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 <[email protected]> wrote:
> On Jun 12, 2:19 pm, peterk <[email protected]> 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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