Re: Exit walsender before confirming remote flush in logical replication

At Wed, 28 Dec 2022 09:15:41 +0000, "Hayato Kuroda (Fujitsu)" <kuroda(dot)hayato(at)fujitsu(dot)com> wrote in
> > Another thing we can investigate here why do we need to ensure that
> > there is no pending send before shutdown.
>
> I have not done yet about it, will continue next year.
> It seems that walsenders have been sending all data before shutting down since ea5516,
> e0b581 and 754baa.
> There were many threads related with streaming replication, so I could not pin
> the specific message that written in the commit message of ea5516.
>
> I have also checked some wiki pages [1][2], but I could not find any design about it.
>
> [1]: https://wiki.postgresql.org/wiki/Streaming_Replication
> [2]: https://wiki.postgresql.org/wiki/Synchronous_Replication_9/2010_Proposal

If I'm grabbing the discussion here correctly, in my memory, it is
because: physical replication needs all records that have written on
primary are written on standby for switchover to succeed. It is
annoying that normal shutdown occasionally leads to switchover
failure. Thus WalSndDone explicitly waits for remote flush/write
regardless of the setting of synchronous_commit. Thus apply delay
doesn't affect shutdown (AFAICS), and that is sufficient since all the
records will be applied at the next startup.

In logical replication apply preceeds write and flush so we have no
indication whether a record is "replicated" to standby by other than
apply LSN. On the other hand, logical recplication doesn't have a
business with switchover so that assurarance is useless. Thus I think
we can (practically) ignore apply_lsn at shutdown. It seems subtly
irregular, though.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center