Horton

Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2025-07-21T06:28:05Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix regression test failure when wal_level is set to minimal.

  2. Toggle logical decoding dynamically based on logical slot presence.

  3. Disallow server start with sync_replication_slots = on and wal_level < logical.

On Mon, Jul 21, 2025 at 10:48 AM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Fri, Jul 18, 2025 at 3:03 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > >
> > > One concern I have is regarding the default setting of
> > > 'force_slot_drop' . I assume the default value of this new DROP-SUB
> > > argument will be 'false' to prevent customers from inadvertently
> > > dropping the last slot on the publisher. But, would this be
> > > acceptable, considering that users may have DROP-SUBSCRIPTION commands
> > > in their scripts which would suddenly stop dropping slot now?
> > >
> >
> > That would only happen when users use this new idea of enabling
> > wal_level to 'logical' on the fly. I think the users having existing
> > setups with pub-sub would have kept the default wal_level to 'logical'
> > on publisher.
> >
>
> Okay, but then we will have to avoid doing the enhancement of getting
> rid of wal_level='logical' as suggested in [1].
>
> Even if we do so, I am not very much convinced for this argument and its value.
> --The value of ''force_slot_drop" will hold its meaning only in a
> conditional scenario. Assuming default is false,  then it will still
> drop the slots until it is last slot and wal_level < logical on
> primary. This behavior can seem a bit unintuitive or confusing from
> the user's perspective.
> --If the user is trying to actually retain the slot by giving
> force_slot_drop=false , then how are we going to track that i.e.
> distinguish from its default.
>
> Bertrand has proposed a similar design in [2]. We can revisit that as well once.
>

I am slightly hesitant to introduce multiple ways to enable logical
decoding/replication unless that is the only path as giving multiple
options to achieve the same thing can confuse users as to which one is
preferable and pros/cons of each.

-- 
With Regards,
Amit Kapila.