Feature toggles

MDN uses feature toggles to integrate un-finished feature changes as early as possible, and to control the behavior of finished features.

Some site features are controlled using django-waffle. You control these features in the django admin site’s waffle section.

Some site features are controlled using constance. You control these features in the django admin site’s constance section.

Waffle features


Waffle switches are simple booleans - they are either on or off.

  • application_ACAO - Enable Access-Control-Allow-Origin=0 header.
  • dumb_doc_urls - (deprecated) Disable the render-time changing of /docs/ URLs to the equivalent Zone URLs (see PR 3331 for reasoning).
  • enable_optimizely - Enable the Optimizely JavaScript.
  • foundation_callout - show Foundation donation tile on homepage.
  • newsletter - Show newsletter sign-up site wide.
  • newsletter_article - Show newsletter sign-up in article footer (must be used in combination with newsletter).
  • redesign_live - show redesign notice for launch period
  • store_revision_ips - Save request data, including the IP address, to enable marking revisions as spam.
  • welcome_email - Send welcome email to new user registrations.
  • wiki_error_on_delete - Throw an error if a user tries to delete a page.
  • wiki_force_immediate_rendering - Force wiki pages to render immediately in the same http request in which they are saved (not in a background process).


Waffle flags control behavior by specific users, groups, percentages, and other advanced criteria.

  • kumabanned - (deprecated) added to users to mark them as banned.
  • kumaediting - Enable/disable wiki editing.
  • line_length - Enable/disable readable line length.
  • page_move - (deprecated) enable/disable page move feature.
  • registration_disabled - Enable/disable new user registration.
  • search_suggestions - Show the advanced search filter suggestions interface.
  • section_edit - Show section edit buttons.
  • sg_task_completion - Enable the Survey Gizmo pop-up.
  • spam_admin_override - Tell Akismet that edits are never spam.
  • spam_spammer_override - Tell Akismet that edits are always spam.
  • spam_testing_mode - Tell Akismet that edits are tests, not real content.
  • spam_checks_enabled - Toggle spam checks site wide.
  • spam_submissions_enabled - Toggle Akismet spam/spam submission ability.
  • sample_frame - Enable .sample-code-frame width, colour, and button styles
  • wiki_spam_exempted - Exempt users and user groups from checking. submissions for spam.
  • wiki_spam_training - Call Akismet to check submissions, but don’t block due to detected spam or Akismet errors.

Constance features

Constance configs let us set operational values for certain features in the database - so we can control them without changing code. They are all listed and documented in the admin site’s constance section.

Using Traffic Cop for A/B Testing

Traffic Cop is a lightweight JavaScript library that allows conducting content experiments on MDN without the need for third party tools such as Optimizely. Traffic Cop also respects user privacy, and will not run if Do Not Track is enabled.

On MDN, Traffic Cop will also not run for logged in users. Here’s how to implement a Traffic Cop content experiment.

The first task is to choose your control page and create your tests pages Here we will use the scenario of testing a new feature on MDN. The new feature involved replacing the static examples on top, with an interactive editor.

First we choose our control page. For this example, we will use:


Next we need our test page. The convention on MDN is to first create a base page that follows a naming convention such as:


With the base page created, we next create out test variation of our chosen page. To do this, navigate to:


You should should now be presented with an editing interface to create the new page.

Switch back to the control page, and click the Edit button. Switch the editor to source mode and copy all of the contents inside the editor. Switch back to our test, ensure the editor is set to source mode, and paste the content you just copied.

You can now go ahead and make the changes you wish to user test. Once you are complete, go ahead and publish the page.

The JavaScript

Let us first add the JavaScript code to initialise and configure Traffic Cop.

Inside kuma\static\js, create a new file called some like experiment-experiment-name.js and paste the following code into the file:

(function() {
    'use strict';

    var cop = new Mozilla.TrafficCop({
        id: 'experiment-experiment-name',
        variations: {
            'v=a': 50, // control
            'v=b': 50 // test


This will initialise Traffic Cop, set up an experiment with the id experiment-experiment-name, and lastly define a 50/50 split between the control, and the test page.

Define your bundle

Next we need to add an entry into kuma\settings\common.py to identify the test, and the related JS that will be injected. Find the following line in common.py:


Just before the closing } add a block such as the following:

    'experiment-experiment-name': {
        'source_filenames': (
        'output_filename': 'build/js/experiment-experiment-name.js',

NOTE: The key here experiment-experiment-name needs to match the id you specified in your JS file above.

Identify your A/B test pages

The final step is to identify the pages to the back-end so it will know where to direct traffic based on the URL parameter that will be added by Traffic Cop. Inside kuma\settings\content_experiments.json add the following:

            "id": "experiment-experiment-name",
            "ga_name": "experiment-name",
            "param": "v",
            "pages": {
                "en-US:Web/JavaScript/Reference/Global_Objects/Array/push": {
                    "a": "Web/JavaScript/Reference/Global_Objects/Array/push",
                    "b": "Experiment:ExperimentName/Array.prototype.push()"

There are a couple of important points to note here:

  1. We are leaving of the domain, as well as the docs/ part of the url.
  2. As with the entry in common.py, the id here matches the id in your JS file, tying it all together.
  3. The param value, needs to match the string you specified inside the variations block in your JS
  4. The first part of our key under pages above, identifies the locale to which this will apply, en-US in this case.
  5. The key for the two pages listed next, needs to match the values you used as the parameter value inside variations in your JS file earlier.

Testing your experiment

With you local instance of Kuma running, navigate to the page you defined as your control. In this example:


NOTE: You should not be logged in to MDN, and ensure that Do Not Track is disabled.

Your experiment JavaScript code bundle defined in common.py should be injected into the page, and Traffic Cop will add a URL parameter to the page that is either v=a or v=b. Depending on which, you will either see the control(a), or the variation(b).

You can also force a specific page to load by appending ?v=a or, ?v=a manually to the control page URL.

If all the above works as expected, open up a pull request, and tag someone on MDN for reivew.