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).