Taking into account syncrep position in flush_lsn reported by apply worker
Hackorum builds and tests every patch posted to the lists, not only commitfest submissions. This is Hackorum's own CI rather than the PostgreSQL project's, and it is still under testing - please report anything that looks wrong.
You can run a PostgreSQL built from this patch straight from Docker, with no checkout and no build:
docker run --rm -p 5432:5432 ghcr.io/hackorum-dev/postgres-patch:t50100psql -h localhost -U postgresBuilt from patchset v2 (message #2), August 17, 2026 at 09:49 AM.
Hey. Currently synchronous_commit is by default disabled for logical
apply worker on the
ground that reported flush_lsn includes only locally flushed data so slot
(publisher) preserves everything higher than this, and so in case of
subscriber
restart no data is lost. However, imagine that subscriber is made highly
available by standby to which synchronous replication is enabled. Then
reported
flush_lsn is ignorant of this synchronous replication progress, and in
case of
failover data loss may occur if subscriber managed to ack flush_lsn ahead of
syncrep. Moreover, it is almost silent due to this
else if (start_lsn < slot->data.confirmed_flush)
{
/*
* It might seem like we should error out in this case, but it's
* pretty common for a client to acknowledge a LSN it doesn't
have to
* do anything for, and thus didn't store persistently, because the
* xlog records didn't result in anything relevant for logical
* decoding. Clients have to be able to do that to support
synchronous
* replication.
*
* Starting at a different LSN than requested might not catch
certain
* kinds of client errors; so the client may wish to check that
* confirmed_flush_lsn matches its expectations.
*/
elog(LOG, "%X/%X has been already streamed, forwarding to %X/%X",
LSN_FORMAT_ARGS(start_lsn),
LSN_FORMAT_ARGS(slot->data.confirmed_flush));
start_lsn = slot->data.confirmed_flush;
}
in logical.c
Attached draft patch fixes this by taking into account syncrep progress in
worker reporting.
-- cheers, arseny
Attachments:
apply_worker_report_syncrep_pos.patchtext/x-patch; charset=UTF-8; name=apply_worker_report_syncrep_pos.patchDownload+11-0
Sorry for the poor formatting of the message above, this should be better:
Hey. Currently synchronous_commit is disabled for logical apply worker
on the ground that reported flush_lsn includes only locally flushed data
so slot (publisher) preserves everything higher than this, and so in
case of subscriber restart no data is lost. However, imagine that
subscriber is made highly available by standby to which synchronous
replication is enabled. Then reported flush_lsn is ignorant of this
synchronous replication progress, and in case of failover data loss may
occur if subscriber managed to ack flush_lsn ahead of syncrep. Moreover,
it is silent due to this
else if (start_lsn < slot->data.confirmed_flush)
{
/*
* It might seem like we should error out in this case, but it's
* pretty common for a client to acknowledge a LSN it doesn't have to
* do anything for, and thus didn't store persistently, because the
* xlog records didn't result in anything relevant for logical
* decoding. Clients have to be able to do that to support synchronous
* replication.
*
* Starting at a different LSN than requested might not catch certain
* kinds of client errors; so the client may wish to check that
* confirmed_flush_lsn matches its expectations.
*/
elog(LOG, "%X/%X has been already streamed, forwarding to %X/%X",
LSN_FORMAT_ARGS(start_lsn),
LSN_FORMAT_ARGS(slot->data.confirmed_flush));
start_lsn = slot->data.confirmed_flush;
}
in logical.c
Attached draft patch fixes this by taking into account syncrep progress
in worker reporting.
Attachments:
apply_worker_report_syncrep_pos.patchtext/x-patch; charset=UTF-8; name=apply_worker_report_syncrep_pos.patchDownload+11-0
On Mon, Aug 12, 2024 at 3:43 PM Arseny Sher <ars@neon.tech> wrote:
Sorry for the poor formatting of the message above, this should be better:
Hey. Currently synchronous_commit is disabled for logical apply worker
on the ground that reported flush_lsn includes only locally flushed data
so slot (publisher) preserves everything higher than this, and so in
case of subscriber restart no data is lost. However, imagine that
subscriber is made highly available by standby to which synchronous
replication is enabled. Then reported flush_lsn is ignorant of this
synchronous replication progress, and in case of failover data loss may
occur if subscriber managed to ack flush_lsn ahead of syncrep.
Won't the same can be achieved by enabling the synchronous_commit
parameter for a subscription?
--
With Regards,
Amit Kapila.
