Re: Logical Replication of sequences

On Tue, Jun 4, 2024 at 8:56 PM Yogesh Sharma
<yogesh.sharma@catprosystems.com> wrote:
>
> On 6/4/24 06:57, Amit Kapila wrote:
>
> > 2. Provide a command say Alter Subscription ...  Replicate Sequences
> > (or something like that) which users can perform before shutdown of
> > the publisher node during upgrade. This will allow copying all the
> > sequences from the publisher node to the subscriber node directly.
> > Similar to previous approach, this could also be inconvenient for
> > users.
>
> This is similar to option 1 except that it is a SQL command now.
>

Right, but I would still prefer a command as it provides clear steps
for the upgrade. Users need to perform (a) Replicate Sequences for a
particular subscription (b) Disable that subscription (c) Perform (a)
and (b) for all the subscriptions corresponding to the publisher we
want to shut down for upgrade.

I agree there are some manual steps involved here but it is advisable
for users to ensure that they have received the required data on the
subscriber before the upgrade of the publisher node, otherwise, they
may not be able to continue replication after the upgrade. For
example, see the "Prepare for publisher upgrades" step in pg_upgrade
docs [1].

>
> > 3. Replicate published sequences via walsender at the time of shutdown
> > or incrementally while decoding checkpoint record. The two ways to
> > achieve this are: (a) WAL log a special NOOP record just before
> > shutting down checkpointer. Then allow the WALsender to read the
> > sequence data and send it to the subscriber while decoding the new
> > NOOP record. (b) Similar to the previous idea but instead of WAL
> > logging a new record directly invokes a decoding callback after
> > walsender receives a request to shutdown which will allow pgoutput to
> > read and send required sequences. This approach has a drawback that we
> > are adding more work at the time of shutdown but note that we already
> > waits for all the WAL records to be decoded and sent before shutting
> > down the walsender during shutdown of the node.
>
> At the time of shutdown a) most logical upgrades don't necessarily call
> for shutdown
>

Won't the major version upgrade expect that the node is down? Refer to
step "Stop both servers" in [1].

>
 b) it will still add to total downtime with large set of
> sequences. Incremental option is better as it will not require a shutdown.
>
> I do see a scenario where sequence of events can lead to loss of sequence
> and generate duplicate sequence values, if subscriber starts consuming
> sequences while publisher is also consuming them. In such cases, subscriber
> shall not be allowed sequence consumption.
>

It would be fine to not allow subscribers to consume sequences that
are being logically replicated but what about the cases where we
haven't sent the latest values of sequences before the shutdown of the
publisher? In such a case, the publisher would have already consumed
some values that wouldn't have been sent to the subscriber and now
when the publisher is down then even if we re-allow the sequence
values to be consumed from the subscriber, it can lead to duplicate
values.

[1] - https://www.postgresql.org/docs/devel/pgupgrade.html

-- 
With Regards,
Amit Kapila.