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

On Tue, Aug 26, 2025 at 12:54 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> I've attached the updated patch that incorporated the comments and is
> rebased to the current HEAD.
>

Thanks for the patch, please find a few comments concerning LOG messages:

1)
slotsync worker gives LOG:
LOG: replication slot synchronization requires logical decoding to be enabled

By this LOG message, the user might not know how to enable logical
decoding. Shall we add HINT/DETAIL similar to other places:
To enable logical decoding on standby, set "wal_level" >= "logical" or
create at least one logical slot on the primary server.

2)
When we try to create a logical slot on standby, it takes some time
until runnign-txns are logged on primary. During that wait-time, if we
drop logical slot on primary disabling logical_deocding on standby,
then slot-creation fails with:

postgres=# SELECT pg_create_logical_replication_slot('st_slot2',
'pgoutput', false, false, false);
ERROR: canceling statement due to conflict with recovery
DETAIL: User was using a logical replication slot that must be invalidated.

Do we need to tweak the message a little bit as this new case is is
not the case of invalidation?

3)
When slot is invalidated on standby, we get message:

LOG: invalidating obsolete replication slot "st_slot"
DETAIL: Logical decoding on standby requires "wal_level" >= "logical"
or to create at least one logical slot on the primary server.

The DETAIL msg looks slightly odd. Shall we make it as:
Logical decoding on standby requires "wal_level" >= "logical" or at
least one logical slot on the primary server.

thanks
Shveta