On 8/13/24 06:35, Amit Kapila wrote:
On Mon, Aug 12, 2024 at 3:43 PM Arseny Sher <ars@neon.tech> wrote:
Sorry for the poor formatting of the message above, this should be better:
Hey. Currently synchronous_commit is disabled for logical apply worker
on the ground that reported flush_lsn includes only locally flushed data
so slot (publisher) preserves everything higher than this, and so in
case of subscriber restart no data is lost. However, imagine that
subscriber is made highly available by standby to which synchronous
replication is enabled. Then reported flush_lsn is ignorant of this
synchronous replication progress, and in case of failover data loss may
occur if subscriber managed to ack flush_lsn ahead of syncrep.Won't the same can be achieved by enabling the synchronous_commit
parameter for a subscription?
Nope, because it would force WAL flush and wait for replication to the
standby in the apply worker, slowing down it. The logic missing
currently is not to wait for the synchronous commit, but still mind its
progress in the flush_lsn reporting.
-- cheers, arseny
On 14/08/2024 16:54, Arseny Sher wrote:
On 8/13/24 06:35, Amit Kapila wrote:
On Mon, Aug 12, 2024 at 3:43 PM Arseny Sher <ars@neon.tech> wrote:
Sorry for the poor formatting of the message above, this should be
better:Hey. Currently synchronous_commit is disabled for logical apply worker
on the ground that reported flush_lsn includes only locally flushed data
so slot (publisher) preserves everything higher than this, and so in
case of subscriber restart no data is lost. However, imagine that
subscriber is made highly available by standby to which synchronous
replication is enabled. Then reported flush_lsn is ignorant of this
synchronous replication progress, and in case of failover data loss may
occur if subscriber managed to ack flush_lsn ahead of syncrep.Won't the same can be achieved by enabling the synchronous_commit
parameter for a subscription?Nope, because it would force WAL flush and wait for replication to the
standby in the apply worker, slowing down it. The logic missing
currently is not to wait for the synchronous commit, but still mind its
progress in the flush_lsn reporting.
I think this patch makes sense. I'm not sure we've actually made any
promises on it, but it feels wrong that the slot's LSN might be advanced
past the LSN that's been has been acknowledged by the replica, if
synchronous replication is configured. I see little downside in making
that promise.
+ /* + * If synchronous replication is configured, take into account its position. + */ + if (SyncRepStandbyNames != NULL && SyncRepStandbyNames[0] != '\0') + { + LWLockAcquire(SyncRepLock, LW_SHARED); + local_flush = Min(local_flush, WalSndCtl->lsn[SYNC_REP_WAIT_FLUSH]); + LWLockRelease(SyncRepLock); + } +
Should probably use the SyncStandbysDefined() macro here. Or check
WalSndCtl->sync_standbys_defined like SyncRepWaitForLSN() does; not sure
which would be more appropriate here.
Should the synchronous_commit setting also affect this?
Please also check if the docs need to be updated, or if a paragraph
should be added somewhere on this behavior.
A TAP test case would be nice. Not sure how complicated it will be, but
if not too complicated, it'd be nice to include it in check-world.
--
Heikki Linnakangas
Neon (https://neon.tech)
On Wed, Aug 21, 2024 at 2:25 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
On 14/08/2024 16:54, Arseny Sher wrote:
On 8/13/24 06:35, Amit Kapila wrote:
On Mon, Aug 12, 2024 at 3:43 PM Arseny Sher <ars@neon.tech> wrote:
Sorry for the poor formatting of the message above, this should be
better:Hey. Currently synchronous_commit is disabled for logical apply worker
on the ground that reported flush_lsn includes only locally flushed data
so slot (publisher) preserves everything higher than this, and so in
case of subscriber restart no data is lost. However, imagine that
subscriber is made highly available by standby to which synchronous
replication is enabled. Then reported flush_lsn is ignorant of this
synchronous replication progress, and in case of failover data loss may
occur if subscriber managed to ack flush_lsn ahead of syncrep.Won't the same can be achieved by enabling the synchronous_commit
parameter for a subscription?Nope, because it would force WAL flush and wait for replication to the
standby in the apply worker, slowing down it. The logic missing
currently is not to wait for the synchronous commit, but still mind its
progress in the flush_lsn reporting.I think this patch makes sense. I'm not sure we've actually made any
promises on it, but it feels wrong that the slot's LSN might be advanced
past the LSN that's been has been acknowledged by the replica, if
synchronous replication is configured. I see little downside in making
that promise.
One possible downside of such a promise could be that the publisher
may slow down for sync replication because it has to wait for all the
configured sync_standbys of subscribers to acknowledge the LSN. I
don't know how many applications can be impacted due to this if we do
it by default but if we feel there won't be any such cases or they
will be in the minority then it is okay to proceed with this.
--
With Regards,
Amit Kapila.
