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

<refentry id="app-pgrewind">
 <indexterm zone="app-pgrewind">
  <primary>pg_rewind</primary>
 </indexterm>

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

 <refnamediv>
  <refname>pg_rewind</refname>
  <refpurpose>synchronize a <productname>PostgreSQL</productname> data directory with another data directory that was forked from it</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
  <cmdsynopsis>
   <command>pg_rewind</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>--target-pgdata</option></arg>
    </group>
    <replaceable> directory</replaceable>
    <group choice="req">
     <arg choice="plain"><option>--source-pgdata=<replaceable>directory</replaceable></option></arg>
     <arg choice="plain"><option>--source-server=<replaceable>connstr</replaceable></option></arg>
    </group>
   </group>
  </cmdsynopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <application>pg_rewind</application> is a tool for synchronizing a PostgreSQL cluster
   with another copy of the same cluster, after the clusters' timelines have
   diverged. A typical scenario is to bring an old master server back online
   after failover as a standby that follows the new master.
  </para>

  <para>
   The result is equivalent to replacing the target data directory with the
   source one. Only changed blocks from relation files are copied;
   all other files are copied in full, including configuration files. The
   advantage of <application>pg_rewind</application> over taking a new base backup, or
   tools like <application>rsync</application>, is that <application>pg_rewind</application> does
   not require reading through unchanged blocks in the cluster. This makes
   it a lot faster when the database is large and only a small
   fraction of blocks differ between the clusters.
  </para>

  <para>
   <application>pg_rewind</application> examines the timeline histories of the source
   and target clusters to determine the point where they diverged, and
   expects to find WAL in the target cluster's <filename>pg_wal</filename> directory
   reaching all the way back to the point of divergence. The point of divergence
   can be found either on the target timeline, the source timeline, or their common
   ancestor. In the typical failover scenario where the target cluster was
   shut down soon after the divergence, this is not a problem, but if the
   target cluster ran for a long time after the divergence, the old WAL
   files might no longer be present. In that case, they can be manually
   copied from the WAL archive to the <filename>pg_wal</filename> directory.
   The use of <application>pg_rewind</application> is not limited to
   failover, e.g., a standby server can be promoted, run some write
   transactions, and then rewound to become a standby again.
  </para>

  <para>
   When the target server is started for the first time after running
   <application>pg_rewind</application>, it will go into recovery mode and replay all
   WAL generated in the source server after the point of divergence.
   If some of the WAL was no longer available in the source server when
   <application>pg_rewind</application> was run, and therefore could not be copied by the
   <application>pg_rewind</application> session, it must be made available when the
   target server is started. This can be done by creating a
   <filename>recovery.conf</filename> file in the target data directory with a
   suitable <varname>restore_command</varname>.
  </para>

  <para>
   <application>pg_rewind</application> requires that the target server either has
   the <xref linkend="guc-wal-log-hints"/> option enabled
   in <filename>postgresql.conf</filename> or data checksums enabled when
   the cluster was initialized with <application>initdb</application>.  Neither of these
   are currently on by default.  <xref linkend="guc-full-page-writes"/>
   must also be set to <literal>on</literal>, but is enabled by default.
  </para>

  <warning>
   <para>
    If <application>pg_rewind</application> fails while processing, then
    the data folder of the target is likely not in a state that can be
    recovered.  In such a case, taking a new fresh backup is recommended.
   </para>

   <para>
    <application>pg_rewind</application> will fail immediately if it finds
    files it cannot write directly to.  This can happen for example when
    the source and the target server use the same file mapping for read-only
    SSL keys and certificates.  If such files are present on the target server
    it is recommended to remove them before running
    <application>pg_rewind</application>.  After doing the rewind, some of
    those files may have been copied from the source, in which case it may
    be necessary to remove the data copied and restore back the set of links
    used before the rewind.
   </para>
  </warning>
 </refsect1>

