<!-- $PostgreSQL: pgsql/doc/src/sgml/pgstandby.sgml,v 2.9 2009/05/14 21:59:22 tgl Exp $ -->

<sect1 id="pgstandby">
 <title>pg_standby</title>

 <indexterm zone="pgstandby">
  <primary>pg_standby</primary>
 </indexterm>

 <para>
  <application>pg_standby</> supports creation of a <quote>warm standby</>
  database server.  It is designed to be a production-ready program, as well
  as a customizable template should you require specific modifications.
 </para>

 <para>
  <application>pg_standby</> is designed to be a waiting
  <literal>restore_command</literal>, which is needed to turn a standard
  archive recovery into a warm standby operation.  Other
  configuration is required as well, all of which is described in the main
  server manual (see <xref linkend="warm-standby">).
 </para>

 <para>
  <application>pg_standby</application> features include:
 </para>
 <itemizedlist>
  <listitem>
   <para>
    Supports copy or link for restoring WAL files
   </para>
  </listitem>
  <listitem>
   <para>
    Written in C, so very portable and easy to install
   </para>
  </listitem>
  <listitem>
   <para>
    Easy-to-modify source code, with specifically designated
    sections to modify for your own needs
   </para>
  </listitem>
  <listitem>
   <para>
    Already tested on Linux and Windows
   </para>
  </listitem>
 </itemizedlist>

 <sect2>
  <title>Usage</title>

  <para>
   To configure a standby
   server to use <application>pg_standby</>, put this into its
   <filename>recovery.conf</filename> configuration file:
  </para>
  <programlisting>
restore_command = 'pg_standby <replaceable>archiveDir</> %f %p %r'
  </programlisting>
  <para>
   where <replaceable>archiveDir</> is the directory from which WAL segment
   files should be restored.
  </para>
  <para>
   The full syntax of <application>pg_standby</>'s command line is
  </para>
  <synopsis>
pg_standby <optional> <replaceable>option</> ... </optional> <replaceable>archivelocation</> <replaceable>nextwalfile</> <replaceable>xlogfilepath</> <optional> <replaceable>restartwalfile</> </optional>
  </synopsis>
  <para>
   When used within <literal>restore_command</literal>, the <literal>%f</> and
   <literal>%p</> macros should be specified for <replaceable>nextwalfile</>
   and <replaceable>xlogfilepath</> respectively, to provide the actual file
   and path required for the restore.
  </para>
  <para>
   If <replaceable>restartwalfile</> is specified, normally by using the
   <literal>%r</literal> macro, then all WAL files logically preceding this
   file will be removed from <replaceable>archivelocation</>. This minimizes
   the number of files that need to be retained, while preserving
   crash-restart capability.  Use of this parameter is appropriate if the
   <replaceable>archivelocation</> is a transient staging area for this
   particular standby server, but <emphasis>not</> when the
   <replaceable>archivelocation</> is intended as a long-term WAL archive area.
  </para>
  <para>
   <application>pg_standby</application> assumes that
   <replaceable>archivelocation</> is a directory readable by the
   server-owning user.  If <replaceable>restartwalfile</> (or <literal>-k</>)
   is specified,
   the <replaceable>archivelocation</> directory must be writable too.
  </para>
  <para>
   There are two ways to fail over to a <quote>warm standby</> database server
   when the master server fails:

