From: | Heikki Linnakangas <hlinnaka(at)iki(dot)fi> |
---|---|
To: | Alexander Korotkov <aekorotkov(at)gmail(dot)com>, Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> |
Cc: | Peter Eisentraut <peter(at)eisentraut(dot)org>, Michael Paquier <michael(at)paquier(dot)xyz>, pgsql-committers(at)lists(dot)postgresql(dot)org |
Subject: | Re: pgsql: Implement pg_wal_replay_wait() stored procedure |
Date: | 2024-10-25 06:06:10 |
Message-ID: | ab0eddce-06d4-4db2-87ce-46fa2427806c@iki.fi |
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Lists: | pgsql-committers pgsql-hackers |
If you call this procedure on a stand-alone server, you get:
postgres=# call pg_wal_replay_wait('1234/0');
ERROR: recovery is not in progress
DETAIL: Recovery ended before replaying target LSN 1234/0; last replay
LSN 0/0.
The DETAIL seems a bit misleading. Recovery never ended, because it
never started in the first place. Last replay LSN is indeed 0/0, but
that's not helpful.
If a standby server has been promoted and you pass an LSN that's earlier
than the last replay LSN, it returns successfully. That makes sense I
guess; if you connect to a standby and wait for it to replay a commit
that you made in the primary, and the standby gets promoted, that seems
correct. But it's a little inconsistent: If the standby crashes
immediately after promotion, and you call pg_wal_replay_wait() after
recovery, it returns success. However, if you shut down the promoted
server and restart it, then last replay LSN is 0/0, and the call will
fail because no recovery happened.
What is the use case for the 'no_error' argument? Why would you ever
want to pass no_error=true ? The separate pg_wal_replay_wait_status()
function feels weird to me. Also it surely shouldn't be marked IMMUTABLE
nor parallel safe.
This would benefit from more documentation, explaining how you would use
this in practice. I believe the use case is that you want "read your
writes" consistency between a primary and a standby. You commit a
transaction in the primary, and you then want to run read-only queries
in a standby, and you want to make sure that you see your own commit,
but you're ok with slightly delayed results otherwise. It would be good
to add a chapter somewhere in the docs to show how to do that in
practice with these functions.
--
Heikki Linnakangas
Neon (https://neon.tech)
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