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

A feature flipper to help you use continuous deployment for Django projects. No longer active. We use django-waffle these days.
https://github.com/tobych/django-feature-flipper

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A feature flipper to help you use continuous deployment for Django projects. No longer active. We use django-waffle these days.

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README 

        
======================

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

============



#. 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



#. Add ``featureflipper`` to your ``INSTALLED_APPS`` setting.



#. Add ``featureflipper.context_processors.features`` to your

   ``TEMPLATE_CONTEXT_PROCESSORS`` setting. It doesn't matter where

   you put it in relation to existing entries.



#. Add ``featureflipper.middleware.FeaturesMiddleware`` to your

   ``MIDDLEWARE_CLASSES`` setting. It doesn't matter where you put it

   in relation to existing entries.



#. 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).



#. 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:



#. The database: the value of the attribute ``enabled`` of the

   ``Feature`` table. You can edit this value using the Django admin

   application.



#. 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.



#. 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 %}

    ...

  {% 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 %}

    Login

  {% 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.



.. _post: http://opensource.washingtontimes.com/blog/2010/nov/28/app-centric-django-development-part-2-app-factory/



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