Re: 2-phase commit

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Tom Lane wrote:
> >> You're not considering the possibility of a transient communication
> >> failure.
> 
> > Can't the master re-send the request after a timeout?
> 
> Not "it can", but "it has to".  The master *must* keep hold of that
> request forever (or until the slave responds, or until we reconfigure
> the system not to consider that slave valid anymore).  Similarly, the
> slave cannot forget the maybe-committed transaction on pain of not being
> a valid slave anymore.  You can make this work, but the resource costs
> are steep.  For instance, in Postgres, you don't get to truncate the WAL
> log, for what could be a really really long time --- more disk space
> than you wanted to spend on WAL anyway.  The locks held by the
> maybe-committed transaction are another potentially unpleasant problem;
> you can't release them, no matter what else they are blocking.

I think we would need a configurable timeout to say a slave is no longer
valid, like 60 seconds, and then let everyone release.  We can let the
administrator decide how long he wants to try to keep two hosts
communicating.  I don't see this as much different from multi-master
replication problems.

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