Go-style expvars endpoint for django apps
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django-expvar

expvar compatible endpoint for django

pip install django-expvar, add expvar to INSTALLED_APPS and add to your urls.py:

from expvar.views import ExpVarView

...

    url('^debug/vars$', ExpVarView.as_view(), name='expvar'),
...

Then, accessing /debug/vars on your app will return a JSON dict with the exposed variables, which you can use for monitoring, debugging, etc.

You can easily expose variables through the expvar endpoint. In your django app, just add a vars.py file with some classes that subclass expvar.ExpVar like so:

import expvar


class Example(expvar.ExpVar):
    name = 'example'

    def value(self):
        return 42

In your django settings, you can optionally specify a EXPVAR_SKIP variable with a list of apps to ignore (ie, any vars.py files in those apps will be ignored).

If multiple variables declare the same name, two different things can happen:

  • if they return scalar values, it's a collision and only one of them will get reported. This is probably not what you intended, so try to stick to unique names
  • if they return dicts as their value, variables with the same name will have their values merged.

plugins:

By default, expvar will only expose the variables that you set up.

There are a few additional packages available though that act as plugins to provide generic data on commandline arguments, process data, etc. Generally, once you have expvar installed, you can pip install them and add them to INSTALLED_APPS and that's enough to use them.

  • django-expvar-cmdline - reports the commandline data for the process (useful for compatability with expvarmon and similar, but may pose a security risk if you pass secrets on the commandline)
  • django-expvar-resource - reports various info on resource usage (memory, interrupts, etc) via a library in the Python's standard lib.
  • django-expvar-psutil - reports even more CPU, memory, swap, and network stats via the 'psutil' library.

future plans/additional notes:

  • TODO: a wrapper on probably either/both django-expvar-resource and/or django-expvar-psutil that exposes the appropriate metrics from those with the exact same names as Go's expvar uses ("Alloc", "HeapInUse", etc.) Ideally, I'd like to be able to just point expvarmon at a django app and have it be useful immediately. Go's internals are so different than Python's though so a lot of the GC related metrics aren't going to carry over.

The Go expvar package has a nice approach of letting you declare exposed variables and set their value, increment, etc. without having to worry about the underlying concurrency issues. I'm on the fence about whether it's appropriate to port this over to Django. Django apps can be deployed in a number of different ways (multi-process, multi-threaded, etc) and it would be non-trivial to make that shared variable sort of approach work across all of them (technically, you can even put a Django app behind CGI so there would be zero persistence across requests). I think the more "pull based" approach that's there now is probably cleanest. You can always just use Django's built in caching functionality if you want to avoid recalculating stuff too much.