Last updated: 2 Oct 2026

publisher: Feature flags

Feature flags in Publisher are managed using the Flipflop gem. The gem is configured with the "cookie" and "default" strategies, plus a lambda strategy that pins a single launched feature on.

Toggling feature flags

A features dashboard is available to make it easy to toggle features on and off during development. This dashboard can be accessed at /flipflop. Note that this can only be used to toggle feature flags for your user.

Pinning a feature on

Flipflop resolves a feature by walking the strategies in the order they are listed and taking the first non-nil answer. A lambda strategy placed above strategy :cookie therefore overrides the dashboard for the features it answers for, and features it returns nil for carry on being toggled per user as normal:

strategy :my_feature_launched do |feature|
  true if feature == :my_feature
end

strategy :cookie
strategy :default

Use this once a feature has launched and you need to stop users switching themselves back onto the old journey, but still want the old code path and its tests in place for a rollback. Removing the strategy restores per-user control.

Note that FeatureConstraint reads the cookie directly rather than going through the strategies, so a pinned feature can still be toggled per user on any route that is constrained on it.

Flipflop::FeatureSet#test! replaces the whole strategy chain with the test strategy, so pinning a feature has no effect on tests, and switch! continues to control both paths.

Testing with feature flags

For testing purposes, feature flags can be configured like so:

  setup do
    test_strategy = Flipflop::FeatureSet.current.test!
    test_strategy.switch!(:feature_name, true)
  end

Creating a feature flag

To create a new feature flag, create an entry in the features.rb file with this format:

feature :feature_name,
      default: false,
      description: "A description of the feature"

Features should default to false while being worked on, meaning users will not see any change without explicitly turning the feature on using the dashboard.

To route the user between different pages based on the status of a feature flag, use the FeatureConstraint class.

For example:

app/routes.rb

constraints FeatureConstraint.new("feature_name") do
  get "path/to/page" => "new_controller#action"
end
get "path/to/page" => "old_controller#action"

This will route GET requests for /path/to/page to new_controller when the feature flag is enabled, and to old_controller when it is disabled.