Feedback-ID: i0fe9450f:Fastmail
Date: Thu, 26 Sep 2024 17:18:49 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Alexander Korotkov <akorotkov@postgresql.org>
Cc: pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Implement pg_wal_replay_wait() stored procedure
Message-ID: <ZvUY6bfTwB0GsyzP@paquier.xyz>
References: <E1sZwuz-002NPQ-Lc@gemulon.postgresql.org>
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Hi Alexander,

On Fri, Aug 02, 2024 at 06:22:21PM +0000, Alexander Korotkov wrote:
> Implement pg_wal_replay_wait() stored procedure
>=20
> pg_wal_replay_wait() is to be used on standby and specifies waiting for
> the specific WAL location to be replayed.  This option is useful when
> the user makes some data changes on primary and needs a guarantee to see
> these changes are on standby.
>=20
> The queue of waiters is stored in the shared memory as an LSN-ordered pai=
ring
> heap, where the waiter with the nearest LSN stays on the top.  During
> the replay of WAL, waiters whose LSNs have already been replayed are dele=
ted
> from the shared memory pairing heap and woken up by setting their latches.
>=20
> pg_wal_replay_wait() needs to wait without any snapshot held.  Otherwise,
> the snapshot could prevent the replay of WAL records, implying a kind of
> self-deadlock.  This is why it is only possible to implement
> pg_wal_replay_wait() as a procedure working without an active snapshot,
> not a function.
>=20
> Catversion is bumped.

I've spotted that this patch uses an OID that should not be used
during the development cycle:
+{ oid =3D> '111',
+  descr =3D> 'wait for the target LSN to be replayed on standby with an op=
tional timeout',=20

Please use something in the 8000-9999 range, as required by
98eab30b93d5.

Thanks,
--
Michael

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