Re: Postgresql Caching

On Mon, 16 Oct 2006, mark(at)mark(dot)mielke(dot)cc wrote:

> On Sun, Oct 15, 2006 at 06:33:36PM -0700, Jeremy Drake wrote:
> > > 2) When updating a PostgreSQL record, I updated the memcache record
> > > to the new value. If another process comes along in parallel before
> > > I commit, that is still looking at an older view, cross-referencing
> > > may not work as expected.
> > Shouldn't you be able to use 2-stage commit for this? Prepare to commit,
> > update the memcache record, then commit? Or am I thinking of something
> > else?
>
> Two stage commits makes the window of error smaller, it can't eliminate it.

Right, I was thinking there was still some raciness there. I think what I
remembered is that if you updated the cache and then the transaction
failed (or rolled back for whatever reason) later on, the cache would have
data that was never committed. The two-phase commit thing is intended to
deal with that eventuality. Which is also a possibility for a consistency
issue.

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