Re: Small fixes needed by high-availability tools

On Fri, May 2, 2025 at 6:30 PM Andrey Borodin <x4mmm@yandex-team.ru> wrote:
>
> 3. Allow reading LSN written by walreciever, but not flushed yet
>
> Problem: if we have synchronous_standby_names = ANY(node1,node2), node2 might be ahead of node1 by flush LSN, but before by written LSN. If we do a failover we choose node2 instead of node1 and loose data recently committed with synchronous_commit=remote_write.
>

In which case, can we rely on written WAL that is not yet flushed?
Because say you decide based on written WAL and choose node-1 in above
case for failover, what if it restarts without flushing the written
WAL?

> Caveat: we already have a function pg_last_wal_receive_lsn(), which in fact returns flushed LSN, not written. I propose to add a new function which returns LSN actually written. Internals of this function are already implemented (GetWalRcvWriteRecPtr()), but unused.
>

It seems to me that this is less controversial than your other two
proposals. So, we can discuss this in a separate thread as well.

-- 
With Regards,
Amit Kapila.