Re: Allow async standbys wait for sync replication

On Tue, Mar 01, 2022 at 11:09:57PM +0530, Bharath Rupireddy wrote:
> On Tue, Mar 1, 2022 at 10:35 PM Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
>> Yes, perhaps the synchronous replication framework will need to alert WAL
>> senders when the syncrep LSN advances so that the WAL is sent to the async
>> standbys. I'm glossing over the details, but I think that should be the
>> general direction.
>
> It's doable. But we can't avoid async walsenders waiting for the flush
> LSN even if we take the SyncRepReleaseWaiters() approach right? I'm
> not sure (at this moment) what's the biggest advantage of this
> approach i.e. (1) backends waking up walsenders after flush lsn is
> updated vs (2) walsenders keep looking for the new flush lsn.

I think there are a couple of advantages. For one, spinning is probably
not the best from a resource perspective. There is no guarantee that the
desired SendRqstPtr will ever be synchronously replicated, in which case
the WAL sender would spin forever. Also, this approach might fit in better
with the existing synchronous replication framework. When a WAL sender
realizes that it can't send up to the current "flush" LSN because it's not
synchronously replicated, it will request to be alerted when it is. In the
meantime, it can send up to the latest syncrep LSN so that the async
standby is as up-to-date as possible.

--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com