Feature flags in the development of GitLab

NOTE: This document explains how to contribute to the development and operations of the GitLab product. If you want to use feature flags to show and hide functionality in your own applications, view this feature flags information instead.

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

WARNING: All newly-introduced feature flags should be used with an actor.

Design documents:

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, see the feature flag lifecycle handbook page.

When to use feature flags

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

Do not use feature flags for long lived settings

Feature flags are meant to be short lived. If you are intending on adding a feature flag so that something can be enabled per user/group/project for a long period of time, consider introducing Cascading Settings or Application Settings instead. Settings offer a way for customers to enable or disable features for themselves on GitLab.com or self-managed and can remain in the codebase as long as needed. In contrast users have no way to enable or disable feature flags for themselves on GitLab.com and only self-managed admins can change the feature flags. Also note that feature flags are not supported in GitLab Dedicated which is another reason you should not use them as a replacement for settings.

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 the existing feature flag because a feature is deemed stable must have the ~"feature flag" label assigned.

When the feature implementation is delivered over multiple merge requests:

Create a new feature flag which is disabled by default, in the first merge request which uses the flag. Flags should not be added separately.
Submit incremental changes via one or more merge requests, ensuring that any new code added can only be reached if the feature flag is enabled. You can keep the feature flag enabled on your local GDK during development.
When the feature is ready to be tested by other team members, create the initial documentation. Include details about the status of the feature flag.
Enable the feature flag for a specific group/project/user and ensure that there are no issues with the implementation. Do not enable the feature flag for a public project like gitlab-org/gitlab if there is no documentation. Team members and contributors might search for documentation on how to use the feature if they see it enabled in a public project.
When the feature is ready for production use, including self-managed instances, open one merge request to:
- Update the documentation to describe the latest flag status.
- Add a changelog entry.
- Remove the feature flag to enable the new behavior, or flip the feature flag to be enabled by default (only for ops and beta feature flags).

When the feature flag removal is delivered over multiple merge requests:

The value change of a feature flag should be the only change in a merge request. As long as the feature flag exists in the codebase, both states should be fully functional (when the feature is on and off).
After all mentions of the feature flag have been removed, legacy code can be removed. Steps in the feature flag roll-out issue should be followed, and if a step needs to be skipped, a comment should be added to the issue detailing why.

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 default branch

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

Types of feature flags

Choose a feature flag type that matches the expected usage.

`gitlab_com_derisk` type

gitlab_com_derisk feature flags are short-lived feature flags, used to de-risk GitLab.com deployments. Most feature flags used at GitLab are of the gitlab_com_derisk type.

Constraints

default_enabled: Must not be set to true. This kind of feature flag is meant to lower the risk on GitLab.com, thus there's no need to keep the flag in the codebase after it's been enabled on GitLab.com. default_enabled: true will not have any effect for this type of feature flag.
Maximum Lifespan: 2 months after it's merged into the default branch
Documentation: This type of feature flag doesn't need to be documented in the All feature flags in GitLab page given they're short-lived and deployment-related
Rollout issue: Must have a rollout issue created from the Feature flag Roll Out template

Usage

The format for gitlab_com_derisk 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 gitlab_com_derisk 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)

`wip` type

Some features are complex and need to be implemented through several MRs. Until they're fully implemented, it needs to be hidden from anyone. In that case, the wip (for "Work In Progress") feature flag allows to merge all the changes to the main branch without actually using the feature yet.

Once the feature is complete, the feature flag type can be changed to the gitlab_com_derisk or beta type depending on how the feature will be presented/documented to customers.

Constraints

default_enabled: Must not be set to true. If needed, this type can be changed to beta once the feature is complete.
Maximum Lifespan: 4 months after it's merged into the default branch
Documentation: This type of feature flag doesn't need to be documented in the All feature flags in GitLab page given they're mostly hiding unfinished code
Rollout issue: Likely no need for a rollout issues, as wip feature flags should be transitioned to another type before being enabled

Usage

# Check if feature flag is enabled
Feature.enabled?(:my_wip_flag, project)

# Check if feature flag is disabled
Feature.disabled?(:my_wip_flag, project)

# Push feature flag to Frontend
push_frontend_feature_flag(:my_wip_flag, project)

`beta` type

We might not be confident we'll be able to scale, support, and maintain a feature in its current form for every designed use case (example). There are also scenarios where a feature is not complete enough to be considered an MVC. Providing a flag in this case allows engineers and customers to disable the new feature until it's performant enough.

