An alternative to the built-in standby mode described in the previous sections is to use a restore_command that polls the archive location. This was the only option available in versions 8.4 and below. See the pg_standby module for a reference implementation of this.

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 master and when the action becomes visible in the standby, corresponding to the time it takes to fill up the WAL file. archive_timeout can be used to make that delay shorter. Also note that you can't combine streaming replication with this method.

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.

The magic that makes the two loosely coupled servers work together is simply a restore_command 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 .history there is no need to wait, and a non-zero return code must be returned. A waiting restore_command 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 restore_command, 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.

Pseudocode for a suitable restore_command is:

triggered = false;
while (!NextWALFileReady() && !triggered)
{
    sleep(100000L);         /* wait for ~0.1 sec */
    if (CheckForExternalTrigger())
        triggered = true;
}
if (!triggered)
        CopyWALFileForRecovery();

A working example of a waiting restore_command is provided in the pg_standby module. It should be used as a reference on how to correctly implement the logic described above. It can also be extended as needed to support specific configurations and environments.

The method for triggering failover is an important part of planning and design. One potential option is the restore_command command. It is executed once for each WAL file, but the process running the restore_command 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 restore_command is not suitable to trigger failover. It is possible to use a simple timeout facility, especially if used in conjunction with a known archive_timeout 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.