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Using Feature Flags with Sculpin

Background

I was asked last week to add a new feature, a Facebook pixel for measuring and building advertising campaigns, to a client's website which I built using the Sculpin static site generator.

The site uses settings within the app/config/sculpin_site.yml file for storing site IDs and usernames. For this, I would add something like:

facebook:
    pixel:
        id: "abc123"

It can then be retrieved with {{ site.facebook.pixel.id }}.

If I then needed to disable the pixel, then I'd typically remove the pixel ID:

facebook:
    pixel:
        id: ~

Introducing feature flags

A technique that I like to use on other projects is using feature flags (aka feature toggles).

Whilst, in this instance, a feature flag wouldn't separate deploying code from toggling a feature - a static site will need to be re-generated and deployed - I thought that there was value in being able to easily toggle a feature without changing its configuration or removing code within the site's templates.

Feature flags in Sculpin

My Sculpin feature flag implementation was to add a feature_flags key within sculpin_site.yml, with each feature's name as the key and a boolean value to set whether it's enabled - similar to how the Drupal 7 version of the Feature Toggle module works.

This is how I added the Facebook pixel feature flag:

feature_flags:
    add_facebook_pixel: true

Using the Facebook pixel feature flag

The Facebook pixel code is stored within it's own partial that I can include from my source/_layouts/app.html.twig layout, including the pixel ID and whether or not the feature flag is enabled.

{% include "facebook-pixel" with {
    is_enabled: site.feature_flags.add_facebook_pixel,
    pixel_id: site.facebook.pixel.id,
} only %}

Within the partial, I can check that both the feature flag is enabled and that there is a Facebook pixel ID, and only add the pixel code if both conditions return a truthy value.

{% if is_enabled and pixel_id %}
    <script>
    !function(f,b,e,v,n,t,s)
    {if(f.fbq)return;n=f.fbq=function(){n.callMethod?
    n.callMethod.apply(n,arguments):n.queue.push(arguments)};
    if(!f._fbq)f._fbq=n;n.push=n;n.loaded=!0;n.version='2.0';
    n.queue=[];t=b.createElement(e);t.async=!0;
    t.src=v;s=b.getElementsByTagName(e)[0];
    s.parentNode.insertBefore(t,s)}(window, document,'script',
    'https://connect.facebook.net/en_US/fbevents.js');
    fbq('init', '{{ pixel_id }}');
    fbq('track', 'PageView');
    </script>
{% endif %}

Now the pixel can be removed just by setting add_facebook_pixel: false in sculpin_site.yml, and without changing any other configuration or templates.

Comments or questions? I'm @opdavies on Twitter.