Re: Logical Replication of sequences

Hi Vignesh, Here are my v20240807-0003 review comments.

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1. GENERAL DOCS.

IMO the replication of SEQUENCES is a big enough topic that it
deserves to have its own section in the docs chapter 31 [1].

Some of the create/alter subscription docs content would stay where it
is in, but a new chapter would just tie everything together better. It
could also serve as a better place to describe the other sequence
replication content like:
(a) getting a WARNING for mismatched sequences and how to handle it.
(b) how can the user know when a subscription refresh is required to
(re-)synchronise sequences
(c) pub/sub examples

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doc/src/sgml/logical-replication.sgml

2. Restrictions

Sequence data is not replicated. The data in serial or identity
columns backed by sequences will of course be replicated as part of
the table, but the sequence itself would still show the start value on
the subscriber. If the subscriber is used as a read-only database,
then this should typically not be a problem. If, however, some kind of
switchover or failover to the subscriber database is intended, then
the sequences would need to be updated to the latest values, either by
executing ALTER SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES or by
copying the current data from the publisher (perhaps using pg_dump) or
by determining a sufficiently high value from the tables themselves.

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2a.
The paragraph starts by saying "Sequence data is not replicated.". It
seems wrong now. Doesn't that need rewording or removing?

~

2b.
Should the info "If, however, some kind of switchover or failover..."
be mentioned in the "Logical Replication Failover" section [2],
instead of here?

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doc/src/sgml/ref/alter_subscription.sgml

3.
Sequence values may occasionally become out of sync due to updates in
the publisher. To verify this, compare the
pg_subscription_rel.srsublsn on the subscriber with the page_lsn
obtained from the pg_sequence_state for the sequence on the publisher.
If the sequence is still using prefetched values, the page_lsn will
not be updated. In such cases, you will need to directly compare the
sequences and execute REFRESH PUBLICATION SEQUENCES if required.

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3a.
This whole paragraph may be better put in the new chapter that was
suggested earlier in review comment #1.

~

3b.
Is it only "Occasionally"? I expected subscriber-side sequences could
become stale quite often.

~

3c.
Is this advice very useful? It's saying if the LSN is different then
the sequence is out of date, but if the LSN is not different then you
cannot tell. Why not ignore LSN altogether and just advise the user to
directly compare the sequences in the first place?

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Also, there are more minor suggestions in the attached nitpicks diff.

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[1] https://www.postgresql.org/docs/current/logical-replication.html
[2] file:///usr/local/pg_oss/share/doc/postgresql/html/logical-replication-failover.html

Kind Regards,
Peter Smith.
Fujitsu Australia