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Postgres FD Implementation
Commit 5c6d1842 authored by Thomas Munro's avatar Thomas Munro
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Remove documentation of waiting restore_command. 

Following the removal of pg_standby, also remove the documentation
section that describes how to write your own "waiting restore_command"
along the same lines.

Discussion: https://postgr.es/m/20201029024412.GP5380%40telsasoft.comReviewed-by: default avatarMichael Paquier <michael@paquier.xyz>
parent 514b411a
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doc/src/sgml/high-availability.sgml
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......@@ -1486,143 +1486,6 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
</para>
</sect1>
<sect1 id="log-shipping-alternative">
<title>Alternative Method for Log Shipping</title>
<para>
An alternative to the built-in standby mode described in the previous
sections is to use a <varname>restore_command</varname> that polls the archive location.
This was the only option available in versions 8.4 and below.
</para>
<para>
Note that in this mode, the server will apply WAL one file at a
time, so if you use the standby server for queries (see Hot Standby),
there is a delay between an action in the primary and when the
action becomes visible in the standby, corresponding to the time it takes
to fill up the WAL file. <varname>archive_timeout</varname> can be used to make that delay
shorter. Also note that you can't combine streaming replication with
this method.
</para>
<para>
The operations that occur on both primary and standby servers are
normal continuous archiving and recovery tasks. The only point of
contact between the two database servers is the archive of WAL files
that both share: primary writing to the archive, standby reading from
the archive. Care must be taken to ensure that WAL archives from separate
primary servers do not become mixed together or confused. The archive
need not be large if it is only required for standby operation.
</para>
<para>
The magic that makes the two loosely coupled servers work together is
simply a <varname>restore_command</varname> used on the standby that,
when asked for the next WAL file, waits for it to become available from
the primary. Normal recovery
processing would request a file from the WAL archive, reporting failure
if the file was unavailable. For standby processing it is normal for
the next WAL file to be unavailable, so the standby must wait for
it to appear. For files ending in
<literal>.history</literal> there is no need to wait, and a non-zero return
code must be returned. A waiting <varname>restore_command</varname> can be
written as a custom script that loops after polling for the existence of
the next WAL file. There must also be some way to trigger failover, which
should interrupt the <varname>restore_command</varname>, break the loop and
return a file-not-found error to the standby server. This ends recovery
and the standby will then come up as a normal server.
</para>
<para>
Pseudocode for a suitable <varname>restore_command</varname> is:
<programlisting>
triggered = false;
while (!NextWALFileReady() &amp;&amp; !triggered)
{
sleep(100000L); /* wait for ~0.1 sec */
if (CheckForExternalTrigger())
triggered = true;
}
if (!triggered)
CopyWALFileForRecovery();
</programlisting>
</para>
<para>
The method for triggering failover is an important part of planning
and design. One potential option is the <varname>restore_command</varname>
command. It is executed once for each WAL file, but the process
running the <varname>restore_command</varname> is created and dies for
each file, so there is no daemon or server process, and
signals or a signal handler cannot be used. Therefore, the
<varname>restore_command</varname> is not suitable to trigger failover.
It is possible to use a simple timeout facility, especially if
used in conjunction with a known <varname>archive_timeout</varname>
setting on the primary. However, this is somewhat error prone
since a network problem or busy primary server might be sufficient
to initiate failover. A notification mechanism such as the explicit
creation of a trigger file is ideal, if this can be arranged.
</para>
<sect2 id="warm-standby-config">
<title>Implementation</title>
<para>
The short procedure for configuring a standby server using this alternative
method is as follows. For
full details of each step, refer to previous sections as noted.
<orderedlist>
<listitem>
<para>
Set up primary and standby systems as nearly identical as
possible, including two identical copies of
<productname>PostgreSQL</productname> at the same release level.
</para>
</listitem>
<listitem>
<para>
Set up continuous archiving from the primary to a WAL archive
directory on the standby server. Ensure that
<xref linkend="guc-archive-mode"/>,
<xref linkend="guc-archive-command"/> and
<xref linkend="guc-archive-timeout"/>
are set appropriately on the primary
(see <xref linkend="backup-archiving-wal"/>).
</para>
</listitem>
<listitem>
<para>
Make a base backup of the primary server (see <xref
linkend="backup-base-backup"/>), and load this data onto the standby.
</para>
</listitem>
<listitem>
<para>
Begin recovery on the standby server from the local WAL
archive, using <varname>restore_command</varname> that waits
as described previously (see <xref linkend="backup-pitr-recovery"/>).
</para>
</listitem>
</orderedlist>
</para>
<para>
Recovery treats the WAL archive as read-only, so once a WAL file has
been copied to the standby system it can be copied to tape at the same
time as it is being read by the standby database server.
Thus, running a standby server for high availability can be performed at
the same time as files are stored for longer term disaster recovery
purposes.
</para>
<para>
For testing purposes, it is possible to run both primary and standby
servers on the same system. This does not provide any worthwhile
improvement in server robustness, nor would it be described as HA.
</para>
</sect2>
</sect1>
<sect1 id="hot-standby">
<title>Hot Standby</title>
......
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