Constraints

default_enabled: Can be set to true so that a feature can be "released" to everyone in beta with the possibility to disable it in the case of scalability issues (ideally it should only be disabled for this reason on specific on-premise installations)
Maximum Lifespan: 6 months after it's merged into the default branch
Documentation: This type of feature flag must be documented in the All feature flags in GitLab page
Rollout issue: Must have a rollout issue created from the Feature flag Roll Out template

Usage

# Check if feature flag is enabled
Feature.enabled?(:my_beta_flag, project)

# Check if feature flag is disabled
Feature.disabled?(:my_beta_flag, project)

# Push feature flag to Frontend
push_frontend_feature_flag(:my_beta_flag, project)

`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.

Remember that using this type should follow a conscious decision not to introduce an instance/group/project/user setting.

While ops type flags have an unlimited lifespan, every 12 months, they must be evaluated to determine if they are still necessary.

Constraints

default_enabled: Should be set to false in most cases, and only enabled to resolve temporary scalability issues or help debug production issues.
Maximum Lifespan: Unlimited, but must be evaluated every 12 months
Documentation: This type of feature flag must be documented in the All feature flags in GitLab page as well as be associated with an operational runbook describing the circumstances when it can be used.
Rollout issue: Likely no need for a rollout issues, as it is hard to predict when they are enabled or disabled

Usage

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

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

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

`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 beta 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.

Constraints

default_enabled: Must not be set to true.
Maximum Lifespan: 6 months after it's merged into the default branch

`worker` type

worker feature flags are special ops flags that allow to control Sidekiq workers behavior, such as deferring Sidekiq jobs.

worker feature flags likely do not have any YAML definition as the name could be dynamically generated using the worker name itself, for example, run_sidekiq_jobs_AuthorizedProjectsWorker. Some examples for using worker type feature flags can be found in deferring Sidekiq jobs.

(Deprecated) `development` type

The development type is deprecated in favor of the gitlab_com_derisk, wip, and beta feature flag types.

Feature flag definition and validation

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 (except for feature flags of the types experiment, worker and undefined).
Not be defined twice. They have to be defined either in FOSS or EE, but not both.
For feature flags that don't have a definition file, use a valid and consistent type: across all invocations.
Have an owner.

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

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

Field	Required	Description
`name`	yes	Name of the feature flag.
`type`	yes	Type of feature flag.
`default_enabled`	yes	The default state of the feature flag.
`introduced_by_url`	yes	The URL to the merge request that introduced the feature flag.
`milestone`	yes	Milestone in which the feature flag was created.
`group`	yes	The group that owns the feature flag.
`feature_issue_url`	no	The URL to the original feature issue.
`rollout_issue_url`	no	The URL to the Issue covering the feature flag rollout.
`log_state_changes`	no	Used to log the state of the feature flag

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

Create a new feature flag

NOTE: GitLab Pages uses a different process for feature flags.

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 feature flag type
?> beta
You picked the type 'beta'

>> Specify the group label to which the feature flag belongs, from the following list:
1. group::group1
2. group::group2
?> 2
You picked the group 'group::group2'

>> URL of the original feature issue (enter to skip):
?> https://gitlab.com/gitlab-org/gitlab/-/issues/435435

>> URL of the MR introducing the feature flag (enter to skip and let Danger provide a suggestion directly in the MR):
?> https://gitlab.com/gitlab-org/gitlab/-/merge_requests/141023

>> Username of the feature flag DRI (enter to skip):
?> bob

>> Is this an EE only feature (enter to skip):
?> [Return]

>> Press any key and paste the issue content that we copied to your clipboard! 🚀
?> [Return automatically opens the "New issue" page where you only have to paste the issue content]

>> URL of the rollout issue (enter to skip):
?> https://gitlab.com/gitlab-org/gitlab/-/issues/437162

create config/feature_flags/beta/my_feature_flag.yml
---
name: my_feature_flag
feature_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/435435
introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/141023
rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/437162
milestone: '16.9'
group: group::composition analysis
type: beta
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

Naming new flags

When choosing a name for a new feature flag, consider the following guidelines:

A long, descriptive name is better than a short but confusing one.
Write the name in snake case (my_cool_feature_flag).
Avoid using disable in the name to avoid having to think (or document) with double negatives. Consider starting the name with hide_, remove_, or disallow_.

