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7th chunk of `doc/src/sgml/ref/pg_createsubscriber.sgml`
dcd383ec8019e6758cf673770e4b68cc4a8345f90c53c7810000000100000d48
 <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>

   <para>
    Replication failures can occur if required WAL files are missing. To prevent
    this, the source server must set
    <xref linkend="guc-max-slot-wal-keep-size"/> to <literal>-1</literal> to
    ensure that required WAL files are not prematurely removed.
   </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.
Title: pg_createsubscriber How It Works: Steps 1-4
Summary
This section describes the initial steps of how `pg_createsubscriber` works. It involves ensuring WAL files are not prematurely removed, starting the target server with specified command-line options, checking if the target server can be converted and verifying source server prerequisites, creating publications and replication slots on the source server for each database using FOR ALL TABLES (with customizable naming patterns if options are not specified), and writing recovery parameters (including a specific LSN and promote action) into the target data directory to start the server in recovery mode until it reaches the specified replication start point and becomes read-write.