<!--
doc/src/sgml/ref/pg_createsubscriber.sgml
PostgreSQL documentation
-->

<refentry id="app-pgcreatesubscriber">
 <indexterm zone="app-pgcreatesubscriber">
  <primary>pg_createsubscriber</primary>
 </indexterm>

 <refmeta>
  <refentrytitle><application>pg_createsubscriber</application></refentrytitle>
  <manvolnum>1</manvolnum>
  <refmiscinfo>Application</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>pg_createsubscriber</refname>
  <refpurpose>convert a physical replica into a new logical replica</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
  <cmdsynopsis>
   <command>pg_createsubscriber</command>
   <arg rep="repeat"><replaceable>option</replaceable></arg>
   <group choice="plain">
    <group choice="req">
     <arg choice="plain"><option>-d</option></arg>
     <arg choice="plain"><option>--database</option></arg>
    </group>
    <replaceable>dbname</replaceable>
    <group choice="req">
     <arg choice="plain"><option>-D</option> </arg>
     <arg choice="plain"><option>--pgdata</option></arg>
    </group>
    <replaceable>datadir</replaceable>
    <group choice="req">
     <arg choice="plain"><option>-P</option></arg>
     <arg choice="plain"><option>--publisher-server</option></arg>
    </group>
    <replaceable>connstr</replaceable>
   </group>
  </cmdsynopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <application>pg_createsubscriber</application> creates a new logical
   replica from a physical standby server.  All tables in the specified
   database are included in the <link linkend="logical-replication">logical
   replication</link> setup.  A pair of
   publication and subscription objects are created for each database.  It
   must be run at the target server.
  </para>

  <para>
   After a successful run, the state of the target server is analogous to a
   fresh logical replication setup.  The main difference between the logical
   replication setup and <application>pg_createsubscriber</application> is how
   the data synchronization is done. <application>pg_createsubscriber</application>
   does not copy the initial table data. It does only the synchronization phase,
   which ensures each table is brought up to a synchronized state.
  </para>

  <para>
   <application>pg_createsubscriber</application> targets large database
   systems because in logical replication setup, most of the time is spent
   doing the initial data copy.  Furthermore, a side effect of this long time
   spent synchronizing data is usually a large amount of changes to be applied
   (that were produced during the initial data copy), which increases even
   more the time when the logical replica will be available. For smaller
   databases, it is recommended to set up logical replication with initial data
   synchronization.  For details, see the <command>CREATE SUBSCRIPTION</command>
   <link linkend="sql-createsubscription-params-with-copy-data">
   <literal>copy_data</literal></link> option.

  </para>
 </refsect1>

 <refsect1>
  <title>Options</title>

  <para>
   <application>pg_createsubscriber</application> accepts the following
   command-line arguments:

