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Re: Incomplete docs for restore_command for hot standby

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Markus Bertheau <mbertheau(dot)pg(at)googlemail(dot)com>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: Incomplete docs for restore_command for hot standby
Date: 2008-02-21 21:18:32
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-bugspgsql-patches
On Thu, 2008-02-21 at 08:01 +0600, Markus Bertheau wrote:
> (I sent this to -docs already, but it didn't get through for some reason.)
> >From the current 8.3 docs:
> Section states about restore_command:
> "The command will be asked for file names that are not present in the
> archive; it must return nonzero when so asked."
> Section 24.4.1 further states:
> "The magic that makes the two loosely coupled servers work together is
> simply a restore_command used on the standby that waits for the next
> WAL file to become available from the primary."
> It is not clear from the first paragraph, whether the non-existing
> file that restore_command is being asked for is a not-yet-generated
> WAL file or something different. If it was a not-yet-generated WAL
> file, restore_command for replication would have to wait for it to
> appear. If it was something different, restore_command for replication
> would have to return an error right away. (Because else it would hang
> indefinitely, waiting for a file that is not going to appear). Yet I
> couldn't find hints in the documentation as to how these two cases can
> be detected by restore_command, i.e. how restore_command should tell a
> request for a WAL file from a request for a non-WAL file.

The two sentences aren't mutually exclusive, especially when you
consider they are discussing two different use cases. Why not read up on
pg_standby anyway?

> Practice (
> shows that this is a problem, and people use unproved heuristics
> ('history' substring in the requested file name).

Old email written during beta. Read at your own peril.

> Additionally, 24.3.3 contains slightly misleading information:
> "It is important that the command return nonzero exit status on
> failure. The command will be asked for log files that are not present
> in the archive; it must return nonzero when so asked. This is not an
> error condition."
> This suggests that all non-existing files that restore_command will be
> asked for are log files. One could therefore reasonably assume that
> restore_command for replication should wait on all non-existing files.
> later corrects this by stating that not only log files may be
> requested, but nevertheless.

If you have some suggested changes, I'd be happy to hear them.

Probably additions are better than just changes though.

  Simon Riggs

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