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help / color / mirror / Atom feedFrom: Bertrand Drouvot <[email protected]>
To: Amit Kapila <[email protected]>
Cc: shveta malik <[email protected]>
Cc: Masahiko Sawada <[email protected]>
Cc: Zhijie Hou (Fujitsu) <[email protected]>
Cc: Dilip Kumar <[email protected]>
Cc: Peter Smith <[email protected]>
Cc: Nisha Moond <[email protected]>
Cc: Hayato Kuroda (Fujitsu) <[email protected]>
Cc: Bharath Rupireddy <[email protected]>
Cc: Peter Eisentraut <[email protected]>
Cc: Bruce Momjian <[email protected]>
Cc: Ashutosh Sharma <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: pgsql-hackers <[email protected]>
Cc: Ajin Cherian <[email protected]>
Cc: Alvaro Herrera <[email protected]>
Subject: Re: Synchronizing slots from primary to standby
Date: Tue, 16 Jan 2024 08:42:11 +0000
Message-ID: <ZaZBYzgM/[email protected]> (raw)
In-Reply-To: <CAA4eK1+q8R=yEpTMiOZ_Xksdd_Jg3SXvBMxRutHQtkCbdeK_yQ@mail.gmail.com>
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Hi,
On Sat, Jan 13, 2024 at 12:53:50PM +0530, Amit Kapila wrote:
> On Fri, Jan 12, 2024 at 5:50 PM shveta malik <[email protected]> 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.
>
Yeah, I think that (2) would be the "ideal" one but (3) is fine too. I think
that if we think/see that (2) is too "complicated"/long to implement maybe we
could do (3) initially and switch to (2) later. What I mean by that is that I
don't think that not doing (2) should be a blocker.
Regards,
--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
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Subject: Re: Synchronizing slots from primary to standby
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