Re: Allow async standbys wait for sync replication

On Sun, Mar 6, 2022 at 1:57 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,
>
> On 2022-03-05 14:14:54 +0530, Bharath Rupireddy wrote:
> > I understand. Even if we use the SyncRepWaitForLSN approach, the async
> > walsenders will have to do nothing in WalSndLoop() until the sync
> > walsender wakes them up via SyncRepWakeQueue.
>
> I still think we should flat out reject this approach. The proper way to
> implement this feature is to change the protocol so that WAL can be sent to
> replicas with an additional LSN informing them up to where WAL can be
> flushed. That way WAL is already sent when the sync replicas have acknowledged
> receipt and just an updated "flush/apply up to here" LSN has to be sent.

I was having this thought back of my mind. Please help me understand these:
1) How will the async standbys ignore the WAL received but
not-yet-flushed by them in case the sync standbys don't acknowledge
flush LSN back to the primary for whatever reasons?
2) When we say the async standbys will receive the WAL, will they just
keep the received WAL in the shared memory but not apply or will they
just write but not apply the WAL and flush the WAL to the pg_wal
directory on the disk or will they write to some other temp wal
directory until they receive go-ahead LSN from the primary?
3) Won't the network transfer cost be wasted in case the sync standbys
don't acknowledge flush LSN back to the primary for whatever reasons?

The proposed idea in this thread (async standbys waiting for flush LSN
from sync standbys before sending the WAL), although it makes async
standby slower in receiving the WAL, it doesn't have the above
problems and is simpler to implement IMO. Since this feature is going
to be optional with a GUC, users can enable it based on the needs.

Regards,
Bharath Rupireddy.