Create an A/B test with Experimentation Module

note
We recommend using GLEX for new experiments.

Implement the experiment

  1. Add the experiment to the Gitlab::Experimentation::EXPERIMENTS hash in experimentation.rb:

    EXPERIMENTS = {
      other_experiment: {
        #...
      },
      # Add your experiment here:
      signup_flow: {
        tracking_category: 'Growth::Activation::Experiment::SignUpFlow' # Used for providing the category when setting up tracking data
      }
    }.freeze
    
  2. Use the experiment in the code.

    Experiments can be performed on a subject. The provided subject should respond to to_global_id or to_s. The resulting string is bucketed and assigned to either the control or the experimental group, so you must always provide the same subject for an experiment to have the same experience.

    1. Use this standard for the experiment in a controller:

      • Experiment run for a user:

        class ProjectController < ApplicationController
          def show
            # experiment_enabled?(:experiment_key) is also available in views and helpers
            if experiment_enabled?(:signup_flow, subject: current_user)
              # render the experiment
            else
              # render the original version
            end
          end
        end
        
      • Experiment run for a namespace:

        if experiment_enabled?(:signup_flow, subject: namespace)
          # experiment code
        else
          # control code
        end
        

      When no subject is given, it falls back to a cookie that gets set and is consistent until the cookie gets deleted.

      class RegistrationController < ApplicationController
        def show
          # falls back to a cookie
          if experiment_enabled?(:signup_flow)
            # render the experiment
          else
            # render the original version
          end
        end
      end
      
    2. Make the experiment available to the frontend in a controller. This example checks whether the experiment is enabled and pushes the result to the frontend:

      before_action do
        push_frontend_experiment(:signup_flow, subject: current_user)
      end
    

    You can check the state of the feature flag in JavaScript:

      import { isExperimentEnabled } from '~/experimentation';
    
      if ( isExperimentEnabled('signupFlow') ) {
        // ...
      }
    

You can also run an experiment outside of the controller scope, such as in a worker:

class SomeWorker
  def perform
    # Check if the experiment is active at all (the percentage_of_time_value > 0)
    return unless Gitlab::Experimentation.active?(:experiment_key)

    # Since we cannot access cookies in a worker, we need to bucket models
    # based on a unique, unchanging attribute instead.
    # It is therefore necessary to always provide the same subject.
    if Gitlab::Experimentation.in_experiment_group?(:experiment_key, subject: user)
      # execute experimental code
    else
      # execute control code
    end
  end
end

Implement tracking events

To determine whether the experiment is a success or not, we must implement tracking events to acquire data for analyzing. We can send events to Snowplow via either the backend or frontend. Read the product intelligence guide for more details.

Track backend events

The framework provides a helper method that is available in controllers:

before_action do
  track_experiment_event(:signup_flow, 'action', 'value', subject: current_user)
end

To test it:

context 'when the experiment is active and the user is in the experimental group' do
  before do
    stub_experiment(signup_flow: true)
    stub_experiment_for_subject(signup_flow: true)
  end

  it 'tracks an event', :snowplow do
    subject

    expect_snowplow_event(
      category: 'Growth::Activation::Experiment::SignUpFlow',
      action: 'action',
      value: 'value',
      label: 'experimentation_subject_id',
      property: 'experimental_group'
    )
  end
end

Track frontend events

The framework provides a helper method that is available in controllers:

before_action do
  push_frontend_experiment(:signup_flow, subject: current_user)
  frontend_experimentation_tracking_data(:signup_flow, 'action', 'value', subject: current_user)
end

This pushes tracking data to gon.experiments and gon.tracking_data.

expect(Gon.experiments['signupFlow']).to eq(true)

expect(Gon.tracking_data).to eq(
  {
    category: 'Growth::Activation::Experiment::SignUpFlow',
    action: 'action',
    value: 'value',
    label: 'experimentation_subject_id',
    property: 'experimental_group'
  }
)

To track it:

import { isExperimentEnabled } from '~/lib/utils/experimentation';
import Tracking from '~/tracking';

document.addEventListener('DOMContentLoaded', () => {
  const signupFlowExperimentEnabled = isExperimentEnabled('signupFlow');

  if (signupFlowExperimentEnabled && gon.tracking_data) {
    const { category, action, ...data } = gon.tracking_data;

