RE: Synchronizing slots from primary to standby - Mailing list pgsql-hackers

On Tuesday, November 28, 2023 11:58 PM Drouvot, Bertrand <bertranddrouvot.pg@gmail.com> wrote:
> 
> Hi,
> 
> On 11/28/23 10:40 AM, shveta malik wrote:
> > On Tue, Nov 28, 2023 at 12:19 PM Drouvot, Bertrand
> > <bertranddrouvot.pg@gmail.com> wrote:
> >>
> >> Hi,
> >>
> >> On 11/28/23 4:13 AM, shveta malik wrote:
> >>> On Mon, Nov 27, 2023 at 4:08 PM Amit Kapila
> <amit.kapila16@gmail.com> wrote:
> >>>>
> >>>> On Mon, Nov 27, 2023 at 2:27 PM Zhijie Hou (Fujitsu)
> >>>> <houzj.fnst@fujitsu.com> wrote:
> >>>>>
> >>>>> Here is the updated version(v39_2) which include all the changes made
> in 0002.
> >>>>> Please use for review, and sorry for the confusion.
> >>>>>
> >>>>
> >>>> --- a/src/backend/replication/logical/launcher.c
> >>>> +++ b/src/backend/replication/logical/launcher.c
> >>>> @@ -8,20 +8,27 @@
> >>>>     *   src/backend/replication/logical/launcher.c
> >>>>     *
> >>>>     * NOTES
> >>>> - *   This module contains the logical replication worker launcher which
> >>>> - *   uses the background worker infrastructure to start the logical
> >>>> - *   replication workers for every enabled subscription.
> >>>> + *   This module contains the replication worker launcher which
> >>>> + *   uses the background worker infrastructure to:
> >>>> + *   a) start the logical replication workers for every enabled
> subscription
> >>>> + *      when not in standby_mode.
> >>>> + *   b) start the slot sync worker for logical failover slots
> synchronization
> >>>> + *      from the primary server when in standby_mode.
> >>>>
> >>>> I was wondering do we really need a launcher on standby to invoke
> >>>> sync-slot worker. If so, why? I guess it may be required for
> >>>> previous versions where we were managing work for multiple
> >>>> slot-sync workers which is also questionable in the sense of
> >>>> whether launcher is the right candidate for the same but now with
> >>>> the single slot-sync worker, it doesn't seem worth having it. What do you
> think?
> >>>>
> >>>> --
> >>>
> >>> Yes, earlier a manager process was needed to manage multiple
> >>> slot-sync workers and distribute load among them, but now that does
> >>> not seem necessary. I gave it a try (PoC) and it seems to work well.
> >>> If  there are no objections to this approach, I can share the patch soon.
> >>>
> >>
> >> +1 on this new approach, thanks!
> >
> > PFA v40. This patch has removed Logical Replication Launcher support
> > to launch slotsync worker.
> 
> Thanks!
> 
> >  The slot-sync worker is now registered as bgworker with postmaster,
> > with bgw_start_time=BgWorkerStart_ConsistentState and
> > bgw_restart_time=60sec.
> >
> > On removal of launcher, now all the validity checks have been shifted
> > to slot-sync worker itself.  This brings us to some point of concerns:
> >
> > a) We still need to maintain  RecoveryInProgress() check in slotsync
> > worker. Since worker has the start time of
> > BgWorkerStart_ConsistentState, it will be started on non-standby as
> > well. So to ensure that it exists on non-standby, "RecoveryInProgress"
> > has been introduced at the beginning of the worker. But once it exits,
> > postmaster will not restart it since it will be clean-exist i.e.
> > proc_exit(0) (the restart logic of postmaster comes into play only
> > when there is an abnormal exit). But to exit for the first time on
> > non-standby, we need that Recovery related check in worker.
> >
> > b) "enable_syncslot" check is moved to slotsync worker now. Since
> > enable_syncslot is PGC_SIGHUP, so proc_exit(1) is currently used to
> > exit the worker if 'enable_syncslot' is found to be disabled.
> > 'proc_exit(1)' has been used in order to ensure that the worker is
> > restarted and GUCs are checked again after restart_time. Downside of
> > this approach is, if someone has kept "enable_syncslot" as disabled
> > permanently even on standby, slotsync worker will keep on restarting
> > and exiting.
> >
> > So to overcome the above pain-points, I think a potential approach
> > will be to start slotsync worker only if 'enable_syncslot' is on and
> > the system is non-standby.
> 
> That makes sense to me.
> 
> > Potential ways (each with some issues) are:
> >
> > 1) Use the current way i.e. register slot-sync worker as bgworker with
> > postmaster, but introduce extra checks in 'maybe_start_bgworkers'. But
> > this seems more like a hack. This will need extra changes as currently
> > once 'maybe_start_bgworkers' is attempted by postmaster, it will
> > attempt again to start any worker only if the worker had abnormal exit
> > and restart_time !=0. The current postmatser will not attempt to start
> > worker on any GUC change.
> >
> > 2) Another way maybe to treat slotsync worker as special case and
> > separate out the start/restart of slotsync worker from bgworker, and
> > follow what we do for autovacuum launcher(StartAutoVacLauncher) to
> > keep starting it in the postmaster loop(ServerLoop). In this way, we
> > may be able to add more checks before starting worker. But by opting
> > this approach, we will have to manage slotsync worker completely by
> > ourself as it will be no longer be part of existing
> > bgworker-registration infra. If this seems okay and there are no other
> > better options, it can be analyzed further in detail.
> >
> > 3) Another approach could be, in order to solve issue (a), introduce a
> > new start_time 'BgWorkerStart_ConsistentState_HotStandby' which means
> > start a bgworker only if consistent state is reached and the system is
> > standby. And for issue (b), lets retain check of enable_syncslot in
> > the worker itself but make it 'PGC_POSTMASTER'. This will ensure we
> > can safely exit the worker(proc_exit(0) if enable_syncslot is disabled
> > and postmaster will not restart it. But I'm not sure if making it
> > "PGC_POSTMASTER" is acceptable from the user's perspective.
> 
> I had the same idea (means make enable_syncslot as 'PGC_POSTMASTER')
> when reading b). I'm +1 on it (at least for V1) as I don't think that this parameter
> value would change frequently. Curious to know what others think too.
> 
> Then as far a) is concerned, I'd vote for introducing a new
> BgWorkerStart_ConsistentState_HotStandby.

