||
|
<note>
|
||||||
|
<para>
|
||||||
|
<option>primary_visibility_consensus</option> <emphasis>must</emphasis> be set to
|
||||||
|
<literal>true</literal> on all nodes for it to be effective.
|
||||||
|
</para>
|
||||||
|
</note>
|
||||||
|
|
||||||
|
<para>
|
||||||
|
The following sample &repmgrd; log output demonstrates the behaviour in a situation
|
||||||
|
where one of three standbys is no longer able to connect to the primary, but <emphasis>can</emphasis>
|
||||||
|
connect to the two other standbys ("sibling nodes"):
|
||||||
|
<programlisting>
|
||||||
|
[2019-05-17 05:36:12] [WARNING] unable to reconnect to node 1 after 3 attempts
|
||||||
|
[2019-05-17 05:36:12] [INFO] 2 active sibling nodes registered
|
||||||
|
[2019-05-17 05:36:12] [INFO] local node's last receive lsn: 0/7006E58
|
||||||
|
[2019-05-17 05:36:12] [INFO] checking state of sibling node "node3" (ID: 3)
|
||||||
|
[2019-05-17 05:36:12] [INFO] node "node3" (ID: 3) reports its upstream is node 1, last seen 1 second(s) ago
|
||||||
|
[2019-05-17 05:36:12] [NOTICE] node 3 last saw primary node 1 second(s) ago, considering primary still visible
|
||||||
|
[2019-05-17 05:36:12] [INFO] last receive LSN for sibling node "node3" (ID: 3) is: 0/7006E58
|
||||||
|
[2019-05-17 05:36:12] [INFO] node "node3" (ID: 3) has same LSN as current candidate "node2" (ID: 2)
|
||||||
|
[2019-05-17 05:36:12] [INFO] checking state of sibling node "node4" (ID: 4)
|
||||||
|
[2019-05-17 05:36:12] [INFO] node "node4" (ID: 4) reports its upstream is node 1, last seen 0 second(s) ago
|
||||||
|
[2019-05-17 05:36:12] [NOTICE] node 4 last saw primary node 0 second(s) ago, considering primary still visible
|
||||||
|
[2019-05-17 05:36:12] [INFO] last receive LSN for sibling node "node4" (ID: 4) is: 0/7006E58
|
||||||
|
[2019-05-17 05:36:12] [INFO] node "node4" (ID: 4) has same LSN as current candidate "node2" (ID: 2)
|
||||||
|
[2019-05-17 05:36:12] [INFO] 2 nodes can see the primary
|
||||||
|
[2019-05-17 05:36:12] [DETAIL] following nodes can see the primary:
|
||||||
|
- node "node3" (ID: 3): 1 second(s) ago
|
||||||
|
- node "node4" (ID: 4): 0 second(s) ago
|
||||||
|
[2019-05-17 05:36:12] [NOTICE] cancelling failover as some nodes can still see the primary
|
||||||
|
[2019-05-17 05:36:12] [NOTICE] election cancelled
|
||||||
|
[2019-05-17 05:36:14] [INFO] node "node2" (ID: 2) monitoring upstream node "node1" (ID: 1) in degraded state</programlisting>
|
||||||
|
In this situation it will cancel the failover and enter degraded monitoring node,
|
||||||
|
waiting for the primary to reappear.
|
||||||
|
</para>
|
||||||
|
</sect1>
|
||||||
|
|
||||||
|
<sect1 id="repmgrd-standby-disconnection-on-failover" xreflabel="Standby disconnection on failover">
|
||||||
|
<title>Standby disconnection on failover</title>
|
||||||
|
|
||||||
|
<indexterm>
|
||||||
|
<primary>repmgrd</primary>
|
||||||
|
<secondary>standby disconnection on failover</secondary>
|
||||||
|
</indexterm>
|
||||||
|
|
||||||
|
<indexterm>
|
||||||
|
<primary>standby disconnection on failover</primary>
|
||||||
|
</indexterm>
|
||||||
|
|
||||||
|
<para>
|
||||||
|
If <option>standby_disconnect_on_failover</option> is set to <literal>true</literal> in
|
||||||
|
<filename>repmgr.conf</filename>, in a failover situation &repmgrd; will forcibly disconnect
|
||||||
|
the local node's WAL receiver before making a failover decision.
