id="guc-synchronous-standby-names" xreflabel="synchronous_standby_names">
      <term><varname>synchronous_standby_names</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>synchronous_standby_names</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies a list of standby servers that can support
        <firstterm>synchronous replication</firstterm>, as described in
        <xref linkend="synchronous-replication"/>.
        There will be one or more active synchronous standbys;
        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
        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-pg-stat-replication-view">
        <structname>pg_stat_replication</structname></link> view).
        Specifying more than one synchronous standby can allow for very high
        availability and protection against data loss.
       </para>
       <para>
        The name of a standby server for this purpose is the
        <varname>application_name</varname> setting of the standby, as set in the
        standby's connection information.  In case of a physical replication
        standby, this should be set in the <varname>primary_conninfo</varname>
        setting; the default is the setting of <xref linkend="guc-cluster-name"/>
        if set, else <literal>walreceiver</literal>.
        For logical replication, this can be set in the connection
        information of the subscription, and it defaults to the
        subscription name.  For other replication stream consumers,
        consult their documentation.
       </para>
       <para>
        This parameter specifies a list of standby servers using
        either of the following syntaxes:
<synopsis>
[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.
        <replaceable class="parameter">num_sync</replaceable>
        must be an integer value greater than zero.
        <literal>FIRST</literal> and <literal>ANY</literal> specify the method to choose
        synchronous standbys from the listed servers.
       </para>
       <para>
        The keyword <literal>FIRST</literal>, 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)</literal> will cause each commit to wait for
        replies from three higher-priority standbys chosen from standby servers
        <literal>s1</literal>, <literal>s2</literal>, <literal>s3</literal> and <literal>s4</literal>.
        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