Creating checks

Conventions

There are four steps to create a check:

1. Create preflight.py file in your application’s directory.
2. In that file create a class which inherits from preflight.Preflight.
3. Add a method to that class with name starting with check_.
4. Register that class by calling preflight.register(ClassName)

Those check methods are used by web interface and by command line one. You can see the results of them being run by either accessing the preflight page or by running $ ./manage.py preflight command.

Check’s description

The docstring of the method is displayed on the web page as a description of the check. You can use it to document what it is checking and what to do in case it fails. Ideally, one should check only one thing, so reason of the failure would be obvious.

Success or Failure

To the check to be considered successful it needs to return a True. Returning False or raising an exception means that the check have failed. The system records the results from all the checks from all of the applications and presents it to the end user.

An Example

Below is an example of simple preflight class which checks if cache is accessible from Django’s process:

# this line is required for Python versions <
from __future__ import absolute_import

import preflight


class AppPreflight(preflight.Preflight):

    def check_cache_is_accessible(self):
        """
        Try to access cache, if it fails that means the caching
        is not properly configured or the cache server is down.
        """
        from django.core.cache import cache
        cache.set('test', 'value')
        if cache.get('test') == 'value':
            cache.delete('test')
            return True
        else:
            return False

preflight.register(AppPreflight)

If everything is properly configured this check method will return True, which means that it succeeded.

But if something is wrong, like, for example, missing python-memcached library, the exception will be raised and will cause this check to fail.

Third option is when this check returns False, which means that the cache is accessible, but is not storing proper values. Not a good sign either.

Note

I’m aware this check can fail for more than one reason. It would be possible to split it into two smaller checks, but that would only made this example more complicated.