|
||||||
|
</para>
|
||||||
|
<note>
|
||||||
|
<para>
|
||||||
|
<option>standby_disconnect_on_failover</option> is available from PostgreSQL 9.5 and later.
|
||||||
|
Additionally this requires that the <literal>repmgr</literal> database user is a superuser.
|
||||||
|
</para>
|
||||||
|
</note>
|
||||||
|
<para>
|
||||||
|
By doing this, it's possible to ensure that, at the point the failover decision is made, no nodes
|
||||||
|
are receiving data from the primary and their LSN location will be static.
|
||||||
|
</para>
|
||||||
|
<important>
|
||||||
|
<para>
|
||||||
|
<option>standby_disconnect_on_failover</option> <emphasis>must</emphasis> be set to the same value on
|
||||||
|
all nodes.
|
||||||
|
</para>
|
||||||
|
</important>
|
||||||
|
<para>
|
||||||
|
Note that when using <option>standby_disconnect_on_failover</option> there will be a delay of 5 seconds
|
||||||
|
plus however many seconds it takes to confirm the WAL receiver is disconnected before
|
||||||
|
&repmgrd; proceeds with the failover decision.
|
||||||
|
</para>
|
||||||
|
<para>
|
||||||
|
Following the failover operation, no matter what the outcome, each node will reconnect its WAL receiver.
|
||||||
|
</para>
|
||||||
|
<para>
|
||||||
|
If using <option>standby_disconnect_on_failover</option>, we recommend that the
|
||||||
|
<option>primary_visibility_consensus</option> option is also used.
|
||||||
|
</para>
|
||||||
|
|
||||||
|
</sect1>
|
||||||
|
|
||||||
|
<sect1 id="repmgrd-failover-validation" xreflabel="Failover validation">
|
||||||
|
<title>Failover validation</title>
|
||||||
|
|
||||||
|
<indexterm>
|
||||||
|
<primary>repmgrd</primary>
|
||||||
|
<secondary>failover validation</secondary>
|
||||||
|
</indexterm>
|
||||||
|
|
||||||
|
<indexterm>
|
||||||
|
<primary>failover validation</primary>
|
||||||
|
</indexterm>
|
||||||
|
|
||||||
|
<para>
|
||||||
|
From <link linkend="release-4.3">repmgr 4.3</link>, &repmgr; makes it possible to provide a script
|
||||||
|
to &repmgrd; which, in a failover situation,
|
||||||
|
will be executed by the promotion candidate (the node which has been selected
|
||||||
|
to be the new primary) to confirm whether the node should actually be promoted.
|
||||||
|
</para>
|
||||||
|
<para>
|
||||||
|
To use this, <option>failover_validation_command</option> in <filename>repmgr.conf</filename>
|
||||||
|
to a script executable by the <literal>postgres</literal> system user, e.g.:
|
||||||
|
<programlisting>
|
||||||
|
failover_validation_command=/path/to/script.sh %n %a</programlisting>
|
||||||
|
</para>
|
||||||
|
<para>
|
||||||
|
The <literal>%n</literal> parameter will be replaced with the node ID, and the
|
||||||
|
<literal>%a</literal> parameter will be replaced by the node name when the script is executed.
|
||||||
|
</para>
|
||||||
|
<para>
|
||||||
|
This script must return an exit code of <literal>0</literal> to indicate the node should promote itself.
|
||||||
|
Any other value will result in the promotion being aborted and the election rerun.
|
||||||
|
There is a pause of <option>election_rerun_interval</option> seconds before the election is rerun.
