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POHigh-concurrency counters without sharding
 

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    <p>This question concerns two implementations of counters which are intended to scale without sharding (with a tradeoff that they might under-count in some situations):</p> <ol> <li><a href="http://appengine-cookbook.appspot.com/recipe/high-concurrency-counters-without-sharding/" rel="noreferrer">http://appengine-cookbook.appspot.com/recipe/high-concurrency-counters-without-sharding/</a> (the code in the comments)</li> <li><a href="http://blog.notdot.net/2010/04/High-concurrency-counters-without-sharding" rel="noreferrer">http://blog.notdot.net/2010/04/High-concurrency-counters-without-sharding</a></li> </ol> <h3>My questions:</h3> <ul> <li>With respect to #1: Running <code>memcache.decr()</code> in a deferred, transactional task seems like overkill. If <code>memcache.decr()</code> is done outside the transaction, I think the worst-case is the transaction fails and we miss counting whatever we decremented. <em>Am I overlooking some other problem that could occur by doing this?</em></li> <li><em>What are the significiant tradeoffs between the two implementations?</em></li> </ul> <h3>Here are the tradeoffs I see:</h3> <ul> <li><h1>2 does not require datastore transactions.</h1></li> <li>To get the counter's value, #2 requires a datastore fetch while with #1 <i>typically</i> only needs to do a <code>memcache.get()</code> and <code>memcache.add()</code>.</li> <li>When incrementing a counter, both call <code>memcache.incr()</code>. Periodically, #2 adds a task to the task queue while #1 transactionally performs a datastore get and put. #1 also always performs <code>memcache.add()</code> (to test whether it is time to persist the counter to the datastore). </li> </ul> <h3><i>Conclusions</i></h3> <p>(without actually running any performance tests):</p> <ul> <li><h1>1 should typically be faster at retrieving a counter (#1 memcache vs #2 datastore). Though #1 has to perform an extra <code>memcache.add()</code> too.</h1></li> <li>However, #2 should be faster when updating counters (#1 datastore get+put vs #2 enqueue a task).</li> <li>On the other hand, with #1 you have to be a bit more careful with the update interval since the task queue quota is almost 100x smaller than either the datastore or memcahce APIs.</li> </ul>
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