Feature flags in the development of GitLab

NOTE: The documentation below covers feature flags used by GitLab to deploy its own features, which is not the same as the feature flags offered as part of the product.

This document provides guidelines on how to use feature flags for the development of GitLab to conditionally and/or incrementally enable features and test them in production/staging.

WARNING: All newly-introduced feature flags should be disabled by default.

NOTE: This document is the subject of continued work as part of an epic to improve internal usage of Feature Flags. Raise any suggestions as new issues and attach them to the epic.

For an overview of the feature flag lifecycle, or if you need help deciding if you should use a feature flag or not, please see the feature flag lifecycle handbook page.

When to use feature flags

Moved to the "When to use feature flags" section in the handbook.

Feature flags in GitLab development

The following highlights should be considered when deciding if feature flags should be leveraged:

• The feature flag must be disabled by default.
• Feature flags should remain in the codebase for as short period as possible to reduce the need for feature flag accounting.
• The person operating the feature flag is responsible for clearly communicating the status of a feature behind the feature flag in the documentation and with other stakeholders. The issue description should be updated with the feature flag name and whether it is defaulted on or off as soon it is evident that a feature flag is needed.
• Merge requests that introduce a feature flag, update its state, or remove them existing feature flag because a feature is deemed stable must have the ~"feature flag" label assigned.

When the feature implementation is delivered among multiple merge requests:

1. Create a new feature flag which is off by default, in the first merge request which uses the flag. Flags should not be added separately.
2. Submit incremental changes via one or more merge requests, ensuring that any new code added can only be reached if the feature flag is on. You can keep the feature flag enabled on your local GDK during development.
3. When the feature is ready to be tested, enable the feature flag for a specific project and ensure that there are no issues with the implementation.
4. When the feature is ready to be announced, create a merge request that adds documentation about the feature, including documentation for the feature flag itself, and a changelog entry. In the same merge request either flip the feature flag to be on by default or remove it entirely in order to enable the new behavior.

One might be tempted to think that feature flags will delay the release of a feature by at least one month (= one release). This is not the case. A feature flag does not have to stick around for a specific amount of time (for example, at least one release), instead they should stick around until the feature is deemed stable. Stable means it works on GitLab.com without causing any problems, such as outages.

Risk of a broken main branch

Feature flags must be used in the MR that introduces them. Not doing so causes a broken main branch scenario due to the rspec:feature-flags job that only runs on the main branch.

Types of feature flags

Choose a feature flag type that matches the expected usage.

development type

development feature flags are short-lived feature flags, used for deploying unfinished code to production. Most feature flags used at GitLab are the development type.

A development feature flag must have a rollout issue created from the Feature Flag Roll Out template.

The format for development feature flags is Feature.<state>(:<dev_flag_name>). To enable and disable them, run on the GitLab Rails console:

# To enable it for the instance:
Feature.enable(:<dev_flag_name>)

# To disable it for the instance:
Feature.disable(:<dev_flag_name>)

# To enable for a specific project:
Feature.enable(:<dev_flag_name>, Project.find(<project id>))

# To disable for a specific project:
Feature.disable(:<dev_flag_name>, Project.find(<project id>))

To check a development feature flag's state:

# Check if the feature flag is enabled
Feature.enabled?(:dev_flag_name)

# Check if the feature flag is disabled
Feature.disabled?(:dev_flag_name)

For development feature flags, the type doesn't need to be specified (they're the default type).

ops type

ops feature flags are long-lived feature flags that control operational aspects of GitLab product behavior. For example, feature flags that disable features that might have a performance impact such as Sidekiq worker behavior.

ops feature flags likely do not have rollout issues, as it is hard to predict when they are enabled or disabled.

To invoke ops feature flags, you must append type: :ops:

# Check if feature flag is enabled
Feature.enabled?(:my_ops_flag, project, type: :ops)

# Check if feature flag is disabled
Feature.disabled?(:my_ops_flag, project, type: :ops)

# Push feature flag to Frontend
push_frontend_feature_flag(:my_ops_flag, project, type: :ops)

experiment type

experiment feature flags are used for A/B testing on GitLab.com.

