Re: synchronized_standby_slots behavior inconsistent with quorum-based synchronous replication

Hi,

Thanks for the review.

On Wed, Mar 11, 2026 at 2:15 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:

> Had a look at the patch. Few concerns:
>
> 1)
> StandbySlotsHaveCaughtup:
>
> + * If a slot listed in synchronized_standby_slots is not found,
> + * report an error.
>
> --
>
> + * If a slot is not physical, report error.
>
> These comments are misleading as we may or may not report ERROR
> depending upon elevel passed by caller.
>
> 2)
> It seems to me (not tested yet) that even in priority based and quorum
> based configuration, if we have found our N slots, still we will end
> up emitting WARNING message for invalidated, missing slots etc such
> as:
>
> a)
> repplication slot \"%s\" specified in parameter \"%s\" does not exist
> Logical replication is waiting on the standby associated with replication slot.
>
> b)
> cannot specify logical replication slot \"%s\" in parameter
> Logical replication is waiting for correction on replication slot.
>
> c)
> physical replication slot \"%s\" specified in parameter \"%s\" has
> been invalidated
> Logical replication is waiting on the standby associated with replication slot.
>
> These messages may give the impression that logical replication is
> actually waiting, even though it might already be progressing normally
> because the required N slots have been found.
>

Agreed. We will need to refactor the code around these messages so
that they are emitted only when necessary.

> OTOH, if we suppress these messages, there could be cases where we
> fail to find the required N valid slots and logical replication
> genuinely ends up waiting, but the user would receive no indication of
> the problem.
>
> One thing for sure is, that we need to emit such messages when
> 'wait_for_all' is true, but for rest, we can not decide inline.
>
> So there are 2 options:
> a) either we change the DETAIL slightly with each such reporting to
> say below or anything better:
>
> DETAIL: logical replication may wait if the required number of standby
> slots is not available.
>
> b)
> Or we collect the slot-names and emit the message at the end only 'if
> (caught_up_slot_num < required)'. Something like:
>
> WARNING: Some replication slots specified in parameter "%s" are not
> valid: invalidated (slot1, slot2), logical (slot3), missing (slot4).
> DETAIL: Logical replication is waiting because the required number of
> standby slots is not available.
>
>
> Thoughts?

For now, 2b seems like the better option to me as well - I will share
here if anything else (better than this) comes to mind while working
on it

--
With Regards,
Ashutosh Sharma.