On 21/08/2024 09:25, Amit Kapila wrote:
On Wed, Aug 21, 2024 at 2:25 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
On 14/08/2024 16:54, Arseny Sher wrote:
On 8/13/24 06:35, Amit Kapila wrote:
On Mon, Aug 12, 2024 at 3:43 PM Arseny Sher <ars@neon.tech> wrote:
Sorry for the poor formatting of the message above, this should be
better:Hey. Currently synchronous_commit is disabled for logical apply worker
on the ground that reported flush_lsn includes only locally flushed data
so slot (publisher) preserves everything higher than this, and so in
case of subscriber restart no data is lost. However, imagine that
subscriber is made highly available by standby to which synchronous
replication is enabled. Then reported flush_lsn is ignorant of this
synchronous replication progress, and in case of failover data loss may
occur if subscriber managed to ack flush_lsn ahead of syncrep.Won't the same can be achieved by enabling the synchronous_commit
parameter for a subscription?Nope, because it would force WAL flush and wait for replication to the
standby in the apply worker, slowing down it. The logic missing
currently is not to wait for the synchronous commit, but still mind its
progress in the flush_lsn reporting.I think this patch makes sense. I'm not sure we've actually made any
promises on it, but it feels wrong that the slot's LSN might be advanced
past the LSN that's been has been acknowledged by the replica, if
synchronous replication is configured. I see little downside in making
that promise.One possible downside of such a promise could be that the publisher
may slow down for sync replication because it has to wait for all the
configured sync_standbys of subscribers to acknowledge the LSN. I
don't know how many applications can be impacted due to this if we do
it by default but if we feel there won't be any such cases or they
will be in the minority then it is okay to proceed with this.
It only slows down updating the flush LSN on the publisher, which is
updated quite lazily anyway.
A more serious scenario is if the sync replica crashes or is not
responding at all. In that case, the flush LSN on the publisher cannot
advance, and WAL starts to accumulate. However, if a sync replica is not
responding, that's very painful for the (subscribing) server anyway: all
commits will hang waiting for the replica. Holding back the flush LSN on
the publisher seems like a minor problem compared to that.
It would be good to have some kind of an escape hatch though. If you get
into that situation, is there a way to advance the publisher's flush LSN
even though the synchronous replica has crashed? You can temporarily
turn off synchronous replication on the subscriber. That will release
any COMMITs on the server too. In theory you might not want that, but in
practice stuck COMMITs are so painful that if you are taking manual
action, you probably do want to release them as well.
--
Heikki Linnakangas
Neon (https://neon.tech)
On Wed, Aug 21, 2024 at 12:40 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
On 21/08/2024 09:25, Amit Kapila wrote:
I think this patch makes sense. I'm not sure we've actually made any
promises on it, but it feels wrong that the slot's LSN might be advanced
past the LSN that's been has been acknowledged by the replica, if
synchronous replication is configured. I see little downside in making
that promise.One possible downside of such a promise could be that the publisher
may slow down for sync replication because it has to wait for all the
configured sync_standbys of subscribers to acknowledge the LSN. I
don't know how many applications can be impacted due to this if we do
it by default but if we feel there won't be any such cases or they
will be in the minority then it is okay to proceed with this.It only slows down updating the flush LSN on the publisher, which is
updated quite lazily anyway.
But doesn't that also mean that if the logical subscriber is
configured in synchronous_standby_names, then the publisher's
transactions also need to wait for such an update? We do update it
lazily but as soon as the operation is applied to the subscriber the
transaction on publisher will be released, however, IIUC the same
won't be true after the patch.
A more serious scenario is if the sync replica crashes or is not
responding at all. In that case, the flush LSN on the publisher cannot
advance, and WAL starts to accumulate. However, if a sync replica is not
responding, that's very painful for the (subscribing) server anyway: all
commits will hang waiting for the replica. Holding back the flush LSN on
the publisher seems like a minor problem compared to that.
Yeah, but as per my understanding that also means holding all the
active work/transactions on the publisher which doesn't sound to be a
minor problem.
It would be good to have some kind of an escape hatch though. If you get
into that situation, is there a way to advance the publisher's flush LSN
even though the synchronous replica has crashed? You can temporarily
turn off synchronous replication on the subscriber. That will release
any COMMITs on the server too. In theory you might not want that, but in
practice stuck COMMITs are so painful that if you are taking manual
action, you probably do want to release them as well.
This will work in the scenario you mentioned.
If the above understanding is correct and you agree that it is not a
good idea to hold back transactions on the publisher then we can think
of a new subscription that allows the apply worker to wait for
synchronous replicas.
--
With Regards,
Amit Kapila.