sharded-counters/generalcounter.py

   1 # Copyright 2008 Google Inc.
   2 #
   3 # Licensed under the Apache License, Version 2.0 (the "License");
   4 # you may not use this file except in compliance with the License.
   5 # You may obtain a copy of the License at
   6 #
   7 #     http://www.apache.org/licenses/LICENSE-2.0
   8 #
   9 # Unless required by applicable law or agreed to in writing, software
  10 # distributed under the License is distributed on an "AS IS" BASIS,
  11 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  12 # See the License for the specific language governing permissions and
  13 # limitations under the License.
  14 #
  15
  16 from google.appengine.api import memcache
  17 from google.appengine.ext import db
  18 import random
  19
  20 class GeneralCounterShardConfig(db.Model):
  21   """Tracks the number of shards for each named counter."""
  22   name = db.StringProperty(required=True)
  23   num_shards = db.IntegerProperty(required=True, default=20)
  24
  25
  26 class GeneralCounterShard(db.Model):
  27   """Shards for each named counter"""
  28   name = db.StringProperty(required=True)
  29   count = db.IntegerProperty(required=True, default=0)
  30
  31
  32 def get_count(name):
  33   """Retrieve the value for a given sharded counter.
  34
  35   Parameters:
  36     name - The name of the counter
  37   """
  38   total = memcache.get(name)
  39   if total is None:
  40     total = 0
  41     for counter in GeneralCounterShard.all().filter('name = ', name):
  42       total += counter.count
  43     memcache.add(name, total, 60)
  44   return total
  45
  46
  47 def increment(name):
  48   """Increment the value for a given sharded counter.
  49
  50   Parameters:
  51     name - The name of the counter
  52   """
  53   config = GeneralCounterShardConfig.get_or_insert(name, name=name)
  54   def txn():
  55     index = random.randint(0, config.num_shards - 1)
  56     shard_name = name + str(index)
  57     counter = GeneralCounterShard.get_by_key_name(shard_name)
  58     if counter is None:
  59       counter = GeneralCounterShard(key_name=shard_name, name=name)
  60     counter.count += 1
  61     counter.put()
  62   db.run_in_transaction(txn)
  63   # does nothing if the key does not exist
  64   memcache.incr(name)
  65
  66
  67 def increase_shards(name, num):
  68   """Increase the number of shards for a given sharded counter.
  69   Will never decrease the number of shards.
  70
  71   Parameters:
  72     name - The name of the counter
  73     num - How many shards to use
  74
  75   """
  76   config = GeneralCounterShardConfig.get_or_insert(name, name=name)
  77   def txn():
  78     if config.num_shards < num:
  79       config.num_shards = num
  80       config.put()
  81   db.run_in_transaction(txn)
  82