   <variablelist>
    <varlistentry>
     <term><option>-d <replaceable class="parameter">dbname</replaceable></option></term>
     <term><option>--database=<replaceable class="parameter">dbname</replaceable></option></term>
     <listitem>
      <para>
       The name of the database in which to create a subscription.  Multiple
       databases can be selected by writing multiple <option>-d</option>
       switches.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-D <replaceable class="parameter">directory</replaceable></option></term>
     <term><option>--pgdata=<replaceable class="parameter">directory</replaceable></option></term>
     <listitem>
      <para>
       The target directory that contains a cluster directory from a physical
       replica.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-n</option></term>
     <term><option>--dry-run</option></term>
     <listitem>
      <para>
       Do everything except actually modifying the target directory.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-p <replaceable class="parameter">port</replaceable></option></term>
     <term><option>--subscriber-port=<replaceable class="parameter">port</replaceable></option></term>
     <listitem>
      <para>
       The port number on which the target server is listening for
       connections.  Defaults to running the target server on port 50432 to
       avoid unintended client connections.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-P <replaceable class="parameter">connstr</replaceable></option></term>
     <term><option>--publisher-server=<replaceable class="parameter">connstr</replaceable></option></term>
     <listitem>
      <para>
       The connection string to the publisher.  For details see <xref
       linkend="libpq-connstring"/>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-s <replaceable class="parameter">dir</replaceable></option></term>
     <term><option>--socketdir=<replaceable class="parameter">dir</replaceable></option></term>
     <listitem>
      <para>
       The directory to use for postmaster sockets on target server.  The
       default is current directory.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-t <replaceable class="parameter">seconds</replaceable></option></term>
     <term><option>--recovery-timeout=<replaceable class="parameter">seconds</replaceable></option></term>
     <listitem>
      <para>
       The maximum number of seconds to wait for recovery to end.  Setting to
       0 disables.  The default is 0.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-U <replaceable class="parameter">username</replaceable></option></term>
     <term><option>--subscriber-username=<replaceable class="parameter">username</replaceable></option></term>
     <listitem>
      <para>
       The user name to connect as on target server.  Defaults to the current
       operating system user name.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-v</option></term>
     <term><option>--verbose</option></term>
     <listitem>
      <para>
       Enables verbose mode.  This will cause
       <application>pg_createsubscriber</application> to output progress
       messages and detailed information about each step to standard error.
       Repeating the option causes additional debug-level messages to appear
       on standard error.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>--config-file=<replaceable class="parameter">filename</replaceable></option></term>
     <listitem>
      <para>
       Use the specified main server configuration file for the target data
       directory.  <application>pg_createsubscriber</application> internally uses
       the <application>pg_ctl</application> command to start and
       stop the target server.  It allows you to specify the actual
       <filename>postgresql.conf</filename> configuration file if it is stored
       outside the data directory.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>--publication=<replaceable class="parameter">name</replaceable></option></term>
     <listitem>
      <para>
       The publication name to set up the logical replication.  Multiple
       publications can be specified by writing multiple
       <option>--publication</option> switches.  The number of publication
       names must match the number of specified databases, otherwise an error
       is reported.  The order of the multiple publication name switches must
       match the order of database switches.  If this option is not specified,
       a generated name is assigned to the publication name.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>--replication-slot=<replaceable class="parameter">name</replaceable></option></term>
     <listitem>
      <para>
       The replication slot name to set up the logical replication.  Multiple
       replication slots can be specified by writing multiple
       <option>--replication-slot</option> switches.  The number of
       replication slot names must match the number of specified databases,
       otherwise an error is reported.  The order of the multiple replication
       slot name switches must match the order of database switches.  If this
       option is not specified, the subscription name is assigned to the
       replication slot name.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>--subscription=<replaceable class="parameter">name</replaceable></option></term>
     <listitem>
      <para>
       The subscription name to set up the logical replication.  Multiple
       subscriptions can be specified by writing multiple
       <option>--subscription</option> switches.  The number of subscription
       names must match the number of specified databases, otherwise an error
       is reported.  The order of the multiple subscription name switches must
       match the order of database switches.  If this option is not specified,
       a generated name is assigned to the subscription name.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-V</option></term>
     <term><option>--version</option></term>
     <listitem>
      <para>
       Print the <application>pg_createsubscriber</application> version and exit.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><option>-?</option></term>
     <term><option>--help</option></term>
     <listitem>
      <para>
       Show help about <application>pg_createsubscriber</application> command
       line arguments, and exit.
      </para>
     </listitem>
    </varlistentry>
    </variablelist>
   </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <refsect2>
   <title>Prerequisites</title>

   <para>
    There are some prerequisites for
    <application>pg_createsubscriber</application> to convert the target server
    into a logical replica.  If these are not met, an error will be reported.
    The source and target servers must have the same major version as the
    <application>pg_createsubscriber</application>.  The given target data
    directory must have the same system identifier as the source data
    directory.  The given database user for the target data directory must have
    privileges for creating <link
    linkend="sql-createsubscription">subscriptions</link> and using <link
    linkend="pg-replication-origin-advance"><function>pg_replication_origin_advance()</function></link>.
   </para>

