Feedback-ID: id4a34324:Fastmail
Date: Thu, 12 Jan 2023 10:16:30 -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 2023-01-12 20:08:55 +0530, Ashutosh Sharma wrote:
> I previously participated in the discussion on "Synchronizing the
> logical replication slots from Primary to Standby" and one of the
> purposes of that project was to synchronize logical slots from primary
> to standby so that if failover occurs, it will not affect the logical
> subscribers of the old primary much. Can someone help me understand
> how we are going to solve this problem with this patch? Are we going
> to encourage users to do LR from standby instead of primary to get rid
> of such problems during failover?

It only provides a building block towards that. The "Synchronizing the logical
replication slots from Primary to Standby" project IMO needs all of the
infrastructure in this patch. With the patch, a logical rep solution can
e.g. maintain one slot on the primary and one on the standby, and occasionally
forward the slot on the standby to the position of the slot on the primary. In
case of a failover it can just start consuming changes from the former
standby, all the necessary changes are guaranteed to be present.


> Also, one small observation:
> 
> I just played around with the latest (v38) patch a bit and found that
> when a new logical subscriber of standby is created, it actually
> creates two logical replication slots for it on the standby server.
> May I know the reason for creating an extra replication slot other
> than the one created by create subscription command? See below:

That's unrelated to this patch. There's no changes to the "higher level"
logical replication code dealing with pubs and subs, it's all on the "logical
decoding" level.

I think this because logical rep wants to be able to concurrently perform
ongoing replication, and synchronize tables added to the replication set. The
pg_16399_sync_16392_7187728548042694423 slot should vanish after the initial
synchronization.

Greetings,

Andres Freund