except for the race condition which might hardly ever be noticed or might be quite an annoyance that other users of memcached don't run into
I didn't get that race condition anyway. Especially the "flush to memcached".
What i'm doing in my forum at the moment is storing a whole thread in memcached as a data structure. Expiry is set to 3 minutes (could also be more, just because I'm storing also user info and signatures, it's set to 3 minutes, so that user info changes get updated faster).
Now, whenever somebody posts a new node or updates their node, the cache entry is explicitly expired immediately. Database transaction, after that delete thread cache entry, then redirect the user to the updated thread or node. And the request for the thread then reads from the database and creates the cache entry again.
Your use of memcached seems a bit different, if I understood that correctly? So the wrong flush to memcached wouldn't happen in my case.

edit: yes, I think that's the point - in the second block of server X you are flushing N2 to memcached, but the current version in the databse is N4. If creating the cache entry only when fetching a thread the RC shouldn't be there.

edit 2:
or maybe I can still create an RC:

X: reads thread with node N1
X: votes N1 -> N2, delete cache entry
X: redirect after POST and reads N2 from DB...

Y: author updates node from N2 to N3
Y: delete cache entry, if exists
Y: redirect after POST and reads N3 from DB
Y: cache N3 to memcached

X: ... cache N2 to memcached

[download]
But that version must have a whole transaction and redirect and memcached operation in between that X blocks. of course, it's still possible, but extremely improbable. Could maybe be prevented by putting the read and creating the cache entry into a transaction with read lock?

In reply to Re^3: PerlMonks Caching (data) by tinita
in thread PerlMonks Caching by ahmad

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