Re: Logical Replication of sequences

On Tue, 19 Aug 2025 at 23:33, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> I imagined something like case 2. For logical replication of tables,
> if we support DDL replication (i.e., CREATE/ALTER/DROP TABLE), all
> changes the apply worker executes are serialized in commit LSN order.
> Therefore, users would not have to be concerned about schema changes
> that happened to the publisher. On the other hand, for sequence
> replication, even if we support DDL replication for sequences (i.e.,
> CREATE/ALTER/DROP SEQUENCES), users would have to execute REFRESH
> PUBLICATION SEQUENCES command after "ALTER SEQUENCE s1 MAXVALUE 12;"
> has been replicated on the subscriber. Otherwise, REFRESH PUBLICATION
> SEQUENCE command would fail because the sequence parameters no longer
> match.

I am summarizing the challenges identified so far (assuming we have
DDL replication implemented through WAL support)
1) Lack of sequence-synchronization resulting in DDL replication
failure/conflict.
On the subscriber, the sequence has advanced to 14:
SELECT currval('s1');
currval
---------
     14

On the publisher, the sequence is reset to 11 and MAXVALUE is changed to 12:
SELECT setval('s1', 11);
ALTER SEQUENCE s1 MAXVALUE 12;
If the subscriber did not execute REFRESH PUBLICATION SEQUENCES, DDL
replication will fail with error.
ERROR:  RESTART value (14) cannot be greater than MAXVALUE (12)

2) Manual DDL on subscriber resulting in sequence synchronization failure.
On the subscriber, the sequence maxvalue is changed:
ALTER SEQUENCE s1 MAXVALUE 12;

On the publisher, the sequence has advanced to 14:
SELECT currval('s1');
currval
---------
     14

REFRESH PUBLICATION SEQUENCES will fail because setting currvalue to
14 is greater than the changed maxvalue 12 in the subscriber.

3) Out of order DDL and REFRESH resulting in synchronization failure.
Initially we have the same sequence on pub and sub. Then lets say pub
has done parameter change:
ALTER SEQUENCE s1 MAXVALUE 12;
Now if this DDL is somehow not replicated on sub, REFRESH PUBLICATION
SEQUENCES will fail initially and may work once DDL is replicated.
~~
Problems 1 and 2 exist in both designs. While the WAL-based REFRESH
may seem slightly better for Problem 3 since REFRESH on the subscriber
will execute only after prior DDLs are replicated—even with the
sequence-sync worker, this isn't a major issue. If a user triggers
REFRESH before the DDL is replicated, the worker will refresh all
sequences except the mismatched one, and keep restarting and retrying
until the DDL is applied. Once that happens, the sequence sync
completes automatically, without the user doing another REFRESH.
Furthermore, the likelihood of a user executing REFRESH exactly during
the window between the DDL execution on the publisher and its
application on the subscriber seems relatively low.

WAL-based approach OTOH introduces several additional challenges that
may outweigh its potential benefits:
1)  Increases load on WAL sender to collect sequence values. We are
talking about all the sequences here which could be huge in number.
2)  Table replication may stall until sequence conflicts are resolved.
The  chances of hitting any conflict/error could be more here as
compared to tables specially when sequence synchronization is not
incremental and the number of sequences are huge. The continuous and
more frequent errors if not handled by users may even end up
invalidating the slot on primary.

The worker approach neither blocks the apply worker in case of errors
nor adds extra load on the WAL sender. On its own, Case 3 doesn’t seem
significant enough to justify switching to a WAL-based design.
Overall, the worker-based approach appears to be less complex and a
better option.

Regards,
Vignesh