An experiment feature flag should conform to the same standards as a development feature flag, although the interface has some differences. An experiment feature flag should have a rollout issue, created using the Experiment Tracking template. More information can be found in the experiment guide.

Feature flag definition and validation

Introduced in GitLab 13.3.

During development (RAILS_ENV=development) or testing (RAILS_ENV=test) all feature flag usage is being strictly validated.

This process is meant to ensure consistent feature flag usage in the codebase. All feature flags must:

• Be known. Only use feature flags that are explicitly defined.
• Not be defined twice. They have to be defined either in FOSS or EE, but not both.
• Use a valid and consistent type: across all invocations.
• Use the same default_enabled: across all invocations.
• Have an owner.

All feature flags known to GitLab are self-documented in YAML files stored in:

• config/feature_flags
• ee/config/feature_flags

Each feature flag is defined in a separate YAML file consisting of a number of fields:

NOTE: All validations are skipped when running in RAILS_ENV=production.

Create a new feature flag

The GitLab codebase provides bin/feature-flag, a dedicated tool to create new feature flag definitions. The tool asks various questions about the new feature flag, then creates a YAML definition in config/feature_flags or ee/config/feature_flags.

Only feature flags that have a YAML definition file can be used when running the development or testing environments.

$ bin/feature-flag my_feature_flag
>> Specify the group introducing the feature flag, like `group::apm`:
?> group::memory

>> URL of the MR introducing the feature flag (enter to skip):
?> https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38602

>> Open this URL and fill in the rest of the details:
https://gitlab.com/gitlab-org/gitlab/-/issues/new?issue%5Btitle%5D=%5BFeature+flag%5D+Rollout+of+%60test-flag%60&issuable_template=Feature+Flag+Roll+Out

>> URL of the rollout issue (enter to skip):
?> https://gitlab.com/gitlab-org/gitlab/-/issues/232533
create config/feature_flags/development/my_feature_flag.yml
---
name: my_feature_flag
introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38602
rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/232533
group: group::memory
type: development
default_enabled: false

All newly-introduced feature flags must be disabled by default.

Features that are developed and merged behind a feature flag should not include a changelog entry. The entry should be added either in the merge request removing the feature flag or the merge request where the default value of the feature flag is set to enabled. If the feature contains any database migrations, it should include a changelog entry for the database changes.

NOTE: To create a feature flag that is only used in EE, add the --ee flag: bin/feature-flag --ee

Risk of a broken master (main) branch

WARNING: Feature flags must be used in the MR that introduces them. Not doing so causes a broken master scenario due to the rspec:feature-flags job that only runs on the master branch.

Delete a feature flag

See cleaning up feature flags for more information about deleting feature flags.

Develop with a feature flag

There are two main ways of using Feature Flags in the GitLab codebase:

• Backend code (Rails)
• Frontend code (VueJS)

Backend

The feature flag interface is defined in lib/feature.rb. This interface provides a set of methods to check if the feature flag is enabled or disabled:

if Feature.enabled?(:my_feature_flag, project)
  # execute code if feature flag is enabled
else
  # execute code if feature flag is disabled
end

if Feature.disabled?(:my_feature_flag, project)
  # execute code if feature flag is disabled
end

In rare cases you may want to make a feature enabled by default. If so, explain the reasoning in the merge request. Use default_enabled: true when checking the feature flag state:

if Feature.enabled?(:feature_flag, project, default_enabled: true)
  # execute code if feature flag is enabled
else
  # execute code if feature flag is disabled
end

if Feature.disabled?(:my_feature_flag, project, default_enabled: true)
  # execute code if feature flag is disabled
end

If not specified, default_enabled is false.

