Horton

Re: pgsql: Implement pg_wal_replay_wait() stored procedure

Michael Paquier <michael@paquier.xyz>

View thread
From: Michael Paquier <michael@paquier.xyz>
To: Alexander Korotkov <akorotkov@postgresql.org>
Cc: pgsql-committers@lists.postgresql.org
Date: 2024-09-26T08:18:49Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Revert pg_wal_replay_wait() stored procedure

  2. Add 'no_error' argument to pg_wal_replay_wait()

  3. Refactor WaitForLSNReplay() to return the result of waiting

  4. Make WaitForLSNReplay() issue FATAL on postmaster death

  5. Move LSN waiting declarations and definitions to better place

  6. Update oid for pg_wal_replay_wait() procedure

  7. Move pg_wal_replay_wait() to xlogfuncs.c

  8. Implement pg_wal_replay_wait() stored procedure

Hi Alexander,

On Fri, Aug 02, 2024 at 06:22:21PM +0000, Alexander Korotkov wrote:
> Implement pg_wal_replay_wait() stored procedure
> 
> 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.
> 
> The queue of waiters is stored in the shared memory as an LSN-ordered pairing
> 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 deleted
> from the shared memory pairing heap and woken up by setting their latches.
> 
> 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.
> 
> Catversion is bumped.

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

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

Thanks,
--
Michael