Re: Logical Replication of sequences

On Tue, Jun 4, 2024 at 4:53 PM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> On Tue, Jun 4, 2024 at 4:27 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > 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.
>
> Thanks. IIUC, both of the above approaches decode the sequences during
> only shutdown. I'm wondering, why not periodically decode and
> replicate the published sequences so that the decoding at the shutdown
> will not take that longer?
>

Even if we decode it periodically (say each time we decode the
checkpoint record) then also we need to send the entire set of
sequences at shutdown. This is because the sequences may have changed
from the last time we sent them.

>
> I can imagine a case where there are tens
> of thousands of sequences in a production server, and surely decoding
> and sending them just during the shutdown can take a lot of time
> hampering the overall server uptime.
>

It is possible but we will send only the sequences that belong to
publications for which walsender is supposed to send the required
data. Now, we can also imagine providing option 2 (Alter Subscription
... Replicate Sequences) so that users can replicate sequences before
shutdown and then disable the subscriptions so that there won't be a
corresponding walsender.

--
With Regards,
Amit Kapila.