Feedback-ID: id4a34324:Fastmail
Date: Wed, 15 Feb 2023 10:18:18 -0800
From: Andres Freund <andres@anarazel.de>
To: Ashutosh Sharma <ashu.coek88@gmail.com>
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>
Subject: Re: Minimal logical decoding on standbys
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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.


> 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.

Greetings,

Andres Freund