Re: pg_receivexlog and feedback message

On Tue, Jun 5, 2012 at 9:53 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> Right now, pg_receivexlog sets:
>                        replymsg->write = InvalidXLogRecPtr;
>                        replymsg->flush = InvalidXLogRecPtr;
>                        replymsg->apply = InvalidXLogRecPtr;
>
> when it sends it's status updates.
>
> I'm thinking it sohuld set replymsg->write = blockpos instad.
>
> Why? That way you can see in pg_stat_replication what has actually
> been received by pg_receivexlog - not just what we last sent. This can
> be useful in combination with an archive_command that can block WAL
> recycling until it has been saved to the standby. And it would be
> useful as a general monitoring thing as well.
>
> I think the original reason was that it shouldn't interefer with
> synchronous replication - but it does take away a fairly useful
> usecase...

I think that not only replaymsg->write but also ->flush should be set to
blockpos in pg_receivexlog. Which allows pg_receivexlog to behave
as synchronous standby, so we can write WAL to both local and remote
synchronously. I believe there are some use cases for synchronous
pg_receivexlog.

OTOH, neither replaymsg->write nor ->flush should be set to
InvalidXLogRecPtr, to prevent pg_basebackup from behaving as
synchronous standby.

Regards,

--
Fujii Masao