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Feature flags

We sometimes employ feature flags to develop features that aren't fully formed at the moment of deployment. They can also be employed to roll out features in production incrementally.

Feature flags are meant to be temporary and part of a rollout plan resulting in their removal.

Creating a new feature flag#

To create a new feature flag, we use data update scripts so that they can be made available over the entire Forem fleet, for example:

bundle exec rails g data_update AddAwesomeAlgorithmFeatureFlag

This will generate a script and its spec. Once done, you can add the new feature flag like this:

module DataUpdateScripts  class AddAwesomeAlgorithmFeatureFlag    def run      FeatureFlag.add(:awesome_algorithm)    end  endend

and then start using it right away.

Guarding a feature behind a flag#

Once the feature flag is added, you can start using it to hide the feature behind a boolean flag:

if FeatureFlag.enabled?(:awesome_algorithm)  # call the new codeelse  # call the previous codeend

or, for example in a view:

<% if FeatureFlag.enabled?(:blue_button) %>  <%= render "blue_button" %><% else %>  <%= render "good_old_button" %><% end %>

Enabling/disabling a feature flag globally#

As mentioned, feature flags can be used to test a work in progress feature or to test a new approach directly in production.

To enable such a feature:

  • Log in to the Forem
  • Go to /admin/feature_flags (it requires the tech_admin role)
  • Click on the flag you want to enable and press "Fully Enable".

A screenshot of the Admin Feature Flag Panel

Removing a feature flag#

Once the feature has been validated and finalized or discarded, please remember to remove its flag with a final data update script, for example:

bundle exec rails g data_update RemoveAwesomeAlgorithmFeatureFlag
module DataUpdateScripts  class AwesomeAlgorithmFeatureFlag    def run      FeatureFlag.remove(:awesome_algorithm)    end  endend

This will ensure that the feature will be removed from all Forems.