To force reading the default_enabled value from the relative YAML definition file, use default_enabled: :yaml:

if Feature.enabled?(:feature_flag, project, default_enabled: :yaml)
  # execute code if feature flag is enabled
end

if Feature.disabled?(:feature_flag, project, default_enabled: :yaml)
  # execute code if feature flag is disabled
end

This allows to use the same feature flag check across various parts of the codebase and maintain the status of default_enabled in the YAML definition file which is the SSOT.

If default_enabled: :yaml is used, a YAML definition is expected or an error is raised in development or test environment, while returning false on production.

If not specified, the default feature flag type for Feature.enabled? and Feature.disabled? is type: development. For all other feature flag types, you must specify the type::

if Feature.enabled?(:feature_flag, project, type: :ops)
  # execute code if ops feature flag is enabled
else
  # execute code if ops feature flag is disabled
end

if Feature.disabled?(:my_feature_flag, project, type: :ops)
  # execute code if feature flag is disabled
end

WARNING: Don't use feature flags at application load time. For example, using the Feature class in config/initializers/* or at the class level could cause an unexpected error. This error occurs because a database that a feature flag adapter might depend on doesn't exist at load time (especially for fresh installations). Checking for the database's existence at the caller isn't recommended, as some adapters don't require a database at all (for example, the HTTP adapter). The feature flag setup check must be abstracted in the Feature namespace. This approach also requires application reload when the feature flag changes. You must therefore ask SREs to reload the Web/API/Sidekiq fleet on production, which takes time to fully rollout/rollback the changes. For these reasons, use environment variables (for example, ENV['YOUR_FEATURE_NAME']) or gitlab.yml instead.

Here's an example of a pattern that you should avoid:

class MyClass
  if Feature.enabled?(:...)
    new_process
  else
    legacy_process
  end
end

Frontend

When using a feature flag for UI elements, make sure to also use a feature flag for the underlying backend code, if there is any. This ensures there is absolutely no way to use the feature until it is enabled.

Use the push_frontend_feature_flag method which is available to all controllers that inherit from ApplicationController. You can use this method to expose the state of a feature flag, for example:

before_action do
  # Prefer to scope it per project or user e.g.
  push_frontend_feature_flag(:vim_bindings, project)
end

def index
  # ...
end

def edit
  # ...
end

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

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

The name of the feature flag in JavaScript is always camelCase, so checking for gon.features.vim_bindings would not work.

See the Vue guide for details about how to access feature flags in a Vue component.

In rare cases you may want to make a feature enabled by default. If so, explain the reasoning in the merge request. Use default_enabled: true when checking the feature flag state:

before_action do
  # Prefer to scope it per project or user e.g.
  push_frontend_feature_flag(:vim_bindings, project, default_enabled: true)
end

If not specified, the default feature flag type for push_frontend_feature_flag is type: development. For all other feature flag types, you must specify the type::

before_action do
  push_frontend_feature_flag(:vim_bindings, project, type: :ops)
end

Feature actors

It is strongly advised to use actors with feature flags. Actors provide a simple way to enable a feature flag only for a given project, group or user. This makes debugging easier, as you can filter logs and errors for example, based on actors. This also makes it possible to enable the feature on the gitlab-org or gitlab-com groups first, while the rest of the users aren't impacted.

Actors also provide an easy way to do a percentage rollout of a feature in a sticky way. If a 1% rollout enabled a feature for a specific actor, that actor will continue to have the feature enabled at 10%, 50%, and 100%.

GitLab currently supports the following models as feature flag actors:

• User
• Project
• Group

The actor is a second parameter of the Feature.enabled? call. The same actor type must be used consistently for all invocations of Feature.enabled?.

Feature.enabled?(:feature_flag, project)
Feature.enabled?(:feature_flag, group)
Feature.enabled?(:feature_flag, user)

Please see Feature flag controls for more details on working with feature flags.

Selectively disable by actor

By default you cannot selectively disable a feature flag by actor.

# This will not work how you would expect.
/chatops run feature set some_feature true
/chatops run feature set --project=gitlab-org/gitlab some_feature false

However, if you add two feature flags, you can write your conditional statement in such a way that the equivalent selective disable is possible.

