Re: Logical Replication of sequences

On Thu, Oct 16, 2025 at 4:53 PM Zhijie Hou (Fujitsu)
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> Regarding whether we can avoid creating slot/origin for seq-only publication.
> I think the main challenge lies in ensuring the apply worker operates smoothly
> without a replication slot. Currently, the apply worker uses the
> START_REPLICATION command with a replication slot to acquire the slot on the
> publisher. To bypass this, it's essential to skip starting the replication and
> specifically, avoid entering the LogicalRepApplyLoop().
>
> To address this, I thought to implement a separate loop dedicated to
> sequence-only subscriptions. Within this loop, the apply worker would only call
> functions like ProcessSyncingSequencesForApply() to manage sequence
> synchronization while periodically checking for any new tables added to the
> subscription. If new tables are detected, the apply worker would exit this loop
> and enter the LogicalRepApplyLoop().
>
> I chose not to consider allowing the START_REPLICATION command to operate
> without a logical slot, as it seems like an unconventional approach requiring
> modifications in walsender and to skip logical decoding and related processes.
>
> Another consideration is whether to address scenarios where tables are
> subsequently removed from the subscription, given that slots and origins would
> already have been created in such cases.
>
> Since it might introduce addition complexity to the patches, and considering
> that we already allow slot/origin to be created for empty subscription, it might
> also be acceptable to allow it to be created for sequence-only subscription. So,
> I chose to add some comments to explain the reason for it in latest version.
>
> Origin case might be slightly easier to handle, but it could also require some
> amount of implementations. Since origin is less harmful than a replication slot
> and maintaining it does not have noticeable overhead, it seems OK to me to
> retain the current behaviour and add some comments in the patch to clarify the
> same.
>

I agree that avoiding to create a slot/origin for sequence-only
subscription is not worth the additional complexity at other places,
especially when we do create them for empty subscriptions.

--
With Regards,
Amit Kapila.