I think there would have to be some divergence between what the counter
should be and what the user will actually see at any given time.. since if
you have a high rate of counts happening for a user.. you'd get contention
when trying to update all the count each time the event being counted
happened.
Of course, you know that part.. since they have all these sharding examples.
So, you gotta decide how stale the count can be...
Once you decide that, since you don't seem to want any potential loss of
counts... you'd probably need two Models to do counting for each user.
(memcache would be out since that allows potential lost counts)
One for each individual count inserted (call it UserCounter) and one for the
count that the user sees (UserTotalCount).
So, if a count-event happens you insert a new row into UserCounter.
Then you should have a task that runs that selects __key__ from UserCounter,
finds out how many entities were returned, updates the UserTotalCount model
with the additional counts, and once that update is successful, it deletes
the keys, entities for those counts that it selected. AND then, once all of
that is successful, have the Task essentially schedule itself to run again
in N seconds or however long you've decided to give it.
Presumably, doing it this way would allow you to make sure that the
counterupdate task is running one at a time for each user (since it can only
start itself up again after it is done counting).. and you would avoid write
contention since the task is the only thing updating the user's counter.
Probably, you could set up two Expando models to do this for all users.. and
each time a new user was created, you'd add a new Column to the Expando
models for that user.
so, you'd have these initial definitions:
class UserCounter(db.Expando):
BobCountEvent = db.BooleanProperty(required=True)
class UserTotalCount(db.Expando):
BobTotalCount = db.IntegerProperty(required=True)
Then, each time user Bob has a count event you do:
bobCount = UserCounter(BobCountEvent = True)
bobCount.put()
And when you want to update Bob's Total Count, you do (I have to do this
quasi-pseudo since it isn't trivial to do):
results = Select __key__ from UserCounter Where BobCountEvent = True
If len(results) > 0:
countResult = Select * from UserTotalCount Where BobTotalCount >= 0
if len(countResult) > 0:
countResult.BobTotalCount += len(results)
db.put(countResults)
else:
newCount = UserTotalCount(BobTotalCount = len(results))
newCount.put()
db.delete(results)
Now, you might wonder... how do I do puts for variable user names? (You can'
t just create new put functions for each new user).. In Python, you can use
exec to do that..
I have not tested how any of this performs... having an expando model may
hurt performance.. but, I don't think so, and I know the method works for
other things (not sure how it'd do on this counter method).
See here for Google's sharded counts example:
http://code.google.com/appengine/articles/sharding_counters.html
On Sat, Nov 28, 2009 at 5:17 PM, peterk <[email protected]> wrote:
> Hey all,
>
> I've been looking at the Task Queue API and counter example. In my
> app, each user will have a couple of counters maintained for them,
> counting various things.
>
> Thing is, these counters need to be accurate. So I'm not sure if the
> example given for the Task Queue API using memcache would be
> appropriate for me - it would not be good, really, if my counters were
> to be inaccurate. My users would expect accurate counts here.
>
> So I was thinking about a sort of modified version whereby each change
> to the counter would be stored in the DS in its own entity. E.g. an
> entity called 'counter_delta' or some such, which holds the delta to
> apply to a counter, and the key to the counter that the delta is to be
> applied to.
>
> Then, using the Task Queue I guess I could hoover up all these delta
> entities, aggregate them, and apply them in one go (or in batches) to
> the counter. And then delete the delta entries.
>
> Thus the task queue is the only thing accessing the counter entity,
> and it does so in a controllable fashion - so no real contention. Each
> change to the counter gets written to the store in its own
> counter_delta entity, so no contention there either. And because the
> deltas are stored in DS and not in memcache, it should be much more
> reliable.
>
> However, I'm not entirely sure how I should actually go about
> implementing this, or specifically, the task queue end of things.
>
> I'm thinking if there is a change to a counter to be made, I should
> check if there's a task already running for this counter, and if so,
> not to do insert any new task, and let the currently running task take
> care of it. If there is no running task for this counter, I would
> instead create one, and set it to run in - say - 60 seconds, allowing
> time for further deltas for this counter to accumulate so the task can
> take care of more than just one delta. This would mean the counter
> might be inaccurate for up to 60 seconds, but I can live with that.
>
> But what I'm wondering is, how can I implement this 'don't insert a
> new task if one for this counter is already in the queue or running'
> behaviour?
>
> I was thinking initially that I could give the task a name based on
> the counter, so that only one such task can exist at any one time.
> However, I believe we have no control over when that name is freed up
> - it isn't necessarily freed up when the task ends, I believe names
> can be reserved for up to 7 days (?) So that wouldn't work. If a name
> could be freed up when a task was really finished then this could
> work, I think.
>
> I was thinking also I could store a flag so that when a counter_delta
> is created, I'd look to see if a flag for this counter was present,
> and if so, do nothing. If not, create the task, and create the flag.
> Then when the task was all done and didn't see any more
> counter_deltas, it'd delete the flag. But I'm worried that there could
> be race conditions here, and some deltas might get overlooked as a
> result? And if I were to use transactions on such a flag, would I not
> fall into the same contention trap I'm trying to avoid in the first
> place?
>
> Help? :| Thanks for any advice/insight...
>
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>
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