Re: Sync Rep v19

On Sat, Mar 5, 2011 at 2:05 PM, Robert Haas <robertmhaas@gmail.com> wrote:

> On Sat, Mar 5, 2011 at 7:49 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
> > If the order is arbitrary, why does it matter if it changes?
> >
> > The user has the power to specify a sequence, yet they have not done so.
> > They are told the results are indeterminate, which is accurate. I can
> > add the words "and may change as new standbys connect" if that helps.
>
> I just don't think that's very useful behavior.  Suppose I have a
> master and two standbys.  Both are local (or both are remote with
> equally good connectivity).  When one of the standbys goes down, there
> will be a hiccup (i.e. transactions will block trying to commit) until
> that guy falls off and the other one takes over.  Now, when he comes
> back up again, I don't want the synchronous standby to change again;
> that seems like a recipe for another hiccup.  I think "who the current
> synchronous standby is" should act as a tiebreak.
>

+1

TLDR part:

The first one might be noticed by users because it takes tens of seconds
before the sync switch. The second hiccup is hardly noticable. However
limiting the # switches of sync standby to the absolute minimum is also good
if e.g. (if there was a hook for it) cluster middleware is notified of the
sync replica change. That might either introduce a race condition or be even
completely unreliable if the notify is sent asynchronous, or it might
introduce a longer lag if the master waits for confirmation of the sync
replica change message. At that point sync replica changes become more
expensive than they are currently.

regards,
Yeb Havinga