On 08.10.2010 01:25, Simon Riggs wrote:
> On Thu, 2010-10-07 at 13:44 -0400, Aidan Van Dyk wrote:
>
>> To get "non-stale" responses, you can only query those k=3 servers.
>> But you've shot your self in the foot because you don't know which
>> 3/10 those will be. The other 7 *are* stale (by definition). They
>> talk about picking the "caught up" slave when the master fails, but
>> you actually need to do that for *every query*.
>
> There is a big confusion around that point and I need to point out that
> statement isn't accurate. It's taken me a long while to understand this.
>
> Asking for k> 1 does *not* mean those servers are time synchronised.
> All it means is that the master will stop waiting after 3
> acknowledgements. There is no connection between the master receiving
> acknowledgements and the standby applying changes received from master;
> the standbys are all independent of one another.
>
> In a bad case, those 3 acknowledgements might happen say 5 seconds apart
> on the worst and best of the 3 servers. So the first standby to receive
> the data could have applied the changes ~4.8 seconds prior to the 3rd
> standby. There is still a chance of reading stale data on one standby,
> but reading fresh data on another server. In most cases the time window
> is small, but still exists.
>
> The other 7 are stale with respect to the first 3. But then so are the
> last 9 compared with the first one. The value of k has nothing
> whatsoever to do with the time difference between the master and the
> last standby to receive/apply the changes. The gap between first and
> last standby (i.e. N, not k) is the time window during which a query
> might/might not see a particular committed result.
>
> So standbys are eventually consistent whether or not the master relies
> on them to provide an acknowledgement. The only place where you can
> guarantee non-stale data is on the master.
Yes, that's a good point. Synchronous replication for load-balancing
purposes guarantees that when *you* perform a commit, after it finishes
it will be visible in all standbys. But if you run the same query across
different standbys, you're not guaranteed get same results. If you just
pick a random server for every query, you might even see time moving
backwards. Affinity is definitely a good idea for the load-balancing
scenario, but even then the anomaly is possible if you get re-routed to
a different server because the one you were bound to dies.
-- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com