Here is the V41 patch set which includes the following changes.

V41-0001:
1) Based on the discussion[1], I update the document to remind user to
change the slot's failover option when ALTER SUBSCRIPTION ... SET
(slot_name = xx).

2) Address comments in [2][3][4].

V41-0002:
1) 'enable_syncslot' is changed from PGC_SIGHUP to PGC_POSTMASTER,
slot-sync worker will now clean exit (proc_exit(0)) if enable_syncslot is
found disabled.

2) BgWorkerStart_ConsistentState_HotStandby is introduced as new
start-time for bgworker. This will start worker only if it is standby_mode
and consistent state is reached.

3) 'SYNCSLOT_STATE_INITIATED' is now set in 'ReplicationSlotCreate' itself
in slot-sync worker case. Earlier it was set at later point of time giving
a window wherein even a synced slot was in 'n' state for quite some time,
which was not correct.

Thanks Shveta for working on the V41-0002.

[1] https://www.postgresql.org/message-id/CAA4eK1Jd9dk%3D5POTKM9p4EyYqYzLXe-AnLzHrUELjzZScLz7mw%40mail.gmail.com
[2] https://www.postgresql.org/message-id/eb09f682-db82-41cd-93bc-5d44e10e1d6d%40gmail.com
[3] https://www.postgresql.org/message-id/CAHut%2BPsuSWjm7U_sVnL8FXZ7ZQcfCcT44kAK7i6qMG35Cwjy3A%40mail.gmail.com
[4] https://www.postgresql.org/message-id/CAFPTHDbFqLgXS6Et%2BshNGPDjCKK66C%2BZSarqFHmQvfnAah3Qsw%40mail.gmail.com

Best Regards,
Hou zj