Re: Postgresql Caching

On Sun, 15 Oct 2006, mark(at)mark(dot)mielke(dot)cc wrote:

> On Sun, Oct 15, 2006 at 08:31:36PM +0530, Merlin Moncure wrote:
> > On 10/15/06, Anon Mous <soundami(at)yahoo(dot)com> wrote:
> > > Would it be possible to combine a special memcache implementation of
> > > memcache with a Postgresql interface wrapper?
> > have you seen
> > http://people.freebsd.org/~seanc/pgmemcache/
>
> Interesting. I note that they don't address the view consistency
> problem any better than an application using memcached directly.
> And that's the real problem with memcached, and why people are
> tempted to 'indulge' by relying on PostgreSQL. Some people value
> the consistency. Others don't. memcached, whether application-side,
> or whether automatically invoked by triggers (pgmemcache) is a
> decision to ignore the consistency.
>
> Using memcache, I've had problems with consistency brought right to
> the front. Both of these have failed me:
>
> 1) When updating a PostgreSQL record, I invalidate the memcache record.
> If another process comes along in parallel before I commit, notices
> that the memcache record is invalidated, it queries the data from
> SQL, and updates the memcache record back to the old value. :-(
>
> 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?

> I'm currently settled on 2), but setting a short timeout (5 seconds) on
> the data. Still an imperfect compromise between speed and accuracy, but
> it isn't causing me problems... yet.

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