Re: Synchronization levels in SR

On Tue, 2010-05-25 at 13:31 -0400, Robert Haas wrote:
> On Tue, May 25, 2010 at 1:10 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> > On Tue, 2010-05-25 at 11:52 -0500, Kevin Grittner wrote:
> >> Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> >> > Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> >> >> If we define robustness at the standby level then robustness
> >> >> depends upon unseen administrators, as well as the current
> >> >> up/down state of standbys. This is action-at-a-distance in its
> >> >> worst form.
> >> >
> >> > Maybe, but I can't help thinking people are going to want some
> >> > form of this. The case where someone wants to do sync rep to the
> >> > machine in the next rack over and async rep to a server at a
> >> > remote site seems too important to ignore.
> >>
> >> I think there may be a terminology issue here -- I took "configure
> >> by standby" to mean that *at the master* you would specify rules for
> >> each standby. I think Simon took it to mean that each standby would
> >> define the rules for replication to it. Maybe this issue can
> >> resolve gracefully with a bit of clarification?
> >
> > The use case of "machine in the next rack over and async rep to a server
> > at a remote site" would require the settings
> >
> > server.nextrack = synch
> > server.remotesite = async
> >
> > which leaves open the question of what happens when "nextrack" is down.
> >
> > In many cases, to give adequate performance in that situation people add
> > an additional server, so the config becomes
> >
> > server.nextrack1 = synch
> > server.nextrack2 = synch
> > server.remotesite = async
> >
> > We then want to specify for performance reasons that we can get a reply
> > from either nextrack1 or nextrack2, so it all still works safely and
> > quickly if one of them is down. How can we express that rule concisely?
> > With some difficulty.
>
> Perhaps the difficulty here is that those still look like per-server
> settings to me. Just maybe with a different set of semantics.

(Those are the per-server settings.)

--
Simon Riggs www.2ndQuadrant.com