transactions waiting for commit will be allowed to proceed after

these standby servers confirm receipt of their data.

The synchronous standbys will be those whose names appear

earlierin this list, and

in this list, and

that are both currently connected and streaming data in real-time

(as shown by a state of <literal>streaming</literal> in the

<link linkend="monitoring-stats-views-table">

<literal>pg_stat_replication</></link> view).

Other standby servers appearing later in this list represent potential

synchronous standbys. If any of the current synchronous

standbys disconnects for whatever reason,

it will be replaced immediately with the next-highest-priority standby.

Specifying more than one standby name can allow very high availability.

Specifying more than one standby names can allow very high availability.

</para>

<para>

This parameter specifies a list of standby servers using

either of the following syntaxes:

<synopsis>

<replaceable class="parameter">num_sync</replaceable> ( <replaceable class="parameter">standby_name</replaceable> [, ...] )

[FIRST] <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="parameter">standby_name</replaceable> [, ...] )

ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="parameter">standby_name</replaceable> [, ...] )

<replaceable class="parameter">standby_name</replaceable> [, ...]

</synopsis>

where <replaceable class="parameter">num_sync</replaceable> is

the number of synchronous standbys that transactions need to

wait for replies from,

and <replaceable class="parameter">standby_name</replaceable>

is the name of a standby server. For example, a setting of

<literal>3 (s1, s2, s3, s4)</> makes transaction commits wait

until their WAL records are received by three higher-priority standbys

chosen from standby servers <literal>s1</>, <literal>s2</>,

<literal>s3</> and <literal>s4</>.

</para>

<para>

The second syntax was used before <productname>PostgreSQL</>

is the name of a standby server.

<literal>FIRST</> and <literal>ANY</> specify the method to choose

synchronous standbys from the listed servers.

</para>

<para>

The keyword <literal>FIRST</>, coupled with

<replaceable class="parameter">num_sync</replaceable>, specifies a

priority-based synchronous replication and makes transaction commits

wait until their WAL records are replicated to

<replaceable class="parameter">num_sync</replaceable> synchronous

standbys chosen based on their priorities. For example, a setting of

<literal>FIRST 3 (s1, s2, s3, s4)</> will cause each commit to wait for

replies from three higher-priority standbys chosen from standby servers

<literal>s1</>, <literal>s2</>, <literal>s3</> and <literal>s4</>.

The standbys whose names appear earlier in the list are given higher

priority and will be considered as synchronous. Other standby servers

appearing later in this list represent potential synchronous standbys.

If any of the current synchronous standbys disconnects for whatever

reason, it will be replaced immediately with the next-highest-priority

standby. The keyword <literal>FIRST</> is optional.

</para>

<para>

The keyword <literal>ANY</>, coupled with

<replaceable class="parameter">num_sync</replaceable>, specifies a

quorum-based synchronous replication and makes transaction commits

wait until their WAL records are replicated to <emphasis>at least</>

<replaceable class="parameter">num_sync</replaceable> listed standbys.

For example, a setting of <literal>ANY 3 (s1, s2, s3, s4)</> will cause

each commit to proceed as soon as at least any three standbys of

<literal>s1</>, <literal>s2</>, <literal>s3</> and <literal>s4</>

reply.

</para>

<para>

<literal>FIRST</> and <literal>ANY</> are case-insensitive. If these

keywords are used as the name of a standby server,

its <replaceable class="parameter">standby_name</replaceable> must

be double-quoted.

</para>

<para>

The third syntax was used before <productname>PostgreSQL</>

version 9.6 and is still supported. It's the same as the first syntax

with <replaceable class="parameter">num_sync</replaceable> equal to 1.

For example, <literal>1 (s1, s2)</> and

<literal>s1, s2</> have the same meaning: either <literal>s1</>

or <literal>s2</> is chosen as a synchronous standby.

with <literal>FIRST</> and

<replaceable class="parameter">num_sync</replaceable> equal to 1.

For example, <literal>FIRST 1 (s1, s2)</> and <literal>s1, s2</> have

the same meaning: either <literal>s1</> or <literal>s2</> is chosen

as a synchronous standby.

</para>

<para>

The name of a standby server for this purpose is the

as synchronous confirm receipt of their data. The number of synchronous

standbys that transactions must wait for replies from is specified in

<varname>synchronous_standby_names</>. This parameter also specifies

a list of standby names, which determines the priority of each standby

for being chosen as a synchronous standby. The standbys whose names

appear earlier in the list are given higher priority and will be considered

as synchronous. Other standby servers appearing later in this list

represent potential synchronous standbys. If any of the current

synchronous standbys disconnects for whatever reason, it will be replaced

immediately with the next-highest-priority standby.

a list of standby names and the method (<literal>FIRST</> and

<literal>ANY</>) to choose synchronous standbys from the listed ones.

</para>

<para>

An example of <varname>synchronous_standby_names</> for multiple

synchronous standbys is:

The method <literal>FIRST</> specifies a priority-based synchronous

replication and makes transaction commits wait until their WAL records are

replicated to the requested number of synchronous standbys chosen based on

their priorities. The standbys whose names appear earlier in the list are

given higher priority and will be considered as synchronous. Other standby

servers appearing later in this list represent potential synchronous

standbys. If any of the current synchronous standbys disconnects for

whatever reason, it will be replaced immediately with the

next-highest-priority standby.

</para>

<para>

An example of <varname>synchronous_standby_names</> for

a priority-based multiple synchronous standbys is:

<programlisting>

synchronous_standby_names = '2 (s1, s2, s3)'

synchronous_standby_names = 'FIRST2 (s1, s2, s3)'

</programlisting>

In this example, if four standby servers <literal>s1</>, <literal>s2</>,

<literal>s3</> and <literal>s4</> are running, the two standbys

<literal>s2</> fails. <literal>s4</> is an asynchronous standby since

its name is not in the list.

</para>

<para>

The method <literal>ANY</> specifies a quorum-based synchronous

replication and makes transaction commits wait until their WAL records

are replicated to <emphasis>at least</> the requested number of

synchronous standbys in the list.

</para>

<para>

An example of <varname>synchronous_standby_names</> for

a quorum-based multiple synchronous standbys is:

<programlisting>

synchronous_standby_names = 'ANY 2 (s1, s2, s3)'

</programlisting>

In this example, if four standby servers <literal>s1</>, <literal>s2</>,

<literal>s3</> and <literal>s4</> are running, transaction commits will

wait for replies from at least any two standbys of <literal>s1</>,

<literal>s2</> and <literal>s3</>. <literal>s4</> is an asynchronous

standby since its name is not in the list.

</para>

<para>

The synchronous states of standby servers can be viewed using

the <structname>pg_stat_replication</structname> view.

<entry><structfield>sync_priority</></entry>

<entry><type>integer</></entry>

<entry>Priority of this standby server for being chosen as the

synchronous standby</entry>

synchronous standby in a priority-based synchronous replication.

This has no effect in a quorum-based synchronous replication.</entry>

</row>

<row>

<entry><structfield>sync_state</></entry>

<literal>sync</>: This standby server is synchronous.

</para>

</listitem>

<listitem>

<para>

<literal>quorum</>: This standby server is considered as a candidate

for quorum standbys.

</para>

</listitem>

</itemizedlist>

</entry>

</row>

syncrep_gram.o: syncrep_scanner.c

syncrep_scanner.c: FLEXFLAGS = -CF -p

syncrep_scanner.c: FLEXFLAGS = -CF -p -i

syncrep_scanner.c: FLEX_NO_BACKUP=yes