Django Waffle

Django Waffle is (yet another) feature flipper for Django. You can define the conditions for which a flag should be active, and use it in a number of ways.

Installation

To start using Waffle, you just need to add it to your INSTALLED_APPS and MIDDLEWARE_CLASSES, and make sure to add the request context processor:

TEMPLATE_CONTEXT_PROCESSORS = (
    # ...
    'django.core.context_processors.request',
    # ...
)

INSTALLED_APPS = (
    # ...
    'waffle',
    # ...
)

MIDDLEWARE_CLASSES = (
    # ...
    'waffle.middleware.WaffleMiddleware',
    # ...
)

Since Waffle will be setting cookies on response objects, you probably want it below any middleware that tweaks cookies before sending them out.

Flags, Switches and Samples

Waffle supports three separate but ultimately similar concepts: Flags and Switches, and Samples.

Basically, a Flag is tied to a request, while Switches and Samples are not. Consequently, Flags are much more complicated, while Switches are just a named boolean in the database, and Samples are just a percentage stored in the database.

Flags

Creating and managing flags is done through the Django admin interface. Each feature flag is represented by a Flag object, which has several properties.

Name:

The name of the flag. Will be used to identify the flag everywhere.

Everyone:

You can flip this flag on (Yes) or off (No) for everyone, overriding all other settings. Leave as Unknown to use normally.

Percent:

A percentage of users for whom the flag will be active. This is maintained through cookies, so clever users can get around it. Still, it's the most common case.

Superusers:

Is this flag always active for superusers?

Staff:

Is this flag always active for staff?

Authenticated:

Is this flag always active for authenticated users?

Groups:

A list of group IDs for which this flag will always be active.

Users:

A list of user IDs for which this flag will always be active.

Rollout:

Activate Rollout mode? (See Rollout Mode below.)

Note:

Describe where the flag is used.

You can combine multiple settings here. For example, you could offer a feature to 12% of users and all superusers. When combining settings, the flag will be active for the user if any of the settings matches for them.

Switches

Switches are also managed through the Django admin. Each Switch object has only two properties:

Name:

The name of the switch.

Active:

Is the switch active or inactive.

Note:

Describe where the switch is used.

Like Flags, Switches can be used in views, templates, or wrapped around entire templates. But because they don't rely on a request objects, Switches can also be used in crons, Celery tasks, daemons--basically anywhere you can access the database.

Samples

Samples, also managed through the Django admin, have two properties:

Name:

The name.

Percent:

A number from 0.0 to 100.0 that determines how often the sample will be active.

Note:

Describe where the sample is used.

Samples are useful for datamining or other "some of the time" tasks that are not linked to a user or request--that is, unlike Flags, they do not set cookies and can't be reliably assumed to be a given value for a given user.

Using Waffle

Flags and Switches can be used in templates, in views, or wrapped around entire views. Samples can be used in templates or views but not to wrap an entire view.

If you try to use a flag or switch that is not defined, it will always be inactive.

Using Waffle in Templates

Jingo/Jinja2

To use a Flag in a Jinja2 template via Jingo, you can simply do:

{% if waffle.flag('flag_name') %}
  Content if flag is active
{% endif %}

You can also add an {% else %} section, of course:

{% if waffle.flag('flag_name') %}
  Flag is active!
{% else %}
  Flag is inactive!
{% endif %}

To use a Switch in a Jinja2 template via Jingo, you can do:

{% if waffle.switch('switch_name') %}
  Content if switch is active
{% endif %}

You can also add an {% else %} section, of course:

{% if waffle.switch('switch_name') %}
  Switch is active!
{% else %}
  Switch is inactive!
{% endif %}

For Samples:

{% if waffle.sample('sample_name') %}
  Sample is active!
{% else %}
  Sample is inactive!
{% endif %}

Django Templates

To use a flag in vanilla Django templates, you can use the flag tag:

{% load waffle_tags %}
{% flag flag_name %}
  Content if flag is active
{% endflag %}

The {% flag %} tag also supports an {% else %} section:

{% flag flag_name %}
  Flag is active!
{% else %}
  Flag is inactive!
{% endflag %}

To use a switch in vanilla Django templates, you can use the switch tag:

{% load waffle_tags %}
{% switch switch_name %}
  Content if switch is active
{% endswitch %}

The {% switch %} tag also supports an {% else %} section:

{% switch switch_name %}
  Switch is active!
{% else %}
  Switch is inactive!
{% endswitch %}

To use a sample, just use the sample tag:

{% sample sample_name %}
  Sample is active!
{% else %} {# Optional `else` section #}
  Sample is inactive!
{% endsample %}

Using Waffle in Views

To use a flag in a view, you just need waffle.flag_is_active:

import waffle

def my_view(request):
    if waffle.flag_is_active(request, 'flag_name'):
        # Behavior if flag is active.
    else:
        # Behavior if flag is inactive.

