Django Feature Flipper

django-feature-flipper helps flip features of your Django site on and off, in the database or per request or session, using URL parameters.

THE SOFTWARE IS ALPHA. THE API IS CHANGING AND THERE ARE NO UNIT TESTS.

This will help you can deploy code and schema changes for upcoming features but hide the features from your users until you're ready. This practice is commonly used in continuous deployment.

The term "feature flipper" seems to have come from Flickr, as described in this often-cited blog post:

http://code.flickr.com/blog/2009/12/02/flipping-out/

Feature flags or switches are becoming more commonly used, it seems.

django-feature-flipper is in part inspired by that post, along with some of the other feature flippers available, including:

• https://github.com/qype/feature_flipper (for Rails, by Florian Munz at Qype)
• https://github.com/grillpanda/dolphin (for Rails, by Matt Johnson)
• http://dhorrigan.com/turnstiles/ (for CodeIgniter, Dan Horrigan)
• https://dev.launchpad.net/LEP/FeatureFlags (for Canonical's Launchpad)

A few days after I first committed django-feature-flipper to github, David Cramer at Disqus has released the "gargoyle" plugin for Django, that offers overlapping functionality. That plugin requires "Nexus", their Django front-end admin replacement.

• http://justcramer.com/2010/12/21/feature-switches-in-django-with-gargoyle/
• https://github.com/disqus/gargoyle

The following post is an interview with Flickr's John Allspaw, author of "The Art of Capacity Planning: Scaling Web Resources."

• http://highscalability.com/blog/2009/6/29/how-to-succeed-at-capacity-planning-without-really-trying-an.html

Includes this quote, which covers feature flags being used to disable features to help "panic gracefully.":

"Of course it's easier to do those things when you have easy config flags to turn things on or off, and a list to run through of what things are acceptable to serve stale and static. We currently have about 195 'features' we can turn off at Flickr in dire circumstances. And we've used those flags when we needed to."

More on feature flags/flippers:

• http://old.nabble.com/Feature-Flipper-to30400696.html
• http://www.alandelevie.com/2010/05/19/feature-flippers-with-rails/
• http://engineering.qype.com/2010/06/03/how-we-work-flipping-features/ (also Florian Munz)
• http://www.reddit.com/r/programming/comments/aehta/flickr_dont_branch/

Continuous deployment:

• http://www.startuplessonslearned.com/2009/06/why-continuous-deployment.html
• http://timothyfitz.wordpress.com/2009/02/10/continuous-deployment-at-imvu-doing-the-impossible-fifty-times-a-day/
• http://radar.oreilly.com/2009/03/continuous-deployment-5-eas.html
• http://en.wikipedia.org/wiki/Lean_Startup

Installation

1. Add the featureflipper directory to your Python path.

   This should work:

   pip install -e git+https://github.com/tobych/django-feature-flipper.git@master#egg=django-feature-flipper
   

2. Add featureflipper to your INSTALLED_APPS setting.

3. Add featureflipper.context_processors.features to your TEMPLATE_CONTEXT_PROCESSORS setting. It doesn't matter where you put it in relation to existing entries.

4. Add featureflipper.middleware.FeaturesMiddleware to your MIDDLEWARE_CLASSES setting. It doesn't matter where you put it in relation to existing entries.

5. Optionally, add a settings.FEATURES_FILE, and set it to the location of a features file (see below) to load after each syncdb (or whenever you'd normally expect fixtures to be loaded).

6. Run ./manage.py syncdb to create the database table.

Limitations

Feature status is currently kept in the database. This is inefficient. They should probably be in Memcached instead.

There is, unforgivably, poor unit test coverage.

What determines a feature's status

A feature's status (enabled or disabled) is determined by, in order:

1. The database: the value of the attribute enabled of the Feature table. You can edit this value using the Django admin application.
2. The session: if a session entry feature_status_myfeature exists, the feature will be enabled if the value is enabled, and disabled otherwise. The middleware will add this entry if the GET parameter session_enable_myfeature is included, as explained below.
3. The request: if a GET parameter enabled_myfeature exists, the feature will enabled for this request, as explained below.

Enabling and disabling features using URLs

Users with permission can_flip_with_url can turn features on and off using URL parameters.

To enable a feature for the current request:

/mypage/?enable_myfeature

To enable a feature for this request and the rest of a session:

/mypage/?session_enable_myfeature

To clear all the features enabled in the session:

/mypage/?session_clear_features

If you want to allow anonymous users to do this, see the section "Authorization for Anonymous Users" here:

http://docs.djangoproject.com/en/dev/topics/auth/

Alternatively (since that looks painful) you can allow anyone to use URLs to flip features by setting FEATURE_FLIPPER_ANONYMOUS_URL_FLIPPING to True in your settings.py.

