gitlab-org--gitlab-foss/doc/development/feature_flags.md

Manage feature flags

Starting from GitLab 9.3 we support feature flags for features in GitLab via Flipper. You should use the Feature class (defined in lib/feature.rb) in your code to get, set and list feature flags.

During runtime you can set the values for the gates via the features API (accessible to admins only).

Feature groups

Starting from GitLab 9.4 we support feature groups via Flipper groups.

Feature groups must be defined statically in lib/feature.rb (in the .register_feature_groups method), but their implementation can obviously be dynamic (querying the DB etc.).

Once defined in lib/feature.rb, you will be able to activate a feature for a given feature group via the feature_group param of the features API

For GitLab.com, team members have access to feature flags through chatops. Only percentage gates are supported at this time. Setting a feature to be used 50% of the time, you should execute /chatops run feature set my_feature_flag 50.

Feature flags for user applications

This document only covers feature flags used in the development of GitLab itself. Feature flags in deployed user applications can be found at Feature Flags

Developing with feature flags

In general, it's better to have a group- or user-based gate, and you should prefer it over the use of percentage gates. This would make debugging easier, as you filter for example logs and errors based on actors too. Furthermore, this allows for enabling for the gitlab-org group first, while the rest of the users aren't impacted.

# Good
Feature.enabled?(:feature_flag, project)

# Avoid, if possible
Feature.enabled?(:feature_flag)

To use feature gates based on actors, the model needs to respond to flipper_id. For example, to enable for the Foo model:

class Foo < ActiveRecord::Base
  include FeatureGate
end

Features that are developed and are intended to be merged behind a feature flag should not include a changelog entry. The entry should be added in the merge request removing the feature flags.

In the rare case that you need the feature flag to be on automatically, use default_enabled: true when checking:

Feature.enabled?(:feature_flag, project, default_enabled: true)

For more information about rolling out changes using feature flags, refer to the Rolling out changes using feature flags guide.

Frontend

For frontend code you can use the method push_frontend_feature_flag, which is available to all controllers that inherit from ApplicationController. Using this method you can expose the state of a feature flag as follows:

before_action do
  push_frontend_feature_flag(:vim_bindings)
end

def index
  # ...
end

def edit
  # ...
end

You can then check for the state of the feature flag in JavaScript as follows:

if ( gon.features.vimBindings ) {
  // ...
}

The name of the feature flag in JavaScript will always be camelCased, meaning that checking for gon.features.vim_bindings would not work.

Specs

In the test environment Feature.enabled? is stubbed to always respond to true, so we make sure behavior under feature flag doesn't go untested in some non-specific contexts.

Whenever a feature flag is present, make sure to test both states of the feature flag.

See the testing guide for information and examples on how to stub feature flags in tests.

Enabling a feature flag (in development)

In the rails console (rails c), enter the following command to enable your feature flag

Feature.enable(:feature_flag_name)

Enabling a feature flag (in production)

Check how to roll out changes using feature flags.