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ITT Mentor Services - Feature flags

General principles

  • We use the gem Flipper to manage feature flags, note that we do not use Flipper cloud.
  • Developers are responsible for changing and maintaining feature flags. The support-only Flipper UI is mounted at /flipper; see routes and claims operations.
  • Confirm the environment, intended targeting and approval before changing a flag. Record its previous state and verify behaviour after the change.

How to

Create a feature flag

  • Create a migration to create your feature flag in the database, you can use Flipper.add(:feature_name) to do this.
  • The feature will be turned off by default.
  • Start using it in your code
  • Use the support UI or an approved, reviewed script to enable or disable flags. This guide does not define a repository-specific rake task contract.

Use in the codebase

Anywhere in the codebase, you can check to see if a feature is enabled. For example, to check if a feature called test_feature is enabled you could use:

Flipper.enabled?(:test_feature)

Enable a feature flag for specific actors

Sometimes we will need to roll out a feature to a selection of our users, but not all of them. We can target specific actors like so:

Flipper.enable(:test_feature, selected_user)

Here selected_user must already be the explicitly verified actor. Do not use the first user in a database as a rollout target.

Note that this does not need to be a user record, for example you may wish to enable a feature for anyone in a certain school. To check if the feature is enabled for an actor you can use:

Flipper.enable(:test_feature, selected_school)

Flipper.enabled?(:test_feature, selected_school)

selected_school must be the explicitly verified school in the intended environment.

Remove a feature flag

Once a feature flag has been turned on globally and everyone is happy that it should be a permanent feature the developer should:

  • Remove all uses from the codebase
  • Remove the flag from Active Record using the approved console/script call Flipper.remove(:feature_name) in all relevant environments, or create a migration. This method call is not a rake task.

Last reviewed

24-02-2025