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    <p>Here's what I'd do (I'm actually considering writing this library after seeing your question. I need it too):</p> <p>Create a <code>taskpool</code> module with the following API.</p> <pre><code>client_id, token = taskpool.get() # Setup a heartbeat in the client JS, maybe every minute. # Also call this every time the client indicates presence taskpool.ping(client_id) taskpool.release(client_id) </code></pre> <p>Implementation: </p> <ul> <li>Store the <code>client_id</code> and <code>token</code> in an entity, with a status indicating whether it's being used, last ping time, and creation time. Let the <code>client_id</code> be the key. Also consider using <a href="http://code.google.com/p/appengine-ndb-experiment/" rel="noreferrer">NDB</a>. Free memcaching. </li> </ul> <p><code>get()</code> checks if there are unused tokens and returns one if it finds it. Otherwise create a new one, store and return it. </p> <p><code>ping()</code> updates the last ping time for that token. Instead of polling, let the client send in a ping every [heartbeat] time. </p> <p><code>release()</code> marks the token as unused. </p> <p>Run a task / cron every [heartbeat] seconds to find the tokens which haven't gotten a ping in a while - and set them as unused. </p> <p>When clients report a closed token, perform a <code>get()</code>.</p> <p>Keep in mind, though, that a loss in security is a by-product of any kind of token pooling. If a malicious client has held on to a token and stopped sending heartbeats, it might later be able to listen in on the messages being passed to the new client once the token is re-purposed. This isn't a problem if you're on a fully public site, but keep it in mind anyway.</p> <p>I will update this answer if and when I write this up as a library. </p>
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    1. COThanks for the creative answer Sudhir, but what is NDB? As far I know the existing App Engine Memcache API was free already? This definitely sounds like a viable, fully-featured solution, but I'd be very curious to see these how these multiple moving parts (crons/tasks and datastore operations) perform in action. Please let us know if you do wind up starting a library (especially if you open up an open-source repository :) ).
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    2. CONDB is the new datastore API that's in 1.6.0 onwards:http://code.google.com/p/appengine-ndb-experiment/. Updated the answer with a link.
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    3. COOh, and by 'free memcaching' I meant NDB takes care of caching for you. No extra lines of code.
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