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.
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