Feature.enabled?(:a_feature, project) && Feature.disabled?(:a_feature_override, project)

# This will enable a feature flag globally, except for gitlab-org/gitlab
/chatops run feature set a_feature true
/chatops run feature set --project=gitlab-org/gitlab a_feature_override true

Enable additional objects as actors

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

Only models that include FeatureGate or expose flipper_id method can be used as an actor for Feature.enabled?.

Feature flags for licensed features

You can't use a feature flag with the same name as a licensed feature name, because it would cause a naming collision. This was widely discussed and removed because it is confusing.

To check for licensed features, add a dedicated feature flag under a different name and check it explicitly, for example:

Feature.enabled?(:licensed_feature_feature_flag, project) &&
  project.feature_available?(:licensed_feature)

Feature groups

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

Once defined in lib/feature.rb, you can to activate a feature for a given feature group via the feature_group parameter of the features API

Enabling a feature flag locally (in development)

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

Feature.enable(:feature_flag_name)

Similarly, the following command disables a feature flag:

Feature.disable(:feature_flag_name)

You can also enable a feature flag for a given gate:

Feature.enable(:feature_flag_name, Project.find_by_full_path("root/my-project"))

Removing a feature flag locally (in development)

When manually enabling or disabling a feature flag from the Rails console, its default value gets overwritten. This can cause confusion when changing the flag's default_enabled attribute.

To reset the feature flag to the default status, you can remove it in the rails console (rails c) as follows:

Feature.remove(:feature_flag_name)

Changelog

• Any change behind a feature flag disabled by default should not have a changelog entry.

  • Exception: database migrations should have a changelog entry.

• Any change related to a feature flag itself (flag removal, default-on setting) should have a changelog entry. Use the flowchart to determine the changelog entry type.

  graph LR
      A[flag: default off] -->|'added' / 'changed'| B(flag: default on)
      B -->|'other'| C(remove flag, keep new code)
      B -->|'removed' / 'changed'| D(remove flag, keep old code)
      A -->|'added' / 'changed'| C
      A -->|no changelog| D

• Any change behind a feature flag that is enabled by default should have a changelog entry.

Feature flags in tests

Introducing a feature flag into the codebase creates an additional code path that should be tested. It is strongly advised to test all code affected by a feature flag, both when enabled and disabled to ensure the feature works properly.

When using the testing environment, all feature flags are enabled by default.

WARNING: This does not apply to end-to-end (QA) tests, which do not enable feature flags by default. There is a different process for using feature flags in end-to-end tests.

To disable a feature flag in a test, use the stub_feature_flags helper. For example, to globally disable the ci_live_trace feature flag in a test:

stub_feature_flags(ci_live_trace: false)

Feature.enabled?(:ci_live_trace) # => false

If you wish to set up a test where a feature flag is enabled only for some actors and not others, you can specify this in options passed to the helper. For example, to enable the ci_live_trace feature flag for a specific project:

project1, project2 = build_list(:project, 2)

# Feature will only be enabled for project1
stub_feature_flags(ci_live_trace: project1)

Feature.enabled?(:ci_live_trace) # => false
Feature.enabled?(:ci_live_trace, project1) # => true
Feature.enabled?(:ci_live_trace, project2) # => false

The behavior of FlipperGate is as follows:

1. You can enable an override for a specified actor to be enabled.
2. You can disable (remove) an override for a specified actor, falling back to the default state.
3. There's no way to model that you explicitly disabled a specified actor.

Feature.enable(:my_feature)
Feature.disable(:my_feature, project1)
Feature.enabled?(:my_feature) # => true
Feature.enabled?(:my_feature, project1) # => true

Feature.disable(:my_feature2)
Feature.enable(:my_feature2, project1)
Feature.enabled?(:my_feature2) # => false
Feature.enabled?(:my_feature2, project1) # => true

have_pushed_frontend_feature_flags

Use have_pushed_frontend_feature_flags to test if push_frontend_feature_flag has added the feature flag to the HTML.

