Chat with Django, Twisted and websockets – addendum 1

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The simplest (to write) solution to the data update race condition I refer to at the end of step 3 in the tutorial, is to use a lock to guard your writes. It’s not necessarily the correct one to use on a production server, but should get the tutorial working properly. To implement it, create a lock:

write_lock = thread.allocate_lock() #forcing synchronized writes between the various kinds of clients

And then use it to guard all of the udpates to shared data-structures (messages in the tutorial); for example:

        ...
        write_lock.acquire()
        self.factory.messages[float(time.time())] = data
        write_lock.release()
        ...
        write_lock.acquire()
        self.wsFactory.messages = self.messages
        write_lock.release()
        ...
        #and so on throughout the file.

You can get a version of the code for this tutorial, including this fix, from here (or by running the following at the command line):

bash: git clone https://github.com/aausch/django_twisted_chat.git
bash: git checkout tags/v0.1.3

PS:

This solution is simple, but, at production levels, might cost you performance for your twisted server. Adding blocking components is generally a bad idea, from a scalability perspective. Odds are that, eventually, you’ll eventually find yourself pushing this kind of synchronization work down into your data store (you might want to look into using something like Redis or MongoDB, and a related non-blocking twisted client library), and probably writing all sorts of fun time based queries to get at it (or maybe use an advanced tool like datomic)

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