   <para>
    The target server must be used as a physical standby.  The target server
    must have <xref linkend="guc-max-replication-slots"/> and <xref
    linkend="guc-max-logical-replication-workers"/> configured to a value
    greater than or equal to the number of specified databases.  The target
    server must have <xref linkend="guc-max-worker-processes"/> configured to a
    value greater than the number of specified databases.  The target server
    must accept local connections.
   </para>

   <para>
    The source server must accept connections from the target server.  The
    source server must not be in recovery. The source server must have <xref
    linkend="guc-wal-level"/> as <literal>logical</literal>.  The source server
    must have <xref linkend="guc-max-replication-slots"/> configured to a value
    greater than or equal to the number of specified databases plus existing
    replication slots.  The source server must have <xref
    linkend="guc-max-wal-senders"/> configured to a value greater than or equal
    to the number of specified databases and existing WAL sender processes.
   </para>
  </refsect2>

  <refsect2>
   <title>Warnings</title>

   <para>
    If <application>pg_createsubscriber</application> fails after the target
    server was promoted, then the data directory is likely not in a state that
    can be recovered.  In such case, creating a new standby server is
    recommended.
   </para>

   <para>
    <application>pg_createsubscriber</application> usually starts the target
    server with different connection settings during transformation.  Hence,
    connections to the target server should fail.
   </para>

   <para>
    Since DDL commands are not replicated by logical replication, avoid
    executing DDL commands that change the database schema while running
    <application>pg_createsubscriber</application>.  If the target server has
    already been converted to logical replica, the DDL commands might not be
    replicated, which might cause an error.
   </para>

   <para>
    If <application>pg_createsubscriber</application> fails while processing,
    objects (publications, replication slots) created on the source server are
    removed.  The removal might fail if the target server cannot connect to
    the source server.  In such a case, a warning message will inform the
    objects left.  If the target server is running, it will be stopped.
   </para>

   <para>
    If the replication is using <xref linkend="guc-primary-slot-name"/>, it
    will be removed from the source server after the logical replication
    setup.
   </para>

   <para>
    If the target server is a synchronous replica, transaction commits on the
    primary might wait for replication while running
    <application>pg_createsubscriber</application>.
   </para>

   <para>
    <application>pg_createsubscriber</application> sets up logical
    replication with two-phase commit disabled.  This means that any
    prepared transactions will be replicated at the time
    of <command>COMMIT PREPARED</command>, without advance preparation.
    Once setup is complete, you can manually drop and re-create the
    subscription(s) with
    the <link linkend="sql-createsubscription-params-with-two-phase"><literal>two_phase</literal></link>
    option enabled.
   </para>

   <para>
    <application>pg_createsubscriber</application> changes the system
    identifier using <application>pg_resetwal</application>.  It would avoid
    situations in which the target server might use WAL files from the source
    server.  If the target server has a standby, replication will break and a
    fresh standby should be created.
   </para>
  </refsect2>

  <refsect2>
   <title>How It Works</title>

   <para>
    The basic idea is to have a replication start point from the source server
    and set up a logical replication to start from this point:
   </para>

   <procedure>
    <step>
     <para>
      Start the target server with the specified command-line options.  If the
      target server is already running,
      <application>pg_createsubscriber</application> will terminate with an
      error.
     </para>
    </step>

    <step>
     <para>
      Check if the target server can be converted.  There are also a few
      checks on the source server.  If any of the prerequisites are not met,
      <application>pg_createsubscriber</application> will terminate with an
      error.
     </para>
    </step>

