Re: 2-phase commit

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Could we allow slaves to check if the backend is still alive, perhaps by
> > asking the postmaster, similar to what we do with the cancel signal ---
> > that way, the slave would never time out and always wait if the master
> > was alive.
> 
> You're not considering the possibility of a transient communication
> failure.  The fact that you cannot currently contact the other guy
> is not proof that he's not still alive.
> 
> Example:
> 
>     Master        Slave
>     ------        -----
>     commit ready-->
>             <--OK
>     commit done->XX
> 
> where "->XX" means the message gets lost due to network failure.  Now
> what?  The slave cannot abort; he promised he could commit, and he does
> not know whether the master has committed or not.  The master does not
> know the slave's state either; maybe he got the second message, and
> maybe he didn't.  Both sides are forced to keep information about the 
> open transaction indefinitely.  Timing out on either side could yield
> the wrong result.

Can't the master re-send the request after a timeout?

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