Date: Tue, 11 Jun 2019 14:36:50 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, Sergei Kornilov <sk@zsrv.org>,
	Amit Khandekar <amitdkhan.pg@gmail.com>,
	tushar <tushar.ahuja@enterprisedb.com>,
	Petr Jelinek <petr.jelinek@2ndquadrant.com>,
	Craig Ringer <craig@2ndquadrant.com>,
	Petr Jelinek <petr@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Minimal logical decoding on standbys
Message-ID: <20190611183650.GA25310@alvherre.pgsql>
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On 2019-May-23, Andres Freund wrote:

> On 2019-05-23 09:37:50 -0400, Robert Haas wrote:
> > On Thu, May 23, 2019 at 9:30 AM Sergei Kornilov <sk@zsrv.org> wrote:
> > > > wal_level is PGC_POSTMASTER.
> > >
> > > But primary can be restarted without restart on standby. We require wal_level replica or highter (currently only logical) on standby. So online change from logical to replica wal_level is possible on standby's controlfile.
> > 
> > That's true, but Amit's scenario involved a change in wal_level during
> > the execution of pg_create_logical_replication_slot(), which I think
> > can't happen.
> 
> I don't see why not - we're talking about the wal_level in the WAL
> stream, not the setting on the standby. And that can change during the
> execution of pg_create_logical_replication_slot(), if a PARAMTER_CHANGE
> record is replayed. I don't think it's actually a problem, as I
> outlined in my response to Amit, though.

I don't know if this is directly relevant, but in commit_ts.c we go to
great lengths to ensure that things continue to work across restarts and
changes of the GUC in the primary, by decoupling activation and
deactivation of the module from start-time initialization.  Maybe that
idea is applicable for this too?

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services