Alex Rivera | Logout

What is so bad with threadlocals

Asked 2009-12-17T07:29:08.737
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Everybody in Django world seems to hate threadlocals(http://code.djangoproject.com/ticket/4280, http://code.djangoproject.com/wiki/CookBookThreadlocalsAndUser). I read Armin's essay on this(http://lucumr.pocoo.org/2006/7/10/why-i-cant-stand-threadlocal-and-others), but most of it hinges on threadlocals is bad because it is inelegant.

I have a scenario where theadlocals will make things significantly easier. (I have a app where people will have subdomains, so all the models need to have access to the current subdomain, and passing them from requests is not worth it, if the only problem with threadlocals is that they are inelegant, or make for brittle code.)

Also a lot of Java frameworks seem to be using threadlocals a lot, so how is their case different from Python/Django 's?

python django thread-local
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I avoid this sort of usage of threadlocals, because it introduces an implicit non-local coupling. I frequently use models in all kinds of non-HTTP-oriented ways (local management commands, data import/export, etc). If I access some threadlocals data in models.py, now I have to find some way to ensure that it is always populated whenever I use my models, and this could get quite ugly.

In my opinion, more explicit code is cleaner and more maintainable. If a model method requires a subdomain in order to operate, that fact should be made obvious by having the method accept that subdomain as a parameter.

If I absolutely could find no way around storing request data in threadlocals, I would at least implement wrapper methods in a separate module that access threadlocals and call the model methods with the needed data. This way the models.py remains self-contained and models can be used without the threadlocals coupling.

answered 2009-12-17T20:53:51.110

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