    <step>
     <para>
      Create a publication and replication slot for each specified database on
      the source server.  Each publication is created using <link
      linkend="sql-createpublication-params-for-all-tables"><literal>FOR ALL
      TABLES</literal></link>.  If the <option>--publication</option> option
      is not specified, the publication has the following name pattern:
      <quote><literal>pg_createsubscriber_%u_%x</literal></quote> (parameter:
      database <parameter>oid</parameter>, random <parameter>int</parameter>).
      If the <option>--replication-slot</option> option is not specified, the
      replication slot has the following name pattern:
      <quote><literal>pg_createsubscriber_%u_%x</literal></quote> (parameters:
      database <parameter>oid</parameter>, random <parameter>int</parameter>).
      These replication slots will be used by the subscriptions in a future
      step.  The last replication slot LSN is used as a stopping point in the
      <xref linkend="guc-recovery-target-lsn"/> parameter and by the
      subscriptions as a replication start point.  It guarantees that no
      transaction will be lost.
     </para>
    </step>

    <step>
     <para>
      Write recovery parameters into the target data directory and restart the
      target server.  It specifies an LSN (<xref
      linkend="guc-recovery-target-lsn"/>) of the write-ahead log location up
      to which recovery will proceed.  It also specifies
      <literal>promote</literal> as the action that the server should take
      once the recovery target is reached.  Additional <link
      linkend="runtime-config-wal-recovery-target">recovery parameters</link>
      are added to avoid unexpected behavior during the recovery process such
      as end of the recovery as soon as a consistent state is reached (WAL
      should be applied until the replication start location) and multiple
      recovery targets that can cause a failure.  This step finishes once the
      server ends standby mode and is accepting read-write transactions.  If
      <option>--recovery-timeout</option> option is set,
      <application>pg_createsubscriber</application> terminates if recovery
      does not end until the given number of seconds.
     </para>
    </step>

    <step>
     <para>
      Create a subscription for each specified database on the target server.
      If the <option>--subscription</option> option is not specified, the
      subscription has the following name pattern:
      <quote><literal>pg_createsubscriber_%u_%x</literal></quote> (parameters:
      database <parameter>oid</parameter>, random <parameter>int</parameter>).
      It does not copy existing data from the source server.  It does not
      create a replication slot.  Instead, it uses the replication slot that
      was created in a previous step.  The subscription is created but it is
      not enabled yet.  The reason is the replication progress must be set to
      the replication start point before starting the replication.
     </para>
    </step>

    <step>
     <para>
      Drop publications on the target server that were replicated because they
      were created before the replication start location.  It has no use on
      the subscriber.
     </para>
    </step>

    <step>
     <para>
      Set the replication progress to the replication start point for each
      subscription.  When the target server starts the recovery process, it
      catches up to the replication start point.  This is the exact LSN to be
      used as a initial replication location for each subscription.  The
      replication origin name is obtained since the subscription was created.
      The replication origin name and the replication start point are used in
      <link
      linkend="pg-replication-origin-advance"><function>pg_replication_origin_advance()</function></link>
      to set up the initial replication location.
     </para>
    </step>

    <step>
     <para>
      Enable the subscription for each specified database on the target server.
      The subscription starts applying transactions from the replication start
      point.
     </para>
    </step>

    <step>
     <para>
      If the standby server was using <xref linkend="guc-primary-slot-name"/>,
      it has no use from now on so drop it.
     </para>
    </step>

    <step>
     <para>
      If the standby server contains <link
      linkend="logicaldecoding-replication-slots-synchronization">failover
      replication slots</link>, they cannot be synchronized anymore, so drop
      them.
     </para>
    </step>

    <step>
     <para>
      Update the system identifier on the target server. The
      <xref linkend="app-pgresetwal"/> is run to modify the system identifier.
      The target server is stopped as a <command>pg_resetwal</command> requirement.
     </para>
    </step>
   </procedure>
  </refsect2>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To create a logical replica for databases <literal>hr</literal> and
   <literal>finance</literal> from a physical replica at
   <literal>foo</literal>:
<screen>
<prompt>$</prompt> <userinput>pg_createsubscriber -D /usr/local/pgsql/data -P "host=foo" -d hr -d finance</userinput>
</screen>
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="app-pgbasebackup"/></member>
  </simplelist>
 </refsect1>
</refentry>
