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: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2025-05-07T06:59:10Z
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 Thu, Apr 24, 2025 at 11:14 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Thu, Apr 24, 2025 at 5:30 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Wed, Apr 23, 2025 at 9:35 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> > > On Wed, Apr 23, 2025 at 5:46 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > BTW, did we consider the idea to automatically transition to 'logical'
> > > > when the first logical slot is created and transition back to
> > > > 'replica' when last logical slot gets dropped? I see some ideas around
> > > > this last time we discussed this topic.
> > >
> > > Yes. Bertrand pointed out that a drawback is that the primary server
> > > needs to create a logical slot in order to execute logical decoding on
> > > the standbys[1].
> > >
> >
> > True, but if we want to avoid that, we can still keep 'logical' as
> > wal_level for the ease of users.
>
> I think we'd like to cover the use case like where users start with
> 'replica' on the primary and execute logical decoding on the standby
> without neither creating a logical slot on the primary nor restarting
> the primary.
>

Okay, if we introduce a SIGHUP GUC like max_wal_level as you are
proposing, the above requirement will be fulfilled, right? The other
way is by API pg_activate_logical_decoding().

> > We can also have another API like the
> > one you originally proposed (pg_activate_logical_decoding) for the
> > ease of users. But the minimum requirement would be that one creates a
> > logical slot to enable logical decoding/replication.
>
> I think we want to avoid the runtime WAL level automatically decreased
> to 'replica' once all logical slots are removed, if users still want
> to execute logical decoding on only the standby. One idea is that if
> users enable logical decoding using pg_activate_logical_decoding(),
> the runtime WAL level doesn't decrease to 'replica' even if all
> logical slots are removed.
>

That makes sense. If we are using an API like
pg_activate_*/pg_deactivate_*, then why add an additional dependency
on the slots?

-- 
With Regards,
Amit Kapila.