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From: shveta malik <shveta.malik@gmail.com>
Date: Tue, 16 Jan 2024 09:03:17 +0530
Message-ID: <CAJpy0uAU+nzufQwSzyB2YmhPHHn71_=1zkauP4Ra32a_eqbcig@mail.gmail.com>
Subject: Re: Synchronizing slots from primary to standby
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, 
	Dilip Kumar <dilipbalaut@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, 
	Peter Smith <smithpb2250@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, 
	"Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, 
	Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, 
	Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Bruce Momjian <bruce@momjian.us>, 
	Ashutosh Sharma <ashu.coek88@gmail.com>, Andres Freund <andres@anarazel.de>, 
	pgsql-hackers <pgsql-hackers@postgresql.org>, Ajin Cherian <itsajin@gmail.com>, 
	Alvaro Herrera <alvherre@alvh.no-ip.org>, shveta malik <shvetamalik@gmail.com>
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On Sat, Jan 13, 2024 at 12:54=E2=80=AFPM Amit Kapila <amit.kapila16@gmail.c=
om> wrote:
>
> On Fri, Jan 12, 2024 at 5:50=E2=80=AFPM shveta malik <shveta.malik@gmail.=
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? 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.

thanks
Shveta