|
||||||
|
</para>
|
||||||
|
<para>
|
||||||
|
Sample &repmgrd; log file output during which the failover validation
|
||||||
|
script rejects the proposed promotion candidate:
|
||||||
|
<programlisting>
|
||||||
|
[2019-03-13 21:01:30] [INFO] visible nodes: 2; total nodes: 2; no nodes have seen the primary within the last 4 seconds
|
||||||
|
[2019-03-13 21:01:30] [NOTICE] promotion candidate is "node2" (ID: 2)
|
||||||
|
[2019-03-13 21:01:30] [NOTICE] executing "failover_validation_command"
|
||||||
|
[2019-03-13 21:01:30] [DETAIL] /usr/local/bin/failover-validation.sh 2
|
||||||
|
[2019-03-13 21:01:30] [INFO] output returned by failover validation command:
|
||||||
|
Node ID: 2
|
||||||
|
|
||||||
|
[2019-03-13 21:01:30] [NOTICE] failover validation command returned a non-zero value: "1"
|
||||||
|
[2019-03-13 21:01:30] [NOTICE] promotion candidate election will be rerun
|
||||||
|
[2019-03-13 21:01:30] [INFO] 1 followers to notify
|
||||||
|
[2019-03-13 21:01:30] [NOTICE] notifying node "node3" (ID: 3) to rerun promotion candidate selection
|
||||||
|
INFO: node 3 received notification to rerun promotion candidate election
|
||||||
|
[2019-03-13 21:01:30] [NOTICE] rerunning election after 15 seconds ("election_rerun_interval")</programlisting>
|
||||||
|
</para>
|
||||||
|
|
||||||
|
|
||||||
|
</sect1>
|
||||||
|
|
||||||
|
<sect1 id="cascading-replication" xreflabel="Cascading replication">
|
||||||
|
<title>repmgrd and cascading replication</title>
|
||||||
|
|
||||||
|
<indexterm>
|
||||||
|
<primary>repmgrd</primary>
|
||||||
|
<secondary>cascading replication</secondary>
|
||||||
|
</indexterm>
|
||||||
|
|
||||||
|
<indexterm>
|
||||||
|
<primary>cascading replication</primary>
|
||||||
|
<secondary>repmgrd</secondary>
|
||||||
|
</indexterm>
|
||||||
|
|
||||||
|
<para>
|
||||||
|
Cascading replication - where a standby can connect to an upstream node and not
|
||||||
|
the primary server itself - was introduced in PostgreSQL 9.2. &repmgr; and
|
||||||
|
&repmgrd; support cascading replication by keeping track of the relationship
|
||||||
|
between standby servers - each node record is stored with the node id of its
|
||||||
|
upstream ("parent") server (except of course the primary server).
|
||||||
|
</para>
|
||||||
|
<para>
|
||||||
|
In a failover situation where the primary node fails and a top-level standby
|
||||||
|
is promoted, a standby connected to another standby will not be affected
|
||||||
|
and continue working as normal (even if the upstream standby it's connected
|
||||||
|
to becomes the primary node). If however the node's direct upstream fails,
|
||||||
|
the "cascaded standby" will attempt to reconnect to that node's parent
|
||||||
|
(unless <varname>failover</varname> is set to <literal>manual</literal> in
|
||||||
|
<filename>repmgr.conf</filename>).
|
||||||
|
</para>
|
||||||
|
|
||||||
|
</sect1>
|
||||||
|
|
||||||
<sect1 id="repmgrd-primary-child-disconnection" xreflabel="Monitoring standby disconnections on the primary">
|
<sect1 id="repmgrd-primary-child-disconnection" xreflabel="Monitoring standby disconnections on the primary">
|
||||||
<title>Monitoring standby disconnections on the primary node</title>
|
<title>Monitoring standby disconnections on the primary node</title>
|
||||||
|
|
||||||
@@ -310,7 +529,7 @@
|
|||||||
[2019-04-24 15:28:19] [NOTICE] node "node3" (ID: 3) has disconnected
|
[2019-04-24 15:28:19] [NOTICE] node "node3" (ID: 3) has disconnected
|
||||||
[2019-04-24 15:28:19] [NOTICE] 1 (of 2) child nodes are connected, but at least 2 child nodes required
|
[2019-04-24 15:28:19] [NOTICE] 1 (of 2) child nodes are connected, but at least 2 child nodes required
|
||||||
