New FlipperSession and FlipperSessionHelper

FlipperSession is designed to be used with
Feature (and Flipper) to allow A/B style testing
for anonymous users.

FlipperSessionHelper leverages FlipperSession.

ee/app/helpers/ee/flipper_session_helper.rb

+# frozen_string_literal: true
+
+module EE
+  module FlipperSessionHelper
+    def flipper_session
+      @flipper_session ||= flipper_session_set? ? get_flipper_session : new_flipper_session
+    end
+
+    private
+
+    def flipper_session_set?
+      session.has_key?(FlipperSession::SESSION_KEY)
+    end
+
+    def get_flipper_session
+      FlipperSession.new(session[FlipperSession::SESSION_KEY])
+    end
+
+    def new_flipper_session
+      FlipperSession.new.tap do |flipper_session|
+        session[FlipperSession::SESSION_KEY] = flipper_session.id
+      end
+    end
+  end
+end

ee/changelogs/unreleased/ashmckenzie-flipper-as-an-ab-testing-framework.yml

+---
+title: Use Flipper as an A/B testing framework
+merge_request: 13755
+author:
+type: added

ee/lib/flipper_session.rb

+# frozen_string_literal: true
+
+require 'securerandom'
+
+class FlipperSession
+  SESSION_KEY = '_flipper_id'.freeze
+
+  attr_reader :id
+
+  def initialize(id = generate_id)
+    @id = id
+  end
+
+  # This method is required by Flipper
+  # https://github.com/jnunemaker/flipper/blob/master/docs/Gates.md#2-individual-actor
+  def flipper_id
+    "flipper_session:#{id}"
+  end
+
+  private
+
+  def generate_id
+    SecureRandom.uuid
+  end
+end

ee/spec/helpers/ee/flipper_session_helper_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe EE::FlipperSessionHelper do
+  describe '.flipper_session' do
+    let(:session) { {} }
+
+    before do
+      allow(helper).to receive(:session).and_return(session)
+    end
+
+    subject { helper.flipper_session }
+
+    context 'when a FlipperSession has not be previously set' do
+      let(:predictable_id) { 'abc123' }
+
+      before do
+        allow_any_instance_of(FlipperSession).to receive(:generate_id).and_return(predictable_id)
+      end
+
+      it 'returns an instance of FlipperSession' do
+        expect(subject).to be_instance_of(FlipperSession)
+      end
+
+      it 'sets a predictable FlipperSession id to session' do
+        subject
+
+        expect(session).to include(FlipperSession::SESSION_KEY => predictable_id)
+      end
+    end
+
+    context 'when a FlipperSession has been previously set' do
+      let(:predictable_id) { 'def456' }
+
+      before do
+        allow_any_instance_of(FlipperSession).to receive(:generate_id).and_return(predictable_id)
+      end
+
+      it 'returns a FlipperSession with the same ID' do
+        # Cannot call subject as it will be cached and not trigger the logic a second time
+        existing_flipper_session = helper.flipper_session
+
+        expect(subject.id).to eq(existing_flipper_session.id)
+      end
+    end
+  end
+end

ee/spec/lib/flipper_session_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+
+describe FlipperSession do
+  describe '#flipper_id' do
+    context 'without passing in an ID' do
+      it 'returns a flipper_session:UUID' do
+        expect(described_class.new.flipper_id).to match(/\Aflipper_session:\w{8}-\w{4}-\w{4}-\w{4}-\w{12}\z/)
+      end
+    end
+
+    context 'passing in an ID' do
+      it 'returns a flipper_session:def456' do
+        id = 'abc123'
+
+        expect(described_class.new(id).flipper_id).to eq("flipper_session:#{id}")
+      end
+    end
+  end
+end