For switches, just use the switch_is_active method:

import waffle

def myview(request):
    if waffle.switch_is_active('myswitch'):
        return 'switch is active'
    return 'switch is inactive'

Because it doesn't need a request object, switch_is_active can be used anywhere.

Similarly, sample_is_active can be used anywhere, since it does not require a request object:

import waffle

def myview(request):
    if waffle.sample_is_active('mysample'):
        # Some percent of requests.

Wrapping a Whole View

You can also wrap an entire view in a flag:

from waffle.decorators import waffle_flag

@waffle_flag('flag_name')
def my_view(request):
    # View only available if flag is active.

...or a switch:

from waffle.decorators import waffle_switch

@waffle_switch('switch_name')
def my_view(request):
    # View only available if switch is active.

If the flag or switch is not active for the request, the view will be a 404.

You can reverse either decorator with an exclamation point at the start of the flag or switch name, for example:

@waffle_flag('!flag_name')
def my_view(request):
    # View is only available if flag is INactive.

Global Settings

There are a few global settings you can define to adjust Waffle's behavior.

WAFFLE_COOKIE:

The format for the cookies Waffle sets. Must contain %s. Defaults to dwf_%s.

WAFFLE_FLAG_DEFAULT:

When a Flag is undefined in the database, Waffle considers it False. Set this to True to make Waffle consider undefined flags True. Defaults to False.

WAFFLE_SWITCH_DEFAULT:

When a Switch is undefined in the database, Waffle considers it False. Set this to True to make Waffle consider undefined switches True. Defaults to False.

WAFFLE_SAMPLE_DEFAULT:

When a Sample is undefined in the database, Waffle considers it False. Set this to True to make Waffle consider undefined samples True. Defaults to False.

WAFFLE_MAX_AGE:

How long should Waffle cookies last? (Integer, in seconds.) See the Cookies section. Defaults to 2529000 (one month).

WAFFLE_OVERRIDE:

Whether Flags can be controlled from the query string. Defaults to False.

WAFFLE_SECURE:

Whether to set the secure flag on cookies. Defaults to False.

WAFFLE_CACHE_PREFIX:

Waffle tries to store objects in cache pretty aggressively. If you ever upgrade and change the shape of the objects (for example upgrading from <0.7.5 to >0.7.5) you'll want to set this to something other than 'waffle:'.

Overriding Flags

If you turn on the WAFFLE_OVERRIDE setting, you can guarantee a flag will be active for a request by putting it in the query string.

For example, if I use the flag example in a view that serves the URL /search, then I can turn on the flag by adding ?example=1 to the query string, or turn it off by adding ?example=0.

By default, WAFFLE_OVERRIDE is off. It may be useful for testing, automated testing in particular.

Switches cannot be overridden at this time.

Cookies

When falling back to percentage of active users, Waffle will set a cookie for every request, setting the flag's value (on or off) for future requests.

If the cookie is set, its value is used (either True or False) and it is re-set. Since cookies are re-set on every request (that uses the flag), you do not need to set WAFFLE_MAX_AGE very high. Just high enough that a typical returning user won't potentially flip back and forth between off and on.

Rollout Mode

Rollout Mode allows you to gradually enable a feature for all users. In "normal" mode, a flag's value will be set in a cookie until WAFFLE_MAX_AGE whether the flag is active or not. In Rollout Mode, an inactive flag will set a session cookie, and an active flag will set a longer-lived cookie.

Every time a user starts a new session, they'll have a chance (determined by the percentage of the flag) to have the feature turned on "permanently". Once it's on, it should stay on, unless they clear their cookies or use a different browser (see To Do).

To guarantee an even rollout, it will likely be necessary to gradually increase the flag's percentage as more and more users get stuck with the active cookie.

Rollout Mode is enabled per flag.

Waffle in JavaScript

Waffle now helps you use flags directly in JavaScript. You need to add the wafflejs view to your urls.py:

from waffle.views import wafflejs

urlpatterns = patterns('',
    # ...
    url(r'^wafflejs$', wafflejs, name='wafflejs'),
    # ...
)

You can then load the Waffle JavaScript in your templates:

<script src="{% url wafflejs %}"></script>

Once you've loaded the JavaScript, you can use the global waffle object. Just pass in a flag name. As in the Python API, if a flag or switch is undefined, it will always be false.

if (waffle.flag('some_flag')) {
    // Flag is active.
} else {
    // Flag is inactive.
}

if (waffle.switch('some_switch')) {
    // Switch is active.
} else {
    // Switch is inactive.
}

if (waffle.sample('some_sample')) {
    // Sample is active.
} else {
    // Sample is inactive.
}

waffle.sample(foo) will return the same value on a given request but that value may not persist across multiple requests.

To Do

• Keep a flag setting for a given user, regardless of browser/device.