In software engineering this problem is known as "negative names for boolean variables". But we can't forbid negative words altogether, to be able to introduce flags as disabled by default, use them to remove a feature by moving it behind a flag, or to selectively disable a flag by actor.

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.

Optionally add a `.patch` file for automated removal of feature flags

The gitlab-housekeeper is able to automatically remove your feature flag code for you using the DeleteOldFeatureFlags keep. The tool will run periodically and automatically clean up old feature flags from the code.

For this tool to automatically remove the usages of the feature flag in your code you can add a .patch file alongside your feature flag YAML file. The file should be exactly the same name except using the .patch extension instead of the .yml extension.

For example you can create a patch file for config/feature_flags/beta/my_feature_flag.yml using the following steps:

Edit the code locally to remove the feature flag my_feature_flag usage assuming that the feature flag is already enabled and we are rolling forward
Run git diff > config/feature_flags/beta/my_feature_flag.patch
Undo the changes to the files where you removed the feature flag usage
Commit this file config/feature_flags/beta/my_feature_flag.patch file to the branch where you are adding the feature flag

Then in future the gitlab-housekeeper will automatically clean up your feature flag for you by applying this patch.

List all the feature flags

To use ChatOps to output all the feature flags in an environment to Slack, you can use the run feature list command. For example:

/chatops run feature list --dev
/chatops run feature list --staging

Toggle a feature flag

See rolling out changes for more information about toggling feature flags.

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

Default behavior for not configured feature flags is controlled by default_enabled: in YAML definition.

If feature flag does not have a YAML definition an error will be raised in development or test environment, while returning false on production.

For feature flags that don't have a definition file (only allowed for the experiment, worker and undefined types), you need to pass their type: when calling Feature.enabled? and Feature.disabled?:

if Feature.enabled?(:experiment_feature_flag, project, type: :experiment)
  # execute code if feature flag is enabled
end

if Feature.disabled?(:worker_feature_flag, project, type: :worker)
  # 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

Recursion detection

When there are many feature flags, it is not always obvious where they are called. Avoid cycles where the evaluation of one feature flag requires the evaluation of other feature flags. If this causes a cycle, it will be broken and the default value will be returned.

To enable this recursion detection to work correctly, always access feature values through Feature::enabled?, and avoid the low-level use of Feature::get. When this happens, we track a Feature::RecursionError exception to the error tracker.

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, for example
  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.

For feature flags that don't have a definition file (only allowed for the experiment, worker and undefined types), you need to pass their type: when calling push_frontend_feature_flag:

before_action do
  push_frontend_feature_flag(:vim_bindings, project, type: :experiment)
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 supports the following feature flag actors:

User model
Project model
Group model
Current request

The actor is a second parameter of the Feature.enabled? call. For example:

Feature.enabled?(:feature_flag, project)

Models which include FeatureGate have an .actor_from_id class method. If you have the model's ID and do not need the model for anything other than checking the feature flag state, you can use .actor_from_id in order check the feature flag state without making a database query to retrieve the model.

# Bad -- Unnecessary query is executed
Feature.enabled?(:feature_flag, Project.find(project_id))

# Good -- No query for projects
Feature.enabled?(:feature_flag, Project.actor_from_id(project_id))

# Good -- Project model is used after feature flag check
project = Project.find(project_id)
return unless Feature.enabled?(:feature_flag, project)
project.update!(column: value)

See Use ChatOps to enable and disable feature flags for details on how to use ChatOps to selectively enable or disable feature flags in GitLab-provided environments, like staging and production.

