Oups, it works :) sorry...
On 15 juin, 17:31, Sylvain <sylvain.viv...@gmail.com> wrote:
> I wonder if there is a bug in slide 40 :
>
> -------
> class CounterHandler(webapp.RequestHandler):
> def post(self):
> key = self.request.get('key')
> if (memcache.incr(key) is None and not memcache.add(key, 1)):
> memcache.incr(key)
>
> if memcache.add(key + '_dirty', 1):
> taskqueue.add(url='/worker',
> params={'key': key})
> -------
>
> I wonder how "memcache.incr(key)" can be executed. ?
>
> because if (memcache.incr(key) is None and not memcache.add(key, 1))
> == true
> then the key doesn't exist ?
>
> On 15 juin, 17:17, Stephen <sdea...@gmail.com> wrote:
>
> > On Jun 12, 2:19 pm, peterk <peter.ke...@gmail.com> wrote:
>
> > > Hey all..
>
> > > Just wondering if I picked this up wrong or if this would indeed need
> > > some changing. Looking at the counter example in this presentation:
>
> > >http://code.google.com/events/io/sessions/OfflineProcessingAppEngine....
>
> > > (From slide 37 in the PDF)
>
> > > Basically, is this example not depending on cache being persistent in
> > > order to work? You're incrementing the counter in cache, then
> > > periodically persisting it to the DB, but if the cache drops your
> > > counter here, you're basically going to reset your counter (and thus
> > > reset the counter in the DB on the next queued update).
>
> > No, IIRC they take this into account.
>
> > If the cache is flushed then the next call to increment will fail,
> > because the memcache API does not automatically start from zero when
> > incrementing a non-existing key.
>
> > At this point you query the db to re-seed the cache, but there is
> > another potential race condition: you have to set the new value of the
> > cache such that if it already exists (another app instance
> > concurrently beat you to the update because it also noticed a missing
> > cache entry) you don't overwrite it.
>
> > At this point the the cache has a value and you can retry the
> > increment call.
>
> > So depending on cache flushing and concurrent activity, incrementing
> > the cache value can be up to a 3-step process.
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