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Improved Sharded Counter for Google AppEngine

From: andrew cooke <andrew@...>

Date: Thu, 4 Aug 2011 20:59:55 -0400

This is based on the code in the examples project, but has some fixes so that
it bootstraps properly after memcache is flushed and the like.



# Copyright 2008 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
# Modified / extended / fixed by andrew@... to work across
# re-deploys, memcache flushes, etc.

from google.appengine.api import memcache 
from google.appengine.ext import db
import random


class GeneralCounterShardConfig(db.Model):
    """Tracks the number of shards for each named counter."""
    name = db.StringProperty(required=True)
    num_shards = db.IntegerProperty(required=True, default=20)


class GeneralCounterShard(db.Model):
    """Shards for each named counter"""
    name = db.StringProperty(required=True)
    count = db.IntegerProperty(required=True, default=0)


def get_count(name):
    """Retrieve the value for a given sharded counter.

    Parameters:
      name - The name of the counter
    """
    total = memcache.get('counter:' + name)
    if total is None:
        total = 0
        for counter in GeneralCounterShard.all().filter('name = ', name):
            total += counter.count
        memcache.add(name, str(total), 60, namespace='counter')
    else:
        total = int(total)
    return total


def increment(name):
    """Increment the value for a given sharded counter.

    Parameters:
      name - The name of the counter
    """
    config = GeneralCounterShardConfig.get_or_insert(name, name=name)

    def txn():
        index = random.randint(0, config.num_shards - 1)
        shard_name = name + str(index)
        counter = GeneralCounterShard.get_by_key_name(shard_name)
        if counter is None:
            counter = GeneralCounterShard(key_name=shard_name, name=name)
        counter.count += 1
        counter.put()

    db.run_in_transaction(txn)
    value = memcache.incr(name, namespace='counter')
    if value is None:
        value = get_count(name)
    return value


def increase_shards(name, num):
    """Increase the number of shards for a given sharded counter.
    Will never decrease the number of shards.

    Parameters:
      name - The name of the counter
      num - How many shards to use

    """
    config = GeneralCounterShardConfig.get_or_insert(name, name=name)

    def txn():
        if config.num_shards < num:
            config.num_shards = num
            config.put()

    db.run_in_transaction(txn)

total = memcache.get('counter:' + name)

From: Steve Olechowski <sjo@...>

Date: Thu, 20 Sep 2012 14:16:29 -0500

from your improved sharded counter post, won't this line:

total = memcache.get('counter:' + name)


always be None because you are setting it everywhere else with   add(
name, namespace='counter') ?  I could be misunderstanding something...

i appreciate you sharing your work here as I have the same needs for a
more resiliant sharded counter!

Thanks!

Steve Olechowski

Re: total = memcache.get('counter:' + name)

From: andrew cooke <andrew@...>

Date: Fri, 21 Sep 2012 08:03:53 -0300

I no longer use this code (or Google App Engine), and I don't remember much
about it, but it certainly look slike you are right.

My best guess is that I modified the code to use namespace (after starting
with a manual prefix), and updated the post, but forgot to update that line.

Thanks for pointing it out,
Andrew

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