<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>