[2019-04-24 15:28:19] [INFO] most recently detached child node was 3 (ca. 0 seconds ago), not triggering "child_nodes_disconnect_command"
|
[2019-04-24 15:28:19] [INFO] most recently detached child node was 3 (ca. 0 seconds ago), not triggering "child_nodes_disconnect_command"
|
||||||
[2019-04-24 15:28:19] [DETAIL] "child_nodes_disconnect_timeout" set to 30 seconds
|
[2019-04-24 15:28:19] [DETAIL] "child_nodes_disconnect_timeout" set To 30 seconds
|
||||||
(...)</programlisting>
|
(...)</programlisting>
|
||||||
</para>
|
</para>
|
||||||
<para>
|
<para>
|
||||||
@@ -646,140 +865,5 @@ $ repmgr cluster event --event=child_nodes_disconnect_command
|
|||||||
|
|
||||||
</sect1>
|
</sect1>
|
||||||
|
|
||||||
<sect1 id="repmgrd-standby-disconnection-on-failover" xreflabel="Standby disconnection on failover">
|
|
||||||
<title>Standby disconnection on failover</title>
|
|
||||||
|
|
||||||
<indexterm>
|
|
||||||
<primary>repmgrd</primary>
|
|
||||||
<secondary>standby disconnection on failover</secondary>
|
|
||||||
</indexterm>
|
|
||||||
|
|
||||||
<indexterm>
|
|
||||||
<primary>standby disconnection on failover</primary>
|
|
||||||
</indexterm>
|
|
||||||
|
|
||||||
<para>
|
|
||||||
If <option>standby_disconnect_on_failover</option> is set to <literal>true</literal> in
|
|
||||||
<filename>repmgr.conf</filename>, in a failover situation &repmgrd; will forcibly disconnect
|
|
||||||
the local node's WAL receiver before making a failover decision.
|
|
||||||
</para>
|
|
||||||
<note>
|
|
||||||
<para>
|
|
||||||
<option>standby_disconnect_on_failover</option> is available from PostgreSQL 9.5 and later.
|
|
||||||
Additionally this requires that the <literal>repmgr</literal> database user is a superuser.
|
|
||||||
</para>
|
|
||||||
</note>
|
|
||||||
<para>
|
|
||||||
By doing this, it's possible to ensure that, at the point the failover decision is made, no nodes
|
|
||||||
are receiving data from the primary and their LSN location will be static.
|
|
||||||
</para>
|
|
||||||
<important>
|
|
||||||
<para>
|
|
||||||
<option>standby_disconnect_on_failover</option> <emphasis>must</emphasis> be set to the same value on
|
|
||||||
all nodes.
|
|
||||||
</para>
|
|
||||||
</important>
|
|
||||||
<para>
|
|
||||||
Note that when using <option>standby_disconnect_on_failover</option> there will be a delay of 5 seconds
|
|
||||||
plus however many seconds it takes to confirm the WAL receiver is disconnected before
|
|
||||||
&repmgrd; proceeds with the failover decision.
|
|
||||||
</para>
|
|
||||||
<para>
|
|
||||||
Following the failover operation, no matter what the outcome, each node will reconnect its WAL receiver.
|
|
||||||
</para>
|
|
||||||
<para>
|
|
||||||
If using <option>standby_disconnect_on_failover</option>, we recommend that the
|
|
||||||
<option>primary_visibility_consensus</option> option is also used.
|
|
||||||
</para>
|
|
||||||
|
|
||||||
</sect1>
|
|
||||||
|
|
||||||
<sect1 id="repmgrd-failover-validation" xreflabel="Failover validation">
|
|
||||||
<title>Failover validation</title>
|
|
||||||
|
|
||||||
<indexterm>
|
|
||||||
<primary>repmgrd</primary>
|
|
||||||
<secondary>failover validation</secondary>
|
|
||||||
</indexterm>
|
|
||||||
|
|
||||||
<indexterm>
|
|
||||||
<primary>failover validation</primary>
|
|
||||||
</indexterm>
|
|
||||||
|
|
||||||
<para>
|
|
||||||
From <link linkend="release-4.3">repmgr 4.3</link>, &repmgr; makes it possible to provide a script
|
|
||||||
to &repmgrd; which, in a failover situation,
|
|
||||||
will be executed by the promotion candidate (the node which has been selected
|
|
||||||
to be the new primary) to confirm whether the node should actually be promoted.