   <variablelist>
    <varlistentry>
     <term>Smart Failover</term>
     <listitem>
      <para>
       In smart failover, the server is brought up after applying all WAL
       files available in the archive. This results in zero data loss, even if
       the standby server has fallen behind, but if there is a lot of
       unapplied WAL it can be a long time before the standby server becomes
       ready. To trigger a smart failover, create a trigger file containing
       the word <literal>smart</>, or just create it and leave it empty.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>Fast Failover</term>
     <listitem>
      <para>
       In fast failover, the server is brought up immediately. Any WAL files
       in the archive that have not yet been applied will be ignored, and
       all transactions in those files are lost. To trigger a fast failover,
       create a trigger file and write the word <literal>fast</> into it.
       <application>pg_standby</> can also be configured to execute a fast
       failover automatically if no new WAL file appears within a defined
       interval.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <table>
   <title><application>pg_standby</> options</title>
   <tgroup cols="3">
     <thead>
     <row>
      <entry>Option</entry>
      <entry>Default</entry>
      <entry>Description</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry><literal>-c</></entry>
      <entry>yes</entry>
      <entry>
       Use <literal>cp</> or <literal>copy</> command to restore WAL files
       from archive.
      </entry>
     </row>
     <row>
      <entry><literal>-d</></entry>
      <entry>no</entry>
      <entry>Print lots of debug logging output on <filename>stderr</>.</entry>
     </row>
     <row>
      <entry><literal>-k</> <replaceable>numfiles</></entry>
      <entry>0</entry>
      <entry>
       Remove files from <replaceable>archivelocation</replaceable> so that
       no more than this many WAL files before the current one are kept in the
       archive.  Zero (the default) means not to remove any files from
       <replaceable>archivelocation</replaceable>.
       This parameter will be silently ignored if
       <replaceable>restartwalfile</replaceable> is specified, since that
       specification method is more accurate in determining the correct
       archive cut-off point.
       Use of this parameter is <emphasis>deprecated</> as of
       <productname>PostgreSQL</> 8.3; it is safer and more efficient to
       specify a <replaceable>restartwalfile</replaceable> parameter.  A too
       small setting could result in removal of files that are still needed
       for a restart of the standby server, while a too large setting wastes
       archive space.
      </entry>
     </row>
     <row>
      <entry><literal>-l</></entry>
      <entry>no</entry>
      <entry>
       Use <literal>ln</> command to restore WAL files from archive.
       Link is more efficient than copy, but the default is copy since link
       will not work in all scenarios.
       On Windows, this option uses the <literal>mklink</> command
       to provide a file-to-file symbolic link. <literal>-l</> will
       not work on versions of Windows prior to Vista.
      </entry>
     </row>
     <row>
      <entry><literal>-r</> <replaceable>maxretries</></entry>
      <entry>3</entry>
      <entry>
        Set the maximum number of times to retry the copy or link command if it
        fails. After each failure, we wait for <replaceable>sleeptime</> *
        <replaceable>num_retries</>
        so that the wait time increases progressively.  So by default,
        we will wait 5 secs, 10 secs, then 15 secs before reporting
        the failure back to the standby server. This will be
        interpreted as end of recovery and the standby will come
        up fully as a result.
      </entry>
     </row>
     <row>
      <entry><literal>-s</> <replaceable>sleeptime</></entry>
      <entry>5</entry>
      <entry>
       Set the number of seconds (up to 60) to sleep between tests to see
       if the WAL file to be restored is available in the archive yet.
       The default setting is not necessarily recommended;
       consult <xref linkend="warm-standby"> for discussion.
      </entry>
     </row>
     <row>
      <entry><literal>-t</> <replaceable>triggerfile</></entry>
      <entry>none</entry>
      <entry>
       Specify a trigger file whose presence should cause failover.
       It is recommended that you use a structured filename to
       avoid confusion as to which server is being triggered
       when multiple servers exist on the same system; for example
       <filename>/tmp/pgsql.trigger.5432</>.
      </entry>
     </row>
     <row>
      <entry><literal>-w</> <replaceable>maxwaittime</></entry>
      <entry>0</entry>
      <entry>
       Set the maximum number of seconds to wait for the next WAL file,
       after which a fast failover will be performed.
       A setting of zero (the default) means wait forever.
       The default setting is not necessarily recommended;
       consult <xref linkend="warm-standby"> for discussion.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect2>

