Re: Minimal logical decoding on standbys

Hi,

On 4/5/23 2:33 AM, Jeff Davis wrote:
> On Tue, 2023-04-04 at 14:55 -0400, Robert Haas wrote:
>> Thanks for your continued work on $SUBJECT. I just took a look at
>> 0004,

Thanks Robert for the feedback!

>> and I think that at the very least the commit message needs
>> work.

Agree.

> Perhaps a commit message like:
>
> "For cascading replication, wake up physical walsenders separately from
> logical walsenders.
>
> Physical walsenders can't send data until it's been flushed; logical
> walsenders can't decode and send data until it's been applied. On the
> standby, the WAL is flushed first, which will only wake up physical
> walsenders; and then applied, which will only wake up logical
> walsenders.
>
> Previously, all walsenders were awakened when the WAL was flushed. That
> was fine for logical walsenders on the primary; but on the standby the
> flushed WAL would not have been applied yet, so logical walsenders were
> awakened too early."

Thanks Jeff for the commit message proposal! It looks good to me
except that I think that "flushed WAL could have been not applied yet" is better than
"flushed WAL would not have been applied yet" but it's obviously open to discussion.

Currently changed it that way and used it in V60 shared up-thread.

>
> For comments, I agree that WalSndWakeup() clearly needs a comment
> update. The call site in ApplyWalRecord() could also use a comment. You
> could add a comment at every call site, but I don't think that's
> necessary if there's a good comment over WalSndWakeup().

Agree, added a comment over WalSndWakeup() and one before calling WalSndWakeup()
in ApplyWalRecord() in V60.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com