How to use the features in templates

The application registers itself with Django's admin app so you can manage the Features. Each feature has a name made up of just alphanumeric characters and hyphens that you can use in templates, views, URLs and elsewhere in your code. Each feature has a boolean enabled property, which is False (disabled) by default. The app also adds a few custom actions to the change list page so you can enable, disable and flip features there.

Features also have a name and description, which aren't currently used anywhere but should help you keep track.

The context processor adds features to the template context, which you can use like this:

{% if feature.search %}
  <form>...</form>
{% endif %}

Here, search is the name of the feature. If the feature referenced doesn't exist, it is silently treated as disabled.

To save you some typing, you can also use a new block tag:

{% load feature_tag %}

{% feature login %}
  <a href="/login/">Login</a>
{% endfeature %}

You can also do this:

{% feature profile %}
  ... will only be output if feature 'profile' is enabled ...
{% disabled %}
  ... will only be output if the feature is disabled ...
{% endfeature %}

How to use the features in views

The middleware adds features, a dict subclass, to each request:

if request.features['search']:
       ...

The middleware also adds features_panel to the request. This object provides more information about the state of each feature than features.

enabled('myfeature') returns True if myfeature is enabled.

source('myfeature') returns a string indicating the source of the final status of the feature:

• site: site-wide, in the Feature instance itself
• session: in the session, set using a URL parameter
• url: per request, set using a URL parameter

source('myfeature) will return another value if a featureflipper plugin is being used (see below).

features and source are also available. They are demonstrated in the example application.

Features file

To make sure you can easily keep features and their default settings under version control, you can load features from a file using the loadfeatures management command (below). If you add FEATURES_FILE to your settings, pointing to a file (typically features.json), features from this file will be loaded each time you do a syncdb. Note that any existing feature of the same name will be overwritten.

The file needs to look like this:

[
  {
    "name": "profile",
    "enabled": true,
    "description": "Allow the user to view and edit their profile."
  },
  {
    "name": "search",
    "enabled": true,
    "description": "Shows the search box on most pages, and the larger one on the home page."
  }
]

Note that for profile above, we're using the description field to describe the feature in general, whereas for search we're describing how and where that feature is make visible to the user. You might end up using a mix of these.

Management commands

• ./manage.py features: List the features in the database, along with their status.
• ./manage.py addfeature: Adds one or more features to the database (leaving them disabled).
• ./manage.py loadfeatures: Loads features from a JSON file (as above), or from the features file defined in settings.FEATURES_FILE.
• ./manage.py dumpfeatures: Outputs features from the database in the same JSON format (although the keys aren't in the same order as the example above).
• ./manage.py enablefeature: Enables the named feature(s).
• ./manage.py disablefeature: Disables the named feature(s).

Signals

Signal featureflipper.signals.feature_defaulted is sent when a feature referred to in a template or view is being defaulted to disabled. This will happen if the feature is not in the database, and hasn't been enabled using URL parameters.

The example project shows how this signal can be used, in views.py.

Note also that featureflipper uses Django's post_syncdb to load a features file when syncdb is run. The connection to the signal is made in featureflipper/management/__init.py__.

Using the example project included in the source

The source tree for django-feature-flipper includes an example project created using the "App Factory" described on a post on the Washington Times open source blog.

The settings.py file stipulates a sqlite3 database, so you'll need sqlite3 to be installed on your system. The database will be created automatically as necessary.

To try the example project:

cd example
./manage.py syncdb
./manage.py runserver

Let syncdb help you create a superuser so you can use the admin to create your own features. If you forget this step you can always run the createsuperuser command to do this. Two features (profile and search) will be loaded from features.json when you do the syncdb. These are referenced in the example template used on the home page. There's no link bank to the home page from the admin so you'll need to hack the URL or open the admin in a separate tab in your browser.

Good practice

• Once you no longer need to flip a feature, remove the feature from the database and all the logic from your template and views.
• If you decide to remove the feature itself from your application, don't leave unused template and view code around. Just delete it. If you later decide to resurect the feature, it'll always be there in your version control repository.

Extending Feature Flipper

The app includes a hook to allow you to add "feature providers" that provide the state of features. On each request, the feature states are collected in turn from any plugins found (the order they're called on is undefined), just after feature states are collected from the database. To add a plugin, you need to create a subclass of featureflipper.FeatureProvider, and make sure it gets compiled along with the rest of your application.

The class attribute source must be a string. This string is what the middeware makes available in request.features_panel.source().

The static method features must return a (possibly empty) list of tuples. The first member is the name of the feature, and the second True if the feature is enabled, and False otherwise. The features returned need not be defined in a Feature instance in the database.

For example:

from featureflipper import FeatureProvider
class UserFeatures(FeatureProvider):
  source = 'user'
  @staticmethod
  def features(request):
    return [('feature1', False), ('feature2', True)]

TODOs and BUGS

See: https://github.com/tobych/django-feature-flipper/issues