|
|
||||||
</para>
|
|
||||||
<para>
|
|
||||||
To use this, <option>failover_validation_command</option> in <filename>repmgr.conf</filename>
|
|
||||||
to a script executable by the <literal>postgres</literal> system user, e.g.:
|
|
||||||
<programlisting>
|
|
||||||
failover_validation_command=/path/to/script.sh %n %a</programlisting>
|
|
||||||
</para>
|
|
||||||
<para>
|
|
||||||
The <literal>%n</literal> parameter will be replaced with the node ID, and the
|
|
||||||
<literal>%a</literal> parameter will be replaced by the node name when the script is executed.
|
|
||||||
</para>
|
|
||||||
<para>
|
|
||||||
This script must return an exit code of <literal>0</literal> to indicate the node should promote itself.
|
|
||||||
Any other value will result in the promotion being aborted and the election rerun.
|
|
||||||
There is a pause of <option>election_rerun_interval</option> seconds before the election is rerun.
|
|
||||||
</para>
|
|
||||||
<para>
|
|
||||||
Sample &repmgrd; log file output during which the failover validation
|
|
||||||
script rejects the proposed promotion candidate:
|
|
||||||
<programlisting>
|
|
||||||
[2019-03-13 21:01:30] [INFO] visible nodes: 2; total nodes: 2; no nodes have seen the primary within the last 4 seconds
|
|
||||||
[2019-03-13 21:01:30] [NOTICE] promotion candidate is "node2" (ID: 2)
|
|
||||||
[2019-03-13 21:01:30] [NOTICE] executing "failover_validation_command"
|
|
||||||
[2019-03-13 21:01:30] [DETAIL] /usr/local/bin/failover-validation.sh 2
|
|
||||||
[2019-03-13 21:01:30] [INFO] output returned by failover validation command:
|
|
||||||
Node ID: 2
|
|
||||||
|
|
||||||
[2019-03-13 21:01:30] [NOTICE] failover validation command returned a non-zero value: "1"
|
|
||||||
[2019-03-13 21:01:30] [NOTICE] promotion candidate election will be rerun
|
|
||||||
[2019-03-13 21:01:30] [INFO] 1 followers to notify
|
|
||||||
[2019-03-13 21:01:30] [NOTICE] notifying node "node3" (ID: 3) to rerun promotion candidate selection
|
|
||||||
INFO: node 3 received notification to rerun promotion candidate election
|
|
||||||
[2019-03-13 21:01:30] [NOTICE] rerunning election after 15 seconds ("election_rerun_interval")</programlisting>
|
|
||||||
</para>
|
|
||||||
|
|
||||||
|
|
||||||
</sect1>
|
|
||||||
|
|
||||||
<sect1 id="cascading-replication" xreflabel="Cascading replication">
|
|
||||||
<title>repmgrd and cascading replication</title>
|
|
||||||
|
|
||||||
<indexterm>
|
|
||||||
<primary>repmgrd</primary>
|
|
||||||
<secondary>cascading replication</secondary>
|
|
||||||
</indexterm>
|
|
||||||
|
|
||||||
<indexterm>
|
|
||||||
<primary>cascading replication</primary>
|
|
||||||
<secondary>repmgrd</secondary>
|
|
||||||
</indexterm>
|
|
||||||
|
|
||||||
<para>
|
|
||||||
Cascading replication - where a standby can connect to an upstream node and not
|
|
||||||
the primary server itself - was introduced in PostgreSQL 9.2. &repmgr; and
|
|
||||||
&repmgrd; support cascading replication by keeping track of the relationship
|
|
||||||
between standby servers - each node record is stored with the node id of its
|
|
||||||
upstream ("parent") server (except of course the primary server).
|
|
||||||
</para>
|
|
||||||
<para>
|
|
||||||
In a failover situation where the primary node fails and a top-level standby
|
|
||||||
is promoted, a standby connected to another standby will not be affected
|
|
||||||
and continue working as normal (even if the upstream standby it's connected
|
|
||||||
to becomes the primary node). If however the node's direct upstream fails,
|
|
||||||
the "cascaded standby" will attempt to reconnect to that node's parent
|
|
||||||
(unless <varname>failover</varname> is set to <literal>manual</literal> in
|
|
||||||
<filename>repmgr.conf</filename>).
|
|
||||||
</para>
|
|
||||||
|
|
||||||
</sect1>
|
|
||||||
|
|
||||||
|
|
||||||
</chapter>
|
</chapter>
|
||||||
|
|||||||
Reference in New Issue
Block a user