Understanding WSGI together with Apache

[Carl Meyer]

Hi Johannes,

On 10/10/2015 08:24 AM, Johannes Bauer wrote:
> I'm running an Apache 2.4 webserver using mod_wsgi 4.3.0. There are two
> different applications running in there running on two completely
> separate vhosts.
> 
> I'm seeing some weird crosstalk between them which I do not understand.
> In particular, crosstalk concerning the locales of the two. One
> application needs to output, e.g., date information using a German
> locale. It uses locale.setlocale to set its LC_ALL to de_DE.UTF-8.
> 
> Now the second application doesn't need nor want to be German. It wants
> to see the C locale everywhere, in particular because at some point it
> uses datetime.datetime.strptime() to parse a datetime.
> 
> Here's where things get weird: Sometimes, my "C" locale process throws
> exceptions, because it's unable to parse a date. When looking why this
> fails, the string looks like de_DE's "Sa, 10 Okt 2015" instead of C's
> "Sat, 10 Oct 2015". This seems to happen depending on which worker
> thread is currently serving the request, i.e. nondeterministically.
> 
> So all in all, this is very weird and I must admit that I don't seem to
> fully understand how WSGI applications are run and served within a
> mod_wsgi framework altogether. In the past it all "just worked" and I
> didn't need to understand it all in-depth. But I think to be able to
> debug such a weird issue, in-depth knowledge of what happens under the
> hood would be helpful.
> 
> So if someone could shed some light on how it works in general or what
> could cause the described issue in particular, I'd really be grateful.

It's been a number of years since I used mod-wsgi (I prefer gunicorn or
uwsgi, in part because I find their process model so much easier to
understand), but as best I understand (hopefully if I get anything
wrong, someone who knows better can correct me) mod-wsgi uses a
little-known CPython feature called "sub-interpreters", meaning that
even multiple mod-wsgi sites on the same server can run in
sub-interpreters of the same Python process, and certain global state
(e.g. os.environ, apparently also the localization context) is shared
between those sub-interpreters, which can cause "crosstalk" issues like
you're seeing.

I'm not sure, but I think _maybe_ using WSGIDaemonProcess [1] and
putting your sites in different WSGIProcessGroup [2] might help?

Carl

 [1]
https://code.google.com/p/modwsgi/wiki/ConfigurationDirectives#WSGIDaemonProcess

[2]
https://code.google.com/p/modwsgi/wiki/ConfigurationDirectives#WSGIDaemonProcess

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