gae_high_performance_counters.py

def incrementCounter(key, update_interval=10):
    """Increments a memcached counter.
    Args:

      key: The key of a datastore entity that contains the counter.
      update_interval: Minimum interval between updates.
    """
    lock_key = "counter_lock:%s" % (key,)
    count_key = "counter_value:%s" % (key,)

    if memcache.add(lock_key, None, time=update_interval):
        # Time to update the DB
        count = int(memcache.get(count_key) or 0) + 1

      def tx():
          entity = db.get(key)
          entity.counter += count
          entity.put()
          deferred.defer(memcache.decr, count_key, delta=count, _transactional=True)
      try:
          db.run_in_transaction(tx)
      except:
          memcache.incr(count_key, initial_value=0)
    else:
        # Just update memcache
        memcache.incr(count_key, initial_value=0)

The initial commit fe5edc is the original recipe by Nick Johnson: http://appengine-cookbook.appspot.com/recipe/high-concurrency-counters-without-sharding

Commit c3fc97 is the revised recipe by evlogimenos.

Commit 8b1063 is the revised recipe by me. It includes a doctest which can be run with nosegae.