Add option to modify sync commit per subscription

This also changes default behaviour of subscription workers to
synchronous_commit = off.

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>

3 files changed, 43 insertions, 0 deletions

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 5883673448c..5254bb3025a 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -6531,6 +6531,16 @@
      </row>
 
      <row>
+      <entry><structfield>subsynccommit</structfield></entry>
+      <entry><type>text</type></entry>
+      <entry></entry>
+      <entry>
+       Contains the value of the <varname>synchronous_commit</varname>
+       setting for the subscription workers.
+      </entry>
+     </row>
+
+     <row>
       <entry><structfield>subconninfo</structfield></entry>
       <entry><type>text</type></entry>
       <entry></entry>
diff --git a/doc/src/sgml/ref/alter_subscription.sgml b/doc/src/sgml/ref/alter_subscription.sgml
index 640fac0a159..f71ee38b40c 100644
--- a/doc/src/sgml/ref/alter_subscription.sgml
+++ b/doc/src/sgml/ref/alter_subscription.sgml
@@ -26,6 +26,7 @@ ALTER SUBSCRIPTION <replaceable class="PARAMETER">name</replaceable> WITH ( <rep
 <phrase>where <replaceable class="PARAMETER">suboption</replaceable> can be:</phrase>
 
     SLOT NAME = <replaceable class="PARAMETER">slot_name</replaceable>
+    | SYNCHRONOUS_COMMIT = <replaceable class="PARAMETER">synchronous_commit</replaceable>
 
 ALTER SUBSCRIPTION <replaceable class="PARAMETER">name</replaceable> SET PUBLICATION <replaceable class="PARAMETER">publication_name</replaceable> [, ...] { REFRESH WITH ( <replaceable class="PARAMETER">puboption</replaceable> [, ... ] ) | NOREFRESH }
 ALTER SUBSCRIPTION <replaceable class="PARAMETER">name</replaceable> REFRESH PUBLICATION WITH ( <replaceable class="PARAMETER">puboption</replaceable> [, ... ] )
@@ -91,6 +92,7 @@ ALTER SUBSCRIPTION <replaceable class="PARAMETER">name</replaceable> DISABLE
    <varlistentry>
     <term><literal>CONNECTION '<replaceable class="parameter">conninfo</replaceable>'</literal></term>
     <term><literal>SLOT NAME = <replaceable class="parameter">slot_name</replaceable></literal></term>
+    <term><literal>SYNCHRONOUS_COMMIT = <replaceable class="PARAMETER">synchronous_commit</replaceable></literal></term>
     <listitem>
      <para>
       These clauses alter properties originally set by
diff --git a/doc/src/sgml/ref/create_subscription.sgml b/doc/src/sgml/ref/create_subscription.sgml
index 3410d6fc8c2..3c51012df8b 100644
--- a/doc/src/sgml/ref/create_subscription.sgml
+++ b/doc/src/sgml/ref/create_subscription.sgml
@@ -32,6 +32,7 @@ CREATE SUBSCRIPTION <replaceable class="PARAMETER">subscription_name</replaceabl
     | CREATE SLOT | NOCREATE SLOT
     | SLOT NAME = <replaceable class="PARAMETER">slot_name</replaceable>
     | COPY DATA | NOCOPY DATA
+    | SYNCHRONOUS_COMMIT = <replaceable class="PARAMETER">synchronous_commit</replaceable>
     | NOCONNECT
 </synopsis>
  </refsynopsisdiv>
@@ -148,6 +149,36 @@ CREATE SUBSCRIPTION <replaceable class="PARAMETER">subscription_name</replaceabl
    </varlistentry>
 
    <varlistentry>
+    <term><literal>SYNCHRONOUS_COMMIT = <replaceable class="PARAMETER">synchronous_commit</replaceable></literal></term>
+    <listitem>
+     <para>
+      The value of this parameter overrides the
+      <xref linkend="guc-synchronous-commit"> setting.  The default value is
+      <literal>off</literal>.
+     </para>
+
+     <para>
+      It is safe to use <literal>off</literal> for logical replication: If the
+      subscriber loses transactions because of missing synchronization, the
+      data will be resent from the publisher.
+     </para>
+
+     <para>
+      A different setting might be appropriate when doing synchronous logical
+      replication.  The logical replication workers report the positions of
+      writes and flushes to the publisher, and when using synchronous
+      replication, the publisher will wait for the actual flush.  This means
+      that setting <literal>SYNCHRONOUS_COMMIT</literal> for the subscriber
+      to <literal>off</literal> when the subscription is used for synchronous
+      replication might increase the latency for <command>COMMIT</command> on
+      the publisher.  In this scenario, it can be advantageous to set
+      <literal>SYNCHRONOUS_COMMIT</literal> to <literal>local</literal> or
+      higher.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
     <term><literal>NOCONNECT</literal></term>
     <listitem>
      <para>