Hi MG,
Sharded counters help solve the concurrent write contention for an
entity.
Time (to the accuracy of milliseconds) helps to provide a one way
arrow towards a unique value.
Each request itself could be associated with an identity (this is up
to you, perhaps time in the request header).
The combination of all 3 may lead to a guaranteed unique number.
Cheers,
Nichole
(p.s. If you do use sharded counters in your approach, you can
configure the datastore
read consistency to be strong:
http://code.google.com/appengine/docs/java/datastore/usingjdo.html)
On Dec 28, 8:33 pm, MG <michael.glas...@gmail.com> wrote:
> Yes, keys are unique, but they are strings, and quite long (20+
> characters, from what I've seen). I do need unique and "growing"
> numeric counter.
>
> On Dec 28, 11:20 pm, nischalshetty <nischalshett...@gmail.com> wrote:
>
>
>
>
>
>
>
> > Have a look at the Datastore "Key" thingy. The keys generated are
> > unique across the system IMO. Not sure though but that's what I
> > remember reading somewhere.
>
> > -N
>
> > On Dec 29, 7:43 am, MG <michael.glas...@gmail.com> wrote:
>
> > > Hello!
>
> > > I need to create a global atomic counter (64-bit) in my AppEngine/J
> > > app. Like
>
> > > long count = MyCounter.increment()
>
> > > that will 100% guarantee that count will never ever be the same for
> > > two different requests, and that I will be able to increment it
> > > several million times/day initially, and much more if/when traffic
> > > increases. I do not really care if it skips a number or two
> > > occasionally: I can live with it returning a value larger than the
> > > actual number of calls, but I do need absolute uniqueness and
> > > reasonably linear growth (i.e. is two consecutive calls from a client
> > > should result in ascending counter values).
>
> > > Is this possible to do with Google AppEngine? Sharded counters can
> > > ensure consistent counting, but not unique counts; memcache counters
> > > can ensure unique increments, but they are perishable and thus
> > > difficult (impossible?) to properly synchronize with persistent
> > > storage. Using one entity to read-update in a transaction will not
> > > scale...
>
> > > Thanks,
> > > MG
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