Re: 2-phase commit

On Saturday 27 September 2003 06:59, Tom Lane wrote:
> Christopher Kings-Lynne <chriskl@familyhealth.com.au> writes:
> >> ... You can make this work, but the resource costs
> >> are steep.
> >
> > So, after 'n' seconds of waiting, we abandon the slave and the slave
> > abandons the master.
>
> [itch...]  But you surely cannot guarantee that the slave and the master
> time out at exactly the same femtosecond.  What happens when the comm
> link comes back online just when one has timed out and the other not?
> (Hint: in either order, it ain't good.  Double plus ungood if, say, the
> comm link manages to deliver the master's "commit confirm" message a
> little bit after the master has timed out and decided to abort after all.)
>
> In my book, timeout-based solutions to this kind of problem are certain
> disasters.

I might be (well, am actually) a bit out of my depth here, but surely what 
happens is if you have machines A,B,C and *any* of them thinks machine C has 
a problem then it does. If C can still communicate with the others then it is 
told to reinitialise/go away/start the sirens. If C can't communicate then 
it's all a bit academic.

Granted, if you have intermittent problems on a link and set your timeouts 
badly then you'll have a very brittle system, but if A thinks C has died, you 
can't just reverse that decision.

--  Richard Huxton Archonet Ltd