 <sect2>
  <title>Examples</title>

  <para>On Linux or Unix systems, you might use:</para>

  <programlisting>
archive_command = 'cp %p .../archive/%f'

restore_command = 'pg_standby -l -d -s 2 -t /tmp/pgsql.trigger.5442 .../archive %f %p %r 2>>standby.log'

recovery_end_command = 'rm -f /tmp/pgsql.trigger.5442'
  </programlisting>
  <para>
   where the archive directory is physically located on the standby server,
   so that the <literal>archive_command</> is accessing it across NFS,
   but the files are local to the standby (enabling use of <literal>ln</>).
   This will:
  </para>
  <itemizedlist>
   <listitem>
    <para>
     use the <literal>ln</> command to restore WAL files from archive
    </para>
   </listitem>
   <listitem>
    <para>
     produce debugging output in <filename>standby.log</>
    </para>
   </listitem>
   <listitem>
    <para>
     sleep for 2 seconds between checks for next WAL file availability
    </para>
   </listitem>
   <listitem>
    <para>
     stop waiting only when a trigger file called
     <filename>/tmp/pgsql.trigger.5442</> appears,
     and perform failover according to its content
    </para>
   </listitem>
   <listitem>
    <para>
     remove the trigger file when recovery ends
    </para>
   </listitem>
   <listitem>
    <para>
     remove no-longer-needed files from the archive directory
    </para>
   </listitem>
  </itemizedlist>

  <para>On Windows, you might use:</para>

  <programlisting>
archive_command = 'copy %p ...\\archive\\%f'

restore_command = 'pg_standby -d -s 5 -t C:\pgsql.trigger.5442 ...\archive %f %p %r 2>>standby.log'

recovery_end_command = 'del C:\pgsql.trigger.5442'
  </programlisting>
  <para>
   Note that backslashes need to be doubled in the
   <literal>archive_command</>, but <emphasis>not</emphasis> in the
   <literal>restore_command</> or <literal>recovery_end_command</>.
   This will:
  </para>
  <itemizedlist>
   <listitem>
    <para>
     use the <literal>copy</> command to restore WAL files from archive
    </para>
   </listitem>
   <listitem>
    <para>
     produce debugging output in <filename>standby.log</>
    </para>
   </listitem>
   <listitem>
    <para>
     sleep for 5 seconds between checks for next WAL file availability
    </para>
   </listitem>
   <listitem>
    <para>
     stop waiting only when a trigger file called
     <filename>C:\pgsql.trigger.5442</> appears,
     and perform failover according to its content
    </para>
   </listitem>
   <listitem>
    <para>
     remove the trigger file when recovery ends
    </para>
   </listitem>
   <listitem>
    <para>
     remove no-longer-needed files from the archive directory
    </para>
   </listitem>
  </itemizedlist>

  <para>
   The <literal>copy</> command on Windows sets the final file size
   before the file is completely copied, which would ordinarly confuse
   <application>pg_standby</application>.  Therefore
   <application>pg_standby</application> waits <literal>sleeptime</>
   seconds once it sees the proper file size.  GNUWin32's <literal>cp</>
   sets the file size only after the file copy is complete.
  </para>

  <para>
   Since the Windows example uses <literal>copy</> at both ends, either
   or both servers might be accessing the archive directory across the
   network.
  </para>

 </sect2>

 <sect2>
  <title>Supported server versions</title>

  <para>
   <application>pg_standby</application> is designed to work with
   <productname>PostgreSQL</> 8.2 and later.
  </para>
  <para>
   <productname>PostgreSQL</> 8.3 provides the <literal>%r</literal> macro,
   which is designed to let <application>pg_standby</application> know the
   last file it needs to keep.  With <productname>PostgreSQL</> 8.2, the
   <literal>-k</literal> option must be used if archive cleanup is
   required.  This option remains available in 8.3, but its use is deprecated.
  </para>
  <para>
   <productname>PostgreSQL</> 8.4 provides the
   <literal>recovery_end_command</literal> option.  Without this option
   a leftover trigger file can be hazardous.
  </para>
 </sect2>

 <sect2>
  <title>Author</title>

  <para>
   Simon Riggs <email>simon@2ndquadrant.com</email>
  </para>
 </sect2>

</sect1>
