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

On Tue, Aug 12, 2025 at 1:26 AM Shlok Kyal <shlok(dot)kyal(dot)oss(at)gmail(dot)com> wrote:
>
>
> Hi Sawada-san,
>
> I have reviewed the patch and have few comments:

Thank you for reviewing the patch!

>
> 1. There are some spelling mistakes in logicaldecoding.sgml
> + and requires waiting for any concurrent transactions to finish, ensureing
> + system-wide conistency. Conversely, when the last logical replication slot
> ensureing -> ensuring
> conistency -> consistency

Will fix.

>
> 2. In publicationcmds.c:
> + errmsg("logical decoding needs to be enabled to
> publish logical changes"),
> + errhint("Set \"wal_level\" >= \"logical\" or create a
> logical replication slot with \"replica\" WAL level before creating
> subscriptions.")));
>
> Should we use something like this for errhint ?
> errhint("Set \"wal_level\" >= \"logical\" or create a logical
> replication slot before creating subscriptions when \"wal_level\" =
> \"replica\".")));

The current message is:

Set "wal_level" >= "logical" or create a logical replication slot with
"replica" WAL level before creating subscriptions.

whereas the proposed message is:

Set "wal_level" >= "logical" or create a logical replication slot
before creating subscriptions when "wal_level" = "replica".

I don't see a big difference between the two sentences but your point
is to use 'when "wal_level" = "replica"' instead of 'with "replica"
WAL level'?

>
>
> 3. In logical.c:
> + errmsg("logical decoding needs to be enabled on the primary"),
> + errhint("Set \"wal_level\" >= \"logical\" or create
> at least one logical slot with \"replica\" WAL level on the
> primary.")));
>
> Should we change the errhint message as below?
> errhint("Set \"wal_level\" >= \"logical\" or create at least one
> logical slot on the primary when \"wal_level\" = \"replica\".")));
>
> 4. In slotsync.c:
> + errmsg("replication slot synchronization requires
> logical decoding to be enabled"),
> + errhint("Set \"wal_level\" >= \"logical\" or create at
> least one logical slot with \"replica\" WAL level on the primary "));
>
> Should we change the errhint message as below?
> errhint("Set \"wal_level\" >= \"logical\" or create at least one
> logical slot on the primary when \"wal_level\" = \"replica\".")));

Please see the above question.

>
> ---------
>
> I have also tested the patch with creating multiple permanent/
> temporary slots in concurrent sessions and I did not find any issue. I
> also tested this patch with a physical replication setup.
> I have a doubt in this case:
> 1. Suppose we have a physical replication setup. wal_level on primary
> is set to 'replica'
> 2. Now we try to create a logical slot on standby, an error is thrown
> as wal_level is set to 'replica' as primary does not have any logical
> slot
> 3. Now we create a temporary logical slot on primary,
> effective_wal_level is set to logical.
> 4. Now we can create slots on standby as effective_wal_level is logical.
> 5. Now we exit the session of the primary server. The temporary slot
> is dropped. This will invalidate the slots on standby as the
> effective_wal_level will be set to 'replica'.
> So we can say that indirectly a temporary slot on primary can control
> the behaviour of permanent slots on standbys.
>
> I checked this behaviour in HEAD. In HEAD also the behaviour is the
> same. If we change the wal_level on primary from 'logical' to
> 'replica', all slots are invalidated on the standby.
> With patch this behaviour can be indirectly controlled by a temporary
> slot. Is it fine? Thoughts?

Your understanding is correct. I've discussed whether we need a way to
keep auto-increased 'logical' WAL level on the primary when standbys
have logical slots. You mentioned temporary logical slots cases but I
think the same is true for the case where users accidentally drop the
last logical slot.

My understanding is that it's fine that logical decoding availability
on standbys is controlled by primary's logical slots (including temp
slots) presence. This essentially is the same behavior as the current
one and users who are concerned about indirectly invalidating the
logical slots on standbys can set wal_level to 'logical' on the
primary. It's a separate discussion (and patch) whether we need to
provide a way for users to keep auto-increased 'logical' WAL level on
the primary.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com