Re: Synchronizing slots from primary to standby

On Tue, Jan 16, 2024 at 9:03 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Sat, Jan 13, 2024 at 12:54 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Fri, Jan 12, 2024 at 5:50 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > >
> > > There are multiple approaches discussed and tried when it comes to
> > > starting a slot-sync worker. I am summarizing all here:
> > >
> > > 1) Make slotsync worker as an Auxiliary Process (like checkpointer,
> > > walwriter, walreceiver etc). The benefit this approach provides is, it
> > > can control begin and stop in a more flexible way as each auxiliary
> > > process could have different checks before starting and can have
> > > different stop conditions. But it needs code duplication for process
> > > management(start, stop, crash handling, signals etc) and currently it
> > > does not support db-connection smoothly (none of the auxiliary process
> > > has one so far)
> > >
> >
> > As slotsync worker needs to perform transactions and access syscache,
> > we can't make it an auxiliary process as that doesn't initialize the
> > required stuff like syscache. Also, see the comment "Auxiliary
> > processes don't run transactions ..." in AuxiliaryProcessMain() which
> > means this is not an option.
> >
> > >
> > > 2) Make slotsync worker as a 'special' process like AutoVacLauncher
> > > which is neither an Auxiliary process nor a bgworker one. It allows
> > > db-connection and also provides flexibility to have start and stop
> > > conditions for a process.
> > >
> >
> > Yeah, due to these reasons, I think this option is worth considering
> > and another plus point is that this allows us to make enable_syncslot
> > a PGC_SIGHUP GUC rather than a PGC_POSTMASTER.
> >
> > >
> > > 3) Make slotysnc worker a bgworker. Here we just need to register our
> > > process as a bgworker (RegisterBackgroundWorker()) by providing a
> > > relevant start_time and restart_time and then the process management
> > > is well taken care of. It does not need any code-duplication and
> > > allows db-connection smoothly in registered process. The only thing it
> > > lacks is that it does not provide flexibility of having
> > > start-condition which then makes us to have 'enable_syncslot' as
> > > PGC_POSTMASTER parameter rather than PGC_SIGHUP. Having said this, I
> > > feel enable_syncslot is something which will not be changed frequently
> > > and with the benefits provided by bgworker infra, it seems a
> > > reasonably good option to choose this approach.
> > >
> >
> > I agree but it may be better to make it a PGC_SIGHUP parameter.
> >
> > > 4) Another option is to have Logical Replication Launcher(or a new
> > > process) to launch slot-sync worker. But going by the current design
> > > where we have only 1 slotsync worker, it may be an overhead to have an
> > > additional manager process maintained.
> > >
> >
> > I don't see any good reason to have an additional launcher process here.
> >
> > >
> > > Thus weighing pros and cons of all these options, we have currently
> > > implemented the bgworker approach (approach 3). Any feedback is
> > > welcome.
> > >
> >
> > I vote to go for (2) unless we face difficulties in doing so but (3)
> > is also okay especially if others also think so.
>
> I am not against any of the approaches but I still feel that when we
> have a standard way of doing things (bgworker) we should not keep
> adding code to do things in a special way unless there is a strong
> reason to do so. Now we need to decide if 'enable_syncslot' being
> PGC_POSTMASTER is a strong reason to go the non-standard way?
>

Agreed and as said earlier I think it is better to make it a
PGC_SIGHUP. Also, not sure we can say it is a non-standard way as
already autovacuum launcher is handled in the same way. One more minor
thing is it will save us for having a new bgworker state
BgWorkerStart_ConsistentState_HotStandby as introduced by this patch.

> If yes,
> then we should think of option 2 else option 3 seems better in my
> understanding (which may be limited due to my short experience here),
> so I am all ears to what others think on this.
>

I also think it would be better if more people share their opinion on
this matter.

--
With Regards,
Amit Kapila.