Sharded, memcached counter for Google App Engine with error tolerance
 
Copyright 2008, William T Katz.
Released under Apache License, version 2.0
 
Should be used for counters that handle a lot of concurrent use.
Follows pattern described in Google I/O talk:
    http://sites.google.com/site/io/building-scalable-web-applications-with-google-app-engine
 
Memcache is used for caching counts and if a cached count is available, it is
the most correct. If there are datastore put issues, we store the un-put 
values into a delayed_incr memcache that will be applied as soon as the next shard put is successful. Changes will only be lost if we lose memcache before 
a successful datastore shard put or there's a failure/error in memcache.
 
Usage:
    hits = Counter('hits')
    hits.increment()
    my_hits = hits.count
    hits.get_count(nocache=True)  # Forces non-cached count of all shards
    hits.count = 6                # Set the counter to arbitrary value
    hits.increment(incr=-1)       # Decrement
    hits.increment(10)
 
For an example of its use (early version), you can see the Tag model in the 
Bloog open-source blog software for App Engine 
(http://github.com/DocSavage/bloog).