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copied!<p>You have many ways to single-thread this kind of thing.</p> <p>One standard approach is <strong>Update First</strong>. You do an update which will seize an exclusive lock on the row; then do your work; and finally commit the change. For this to work, you need to bypass the ORM's caching. </p> <p>Another standard approach is to have a separate, single-threaded application server that isolates the Web transactions from the complex calculation. </p> <ul> <li><p>Your web application can create a queue of scoring requests, spawn a separate process, and then write the scoring requests to this queue. The spawn can be put in Django's <code>urls.py</code> so it happens on web-app startup. Or it can be put into separate <code>manage.py</code> admin script. Or it can be done "as needed" when the first scoring request is attempted.</p></li> <li><p>You can also create a separate WSGI-flavored web server using Werkzeug which accepts WS requests via urllib2. If you have a single port number for this server, requests are queued by TCP/IP. If your WSGI handler has one thread, then, you've achieved serialized single-threading. This is slightly more scalable, since the scoring engine is a WS request and can be run anywhere.</p></li> </ul> <p>Yet another approach is to have some other resource that has to be acquired and held to do the calculation. </p> <ul> <li><p>A Singleton object in the database. A single row in a unique table can be updated with a session ID to seize control; update with session ID of <code>None</code> to release control. The essential update has to include a <code>WHERE SESSION_ID IS NONE</code> filter to assure that the update fails when the lock is held by someone else. This is interesting because it's inherently race-free -- it's a single update -- not a SELECT-UPDATE sequence.</p></li> <li><p>A garden-variety semaphore can be used outside the database. Queues (generally) are easier to work with than a low-level semaphore.</p></li> </ul>

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