Re: Sync Rep v19

On Sat, 2011-03-05 at 14:44 +0100, Yeb Havinga wrote:
> On Sat, Mar 5, 2011 at 2:05 PM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
> On Sat, Mar 5, 2011 at 7:49 AM, Simon Riggs
> <simon(at)2ndquadrant(dot)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.

I'm not in favour.

If the user has a preferred order, they can specify it. If there is no
preferred order, how will we maintain that order?

What are the rules for maintaining this arbitrary order?

No, this is not something we need prior to commit, if ever.

--
Simon Riggs http://www.2ndQuadrant.com/books/
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