For example,

stub_feature_flags(value_stream_analytics_path_navigation: false)

visit group_analytics_cycle_analytics_path(group)

expect(page).to have_pushed_frontend_feature_flags(valueStreamAnalyticsPathNavigation: false)

stub_feature_flags vs Feature.enable*

It is preferred to use stub_feature_flags to enable feature flags in the testing environment. This method provides a simple and well described interface for simple use cases.

However, in some cases more complex behavior needs to be tested, like percentage rollouts of feature flags. This can be done using .enable_percentage_of_time or .enable_percentage_of_actors:

# Good: feature needs to be explicitly disabled, as it is enabled by default if not defined
stub_feature_flags(my_feature: false)
stub_feature_flags(my_feature: true)
stub_feature_flags(my_feature: project)
stub_feature_flags(my_feature: [project, project2])

# Bad
Feature.enable(:my_feature_2)

# Good: enable my_feature for 50% of time
Feature.enable_percentage_of_time(:my_feature_3, 50)

# Good: enable my_feature for 50% of actors/gates/things
Feature.enable_percentage_of_actors(:my_feature_4, 50)

Each feature flag that has a defined state is persisted during test execution time:

Feature.persisted_names.include?('my_feature') => true
Feature.persisted_names.include?('my_feature_2') => true
Feature.persisted_names.include?('my_feature_3') => true
Feature.persisted_names.include?('my_feature_4') => true

Stubbing actor

When you want to enable a feature flag for a specific actor only, you can stub its representation. A gate that is passed as an argument to Feature.enabled? and Feature.disabled? must be an object that includes FeatureGate.

In specs you can use the stub_feature_flag_gate method that allows you to quickly create a custom actor:

gate = stub_feature_flag_gate('CustomActor')

stub_feature_flags(ci_live_trace: gate)

Feature.enabled?(:ci_live_trace) # => false
Feature.enabled?(:ci_live_trace, gate) # => true

You can also disable a feature flag for a specific actor:

gate = stub_feature_flag_gate('CustomActor')

stub_feature_flags(ci_live_trace: false, thing: gate)

Controlling feature flags engine in tests

Our Flipper engine in the test environment works in a memory mode Flipper::Adapters::Memory. production and development modes use Flipper::Adapters::ActiveRecord.

You can control whether the Flipper::Adapters::Memory or ActiveRecord mode is being used.

stub_feature_flags: true (default and preferred)

In this mode Flipper is configured to use Flipper::Adapters::Memory and mark all feature flags to be on-by-default and persisted on a first use. This overwrites the default_enabled: of Feature.enabled? and Feature.disabled? returning always true unless feature flag is persisted.

Make sure behavior under feature flag doesn't go untested in some non-specific contexts.

stub_feature_flags: false

This disables a memory-stubbed flipper, and uses Flipper::Adapters::ActiveRecord a mode that is used by production and development.

You should use this mode only when you really want to tests aspects of Flipper with how it interacts with ActiveRecord.

End-to-end (QA) tests

Toggling feature flags works differently in end-to-end (QA) tests. The end-to-end test framework does not have direct access to Rails or the database, so it can't use Flipper. Instead, it uses the public API. Each end-to-end test can enable or disable a feature flag during the test. Alternatively, you can enable or disable a feature flag before one or more tests when you run them from your GitLab repository's qa directory, or if you run the tests via GitLab QA.

As noted above, feature flags are not enabled by default in end-to-end tests. This means that end-to-end tests will run with feature flags in the default state implemented in the source code, or with the feature flag in its current state on the GitLab instance under test, unless the test is written to enable/disable a feature flag explicitly.

When a feature flag is changed on Staging or on GitLab.com, a Slack message will be posted to the #qa-staging or #qa-production channels to inform the pipeline triage DRI so that they can more easily determine if any failures are related to a feature flag change. However, if you are working on a change you can help to avoid unexpected failures by confirming that the end-to-end tests pass with a feature flag enabled.