Allow time delayed standbys and recovery

Set min_recovery_apply_delay to force a delay in recovery apply for commit and
restore point WAL records. Other records are replayed immediately. Delay is
measured between WAL record time and local standby time.

Robert Haas, Fabrízio de Royes Mello and Simon Riggs
Detailed review by Mitsumasa Kondo

1 files changed, 50 insertions, 0 deletions

diff --git a/doc/src/sgml/recovery-config.sgml b/doc/src/sgml/recovery-config.sgml
index 9d80256a556..ee5dc8687e2 100644
--- a/doc/src/sgml/recovery-config.sgml
+++ b/doc/src/sgml/recovery-config.sgml
@@ -142,6 +142,56 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
       </listitem>
      </varlistentry>
 
+     <varlistentry id="min-recovery-apply-delay" xreflabel="min_recovery_apply_delay">
+      <term><varname>min_recovery_apply_delay</varname> (<type>integer</type>)</term>
+      <indexterm>
+        <primary><varname>min_recovery_apply_delay</> recovery parameter</primary>
+      </indexterm>
+      <listitem>
+       <para>
+        By default, a standby server keeps restoring WAL records from the
+        primary as soon as possible. It may be useful to have a time-delayed
+        copy of the data, offering various options to correct data loss errors.
+        This paramater allows you to delay recovery by a fixed period of time,
+        specified in milliseconds if no unit is specified.  For example, if
+        you set this parameter to <literal>5min</literal>, the standby will
+        replay each transaction commit only when the system time on the standby
+        is at least five minutes past the commit time reported by the master.
+       </para>
+       <para>
+        It is possible that the replication delay between servers exceeds the
+        value of this parameter, in which case no delay is added.
+        Note that the delay is calculated between the WAL timestamp as written
+        on master and the time on the current standby. Delays
+        in transfer because of networks or cascading replication configurations
+        may reduce the actual wait time significantly. If the system
+        clocks on master and standby are not synchronised, this may lead to
+        recovery applying records earlier than expected but is not a major issue
+        because the useful settings of the parameter are much larger than
+        typical time deviation between the servers. Be careful to allow for
+        different timezone settings on master and standby.
+       </para>
+       <para>
+        The delay occurs only on WAL records for COMMIT and Restore Points.
+        Other records may be replayed earlier than the specified delay, which
+        is not an issue for MVCC though may potentially increase the number
+        of recovery conflicts generated.
+       </para>
+       <para>
+        The delay occurs until the standby is promoted or triggered. After that
+        the standby will end recovery without further waiting.
+       </para>
+       <para>
+        This parameter is intended for use with streaming replication deployments,
+        however, if the parameter is specified it will be honoured in all cases.
+        Synchronous replication is not affected by this setting because there is
+        not yet any setting to request synchronous apply of transaction commits.
+        <varname>hot_standby_feedback</> will be delayed by use of this feature
+        which could lead to bloat on the master; use both together with care.
+       </para>
+      </listitem>
+     </varlistentry>
+
     </variablelist>
 
   </sect1>