    Tracking.event(category, action, data);
  }
}

To test it in Jest:

import { withGonExperiment } from 'helpers/experimentation_helper';
import Tracking from '~/tracking';

describe('event tracking', () => {
  describe('with tracking data', () => {
    withGonExperiment('signupFlow');

    beforeEach(() => {
      jest.spyOn(Tracking, 'event').mockImplementation(() => {});

      gon.tracking_data = {
        category: 'Growth::Activation::Experiment::SignUpFlow',
        action: 'action',
        value: 'value',
        label: 'experimentation_subject_id',
        property: 'experimental_group'
      };
    });

    it('should track data', () => {
      performAction()

      expect(Tracking.event).toHaveBeenCalledWith(
        'Growth::Activation::Experiment::SignUpFlow',
        'action',
        {
          value: 'value',
          label: 'experimentation_subject_id',
          property: 'experimental_group'
        },
      );
    });
  });
});

Record experiment user

In addition to the anonymous tracking of events, we can also record which users have participated in which experiments, and whether they were given the control experience or the experimental experience.

The record_experiment_user helper method is available to all controllers, and it enables you to record these experiment participants (the current user) and which experience they were given:

before_action do
  record_experiment_user(:signup_flow)
end

Subsequent calls to this method for the same experiment and the same user have no effect unless the user is then enrolled into a different experience. This happens when we roll out the experimental experience to a greater percentage of users.

This data is completely separate from the events tracking data. They are not linked together in any way.

Add context

You can add arbitrary context data in a hash which gets stored as part of the experiment user record. New calls to the record_experiment_user with newer contexts are merged deeply into the existing context.

This data can then be used by data analytics dashboards.

before_action do
  record_experiment_user(:signup_flow, foo: 42, bar: { a: 22})
  # context is { "foo" => 42, "bar" => { "a" => 22 }}
end

# Additional contexts for newer record calls are merged deeply
record_experiment_user(:signup_flow, foo: 40, bar: { b: 2 }, thor: 3)
# context becomes { "foo" => 40, "bar" => { "a" => 22, "b" => 2 }, "thor" => 3}

Record experiment conversion event

Along with the tracking of backend and frontend events and the recording of experiment participants, we can also record when a user performs the desired conversion event action. For example:

  • Experimental experience: Show an in-product nudge to test if the change causes more people to sign up for trials.
  • Conversion event: The user starts a trial.

The record_experiment_conversion_event helper method is available to all controllers. Use it to record the conversion event for the current user, regardless of whether the user is in the control or experimental group:

before_action do
  record_experiment_conversion_event(:signup_flow)
end

Note that the use of this method requires that we have first recorded the user as being part of the experiment.

Enable the experiment

After all merge requests have been merged, use ChatOps in the appropriate channel to start the experiment for 10% of the users. The feature flag should have the name of the experiment with the _experiment_percentage suffix appended. For visibility, share any commands run against production in the #s_growth channel:

  /chatops run feature set signup_flow_experiment_percentage 10

If you notice issues with the experiment, you can disable the experiment by removing the feature flag:

  /chatops run feature delete signup_flow_experiment_percentage

Add user to experiment group manually

To force the application to add your current user into the experiment group, add a query string parameter to the path where the experiment runs. If you add the query string parameter, the experiment works only for this request, and doesn’t work after following links or submitting forms.

For example, to forcibly enable the EXPERIMENT_KEY experiment, add force_experiment=EXPERIMENT_KEY to the URL:

https://gitlab.com/<EXPERIMENT_ENTRY_URL>?force_experiment=<EXPERIMENT_KEY>

You can force the current user into the experiment group for <EXPERIMENT_KEY> during the browser session by using your browser’s developer tools:

document.cookie = "force_experiment=<EXPERIMENT_KEY>; path=/";

Use a comma to list more than one experiment to be forced:

document.cookie = "force_experiment=<EXPERIMENT_KEY>,<ANOTHER_EXPERIMENT_KEY>; path=/";

To clear the experiments, unset the force_experiment cookie:

document.cookie = "force_experiment=; path=/";

Testing and test helpers

RSpec

Use the following in RSpec to mock the experiment:

context 'when the experiment is active' do
  before do
    stub_experiment(signup_flow: true)
  end

  context 'when the user is in the experimental group' do
    before do
      stub_experiment_for_subject(signup_flow: true)
    end

    it { is_expected.to do_experimental_thing }
  end

  context 'when the user is in the control group' do
    before do
      stub_experiment_for_subject(signup_flow: false)
    end

    it { is_expected.to do_control_thing }
  end
end

Jest

Use the following in Jest to mock the experiment:

import { withGonExperiment } from 'helpers/experimentation_helper';

describe('given experiment is enabled', () => {
  withGonExperiment('signupFlow');

  it('should do the experimental thing', () => {
    expect(wrapper.find('.js-some-experiment-triggered-element')).toEqual(expect.any(Element));
  });
});