Re: Synchronizing slots from primary to standby

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.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com