Flag state is not inherited from a group by its subgroups or projects. If you need a flag state to be consistent for an entire group hierarchy, consider using the top-level group as the actor. This group can be found by calling #root_ancestor on any group or project.

Feature.enabled?(:feature_flag, group.root_ancestor)

Mixing actor types

Generally you should use only one type of actor in all invocations of Feature.enabled? for a particular feature flag, and not mix different actor types.

Mixing actor types can lead to a feature being enabled or disabled inconsistently in ways that can cause bugs. For example, if at the controller level a flag is checked using a group actor and at the service level it is checked using a user actor, the feature may be both enabled, and disabled at different points in the same request.

In some situations it is safe to mix actor types if you know that it won't lead to inconsistent results. For example, a webhook can be associated with either a group or a project, and so a feature flag for a webhook might leverage this to rollout a feature for group and project webhooks using the same feature flag.

If you need to use different actor types and cannot safely mix them in your situation you should use separate flags for each actor type instead. For example:

Feature.enabled?(:feature_flag_group, group)
Feature.enabled?(:feature_flag_user, user)

Instance actor

WARNING: Instance-wide feature flags should only be used when a feature is tied in to an entire instance. Always prioritize other actors first.

In some cases, you may want a feature flag to be enabled for an entire instance and not based on an actor. A great example are the Admin settings, where it would be impossible to enable the Feature Flag based on a group or a project since they are both undefined.

The user actor would cause confusion since a Feature Flag might be enabled for a user who is not an admin, but disabled for a user who is.

Instead, it is possible to use the :instance symbol as the second argument to Feature.enabled?, which will be sanitized as a GitLab instance.

Feature.enabled?(:feature_flag, :instance)

Current request actor

Introduced in GitLab 16.5

It is not recommended to use percentage of time rollout, as each call may return inconsistent results.

Rather it is advised to use the current request as an actor.

# Bad
Feature.enable_percentage_of_time(:feature_flag, 40)
Feature.enabled?(:feature_flag)

# Good
Feature.enable_percentage_of_actors(:feature_flag, 40)
Feature.enabled?(:feature_flag, Feature.current_request)

When using the current request as the actor, the feature flag should return the same value within the context of a request. As the current request actor is implemented using SafeRequestStore, we should have consistent feature flag values within:

a Rack request
a Sidekiq worker execution
an ActionCable worker execution

To migrate an existing feature from percentage of time to the current request actor, it is recommended that you create a new feature flag. This is because it is difficult to control the timing between existing percentage_of_time values, the deployment of the code change, and switching to use percentage_of_actors.

Use actors for verifying in production

WARNING: Using production as a testing environment is not recommended. Use our testing environments for testing features that are not production-ready.

While the staging environment provides a way to test features in an environment that resembles production, it doesn't allow you to compare before-and-after performance metrics specific to production environment. It can be useful to have a project in production with your development feature flag enabled, to allow tools like Sitespeed reports to reveal the metrics of the new code under a feature flag.

This approach is even more useful if you're already tracking the old codebase in Sitespeed, enabling you to compare performance accurately before and after the feature flag's rollout.

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

The available feature groups are:

Group name	Scoped to	Description
`gitlab_team_members`	Users	Enables the feature for users who are members of `gitlab-com`

Feature groups can be enabled via the group name:

Feature.enable(:feature_flag_name, :gitlab_team_members)

Controlling feature flags locally

On rails console

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"))

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:

Feature.remove(:feature_flag_name)

On your browser

Access http://gdk.test:3000/rails/features to see the manage locally the feature flag.

Logging

Usage and state of the feature flag is logged if either:

log_state_changes is set to true in the feature flag definition.
milestone refers to a milestone that is greater than or equal to the current GitLab version.

When the state of a feature flag is logged, it can be identified by using the "json.feature_flag_states": "feature_flag_name:1" or "json.feature_flag_states": "feature_flag_name:0" condition in Kibana. You can see an example in this link.

NOTE: Only 20% of the requests log the state of the feature flags. This is controlled with the feature_flag_state_logs feature flag.

Changelog

We want to avoid introducing a changelog when features are not accessible by an end-user either