I've now setup a warm-standby machine by using wal archiving. The 
restore_command on the
warm-standby machine loops until the wal requested by postgres appears, instead 
returning 1. Additionally, restore_command check for two special flag-files 
and "take_online". If "take_online" exists, then it exists with code 1 in case 
of a
non-existant wal - this allows me to take the slave online if the master fails.

This methods seems to work, but it is neither particularly fool-proof nor
administrator friendly. It's not possible e.g. to reboot the slave without 
abortint the recovery, and therefor processing all wals generated since the last
backup all over again.

Monitoring this system is hard too, since there is no easy way to detect errors
while restoring a particular wal.

I think that all those problems could be solved if postgres provided a 
standalone application
that could restore one wal into a specified data-dir. It should be possible to 
call this
application repeatedly to restore wals as they are received from the master. Since 
would be call seperately for every wal, I'd be easy to detect errors recovering 
a specific wal.

Do you think this idea is feaseable? How hard would it be to turn the current 
into a standalone executable (That of course needs to be called when postgres 
is _not_ running.)

greetings, Florian Pflug

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