Python Django Global Variables

Question

Hi all,

I'm looking for simple but recommended way in Django to store a variable in memory only. When Apache restarts or the Django development server restarts, the variable is reset back to 0. More specifically, I want to count how many times a particular action takes place on each model instance (database record), but for performance reasons, I don't want to store these counts in the database. I don't care if the counts disappear after a server restart. But as long as the server is up, I want these counts to be consistent between the Django shell and the web interface, and I want to be able to return how many times the action has taken place on each model instance.

I don't want the variables to be associated with a user or session because I might want to return these counts without being logged in (and I want the counts to be consistent no matter what user is logged in). Am I describing a global variable? If so, how do I use one in Django? I've noticed the files like the urls.py, settings.py and models.py seems to be parsed only once per server startup (by contrast to the views.py which seems to be parsed eache time a request is made). Does this mean I should declare my variables in one of those files? Or should I store it in a model attribute somehow (as long as it sticks around for as long as the server is running)? This is probably an easy question, but I'm just not sure how it's done in Django.

Any comments or advice is much appreciated. Thanks, Joe

Answer 1

You mustn't declare global variables. Settings (constants) are OK if done right. But variables violate with shared-nothing architecture and might cause a lot of trouble. (best case they'll be inconsistent)

I would simply store those statistics in the cache. (Well, actually I would store them in the database but you made clear you believe it will have a negative impact on performance, so...)

The new incr() and decr() methods are especially suitable for counting. See docs for more info.

Answer 2

Why one mustn't declare global variables? O_o. It just looks like a propaganda. If the author knows what he wants and what side-effects will be, why not. Maybe it's just a quick experiment.

You could declare your counter as a model class-member. Then to deal with race condition you have to add a method that will wait if some other client, from another thread works with counter. Something like this:

import threading

class MyModel(ModelBase):
    _counter = 0
    _counter_lock = threading.Lock()

    @classmethod
    def increment_counter(cls):
        with cls._counter_lock:
            cls._counter += 1

    def some_action(self):
        # core code
        self.increment_counter()


# somewhere else
print MyModel._counter

Remember however: you have to have your application in one process. So if you've deployed the application under Apache, be sure it is configured to spawn many threads, but not many processes. If you're experimenting with ./manage.py run no actions are required.