GAE数据存储区中的运行条件

Question

class ModelCount(db.Model):
    type = db.StringProperty(required=True,default='Default-type') 
    count = db.IntegerProperty(required=True, default=0) #Current counter

    def to_dict(self):
       d = dict([(p, unicode(getattr(self, p))) for p in self.properties()])
       d["id"] = self.key().id()
       return d

    #Increments counter
    @staticmethod
    def increment_counter(en_name):
         modelCount = ModelCount.all().filter('type',en_name).get()
         if modelCount:
            modelCount.count += 1
            modelCount.put()
         else:
            modelCount = ModelCount(type=en_name, count=1)
            modelCount.put()

In the above code ( increment_counter ), I am reading the count from ModelCount and incrementing it by one. I face run condition in increment_counter method when the server receives multiple requests. So I want to make increment_counter atomic. If I use @db.transactional on increment_counter , I get "Only ancestor queries are allowed inside transactions" error.

How can I fix this and make it atomic?

Answer 1

you could try to use sharding https://developers.google.com/appengine/articles/sharding_counters

full source available at https://github.com/GoogleCloudPlatform/appengine-sharded-counters-python