 <refsect1>
  <title>Options</title>

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

    <variablelist>
     <varlistentry>
      <term><option>-D <replaceable class="parameter">directory</replaceable></option></term>
      <term><option>--target-pgdata=<replaceable class="parameter">directory</replaceable></option></term>
      <listitem>
       <para>
        This option specifies the target data directory that is synchronized
        with the source. The target server must be shut down cleanly before
        running <application>pg_rewind</application>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>--source-pgdata=<replaceable class="parameter">directory</replaceable></option></term>
      <listitem>
       <para>
        Specifies the file system path to the data directory of the source
        server to synchronize the target with. This option requires the
        source server to be cleanly shut down.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>--source-server=<replaceable class="parameter">connstr</replaceable></option></term>
      <listitem>
       <para>
        Specifies a libpq connection string to connect to the source
        <productname>PostgreSQL</productname> server to synchronize the target
        with. The connection must be a normal (non-replication) connection
        with a role having sufficient permissions to execute the functions
        used by <application>pg_rewind</application> on the source server
        (see Notes section for details) or a superuser role.  This option
        requires the source server to be running and not in recovery mode.
       </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</option></term>
      <term><option>--progress</option></term>
      <listitem>
       <para>
        Enables progress reporting. Turning this on will deliver an approximate
        progress report while copying data from the source cluster.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>--debug</option></term>
      <listitem>
       <para>
        Print verbose debugging output that is mostly useful for developers
        debugging <application>pg_rewind</application>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><option>-V</option></term>
      <term><option>--version</option></term>
      <listitem><para>Display version information, then exit.</para></listitem>
     </varlistentry>

     <varlistentry>
      <term><option>-?</option></term>
      <term><option>--help</option></term>
      <listitem><para>Show help, then exit.</para></listitem>
     </varlistentry>

    </variablelist>
   </para>
 </refsect1>

 <refsect1>
  <title>Environment</title>

  <para>
   When <option>--source-server</option> option is used,
   <application>pg_rewind</application> also uses the environment variables
   supported by <application>libpq</application> (see <xref linkend="libpq-envars"/>).
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   When executing <application>pg_rewind</application> using an online
   cluster as source, a role having sufficient permissions to execute the
   functions used by <application>pg_rewind</application> on the source
   cluster can be used instead of a superuser.  Here is how to create such
   a role, named <literal>rewind_user</literal> here:
<programlisting>
CREATE USER rewind_user LOGIN;
GRANT EXECUTE ON function pg_catalog.pg_ls_dir(text, boolean, boolean) TO rewind_user;
GRANT EXECUTE ON function pg_catalog.pg_stat_file(text, boolean) TO rewind_user;
GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text) TO rewind_user;
GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text, bigint, bigint, boolean) TO rewind_user;
</programlisting>  
  </para>

  <para>
   When executing <application>pg_rewind</application> using an online
   cluster as source which has been recently promoted, it is necessary
   to execute a <command>CHECKPOINT</command> after promotion so as its
   control file reflects up-to-date timeline information, which is used by
   <application>pg_rewind</application> to check if the target cluster
   can be rewound using the designated source cluster.
  </para>

  <refsect2>
   <title>How it works</title>

   <para>
    The basic idea is to copy all file system-level changes from the source
    cluster to the target cluster:
   </para>

   <procedure>
    <step>
     <para>
      Scan the WAL log of the target cluster, starting from the last
      checkpoint before the point where the source cluster's timeline
      history forked off from the target cluster. For each WAL record,
      record each data block that was touched. This yields a list of all
      the data blocks that were changed in the target cluster, after the
      source cluster forked off.
     </para>
    </step>
    <step>
     <para>
      Copy all those changed blocks from the source cluster to
      the target cluster, either using direct file system access
      (<option>--source-pgdata</option>) or SQL (<option>--source-server</option>).
     </para>
    </step>
    <step>
     <para>
      Copy all other files such as <filename>pg_xact</filename> and
      configuration files from the source cluster to the target cluster
      (everything except the relation files). Similarly to base backups,
      the contents of the directories <filename>pg_dynshmem/</filename>,
      <filename>pg_notify/</filename>, <filename>pg_replslot/</filename>,
      <filename>pg_serial/</filename>, <filename>pg_snapshots/</filename>,
      <filename>pg_stat_tmp/</filename>, and
      <filename>pg_subtrans/</filename> are omitted from the data copied
      from the source cluster. Any file or directory beginning with
      <filename>pgsql_tmp</filename> is omitted, as well as are
      <filename>backup_label</filename>,
      <filename>tablespace_map</filename>,
      <filename>pg_internal.init</filename>,
      <filename>postmaster.opts</filename> and
      <filename>postmaster.pid</filename>.
     </para>
    </step>
    <step>
     <para>
      Apply the WAL from the source cluster, starting from the checkpoint
      created at failover. (Strictly speaking, <application>pg_rewind</application>
      doesn't apply the WAL, it just creates a backup label file that
      makes <productname>PostgreSQL</productname> start by replaying all WAL from
      that checkpoint forward.)
     </para>
    </step>
   </procedure>
  </refsect2>
 </refsect1>

</refentry>
