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In-Reply-To: <20230215181818.mhmwp3pvt3crtalo@awork3.anarazel.de>
From: Ashutosh Sharma <ashu.coek88@gmail.com>
Date: Thu, 16 Feb 2023 10:09:22 +0530
Message-ID: <CAE9k0PmfMp2K5GGtJHbthtv4UA2f+VD2prnRSC14oU9HdDWy0A@mail.gmail.com>
Subject: Re: Minimal logical decoding on standbys
To: Andres Freund <andres@anarazel.de>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Robert Haas <robertmhaas@gmail.com>, 
	Thomas Munro <thomas.munro@gmail.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, 
	Ibrar Ahmed <ibrar.ahmad@gmail.com>, Amit Khandekar <amitdkhan.pg@gmail.com>, fabriziomello@gmail.com, 
	tushar <tushar.ahuja@enterprisedb.com>, Rahila Syed <rahila.syed@2ndquadrant.com>, 
	pgsql-hackers <pgsql-hackers@postgresql.org>
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On Wed, Feb 15, 2023 at 11:48 PM Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2023-02-15 18:02:11 +0530, Ashutosh Sharma wrote:
> > Thanks Andres. I have one more query (both for you and Bertrand). I
> > don't know if this has already been answered somewhere in this mail
> > thread, if yes, please let me know the mail that answers this query.
> >
> > Will there be a problem if we mandate the use of physical replication
> > slots and hot_standby_feedback to support minimum LD on standby. I
> > know people can do a physical replication setup without a replication
> > slot or even with hot_standby_feedback turned off, but are we going to
> > have any issue if we ask them to use a physical replication slot and
> > turn on hot_standby_feedback for LD on standby. This will reduce the
> > code changes required to do conflict handling for logical slots on
> > standby which is being done by v50-0001 and v50-0002* patches
> > currently.
>
> I don't think it would. E.g. while restoring from archives we can't rely on
> knowing that the slot still exists on the primary.
>
> We can't just do corrupt things, including potentially crashing, when the
> configuration is wrong. We can't ensure that the configuration is accurate all
> the time. So we need to detect this case. Hence needing to detect conflicts.
>

OK. Got it, thanks.

>
> > IMHO even in normal scenarios i.e. when we are not doing LD on
> > standby, we should mandate the use of a physical replication slot.
>
> I don't think that's going to fly. There plenty scenarios where you e.g. don't
> want to use a slot, e.g. when you want to limit space use on the primary.
>

I think this can be controlled via max_slot_wal_keep_size GUC, if I
understand it correctly.

--
With Regards,
Ashutosh Sharma.