Continuous Archiving for Multiple Warm Standby Servers - Mailing list pgsql-general

I'm attempting to design a postgres system whereby an authoritative primary server simultaneously feeds continuous archives to a number of warm standby servers that live both on the local network and on remote networks.

The sticking point in my current thinking about such a system is what to do in the event that any of an array of possible nodes becomes unreachable. I would expect a custom archive_command to have the intelligence about network reachability and to report a nonzero status if it was unable to submit an archive to any particular node.

The way I understand it, postgres would then resubmit the file that caused the nonzero status, which, if connectivity has been restored, is no problem for the node that caused the nonzero status in the first place. But then the issue becomes what to do with the nodes that were fine when the nonzero status.

From the docs <http://www.postgresql.org/docs/8.2/static/continuous-archiving.html#BACKUP-ARCHIVING-WAL>:

"It is advisable to test your proposed archive command to ensure that it indeed does not overwrite an existing file, and that it returns nonzero status in this case. We have found that cp -i does this correctly on some platforms but not others. If the chosen command does not itself handle this case correctly, you should add a command to test for pre-existence of the archive file."

What is the advised remedy for this scenario in general? And then what is it if nonzero status is returned by archive_command because the file already exists on nodes that stayed up after a scenario where nonzero status is returned because one or more nodes became unreachable?

A follow-on question is: Does it become the responsibility of archive_command in a scenario like this to track which files have been archived on which nodes? Is there any introspective way for a standby server to know that a file has been archived by primary? If not, is it safe to reply on using sequential numbering of WAL files for implicit introspection? I don't see any functions that provide introspection of this nature. I ask because it seems like network-to-network failures are a common enough occurrence that some mechanism for archive verification is a must-have. I'm just trying to determine how much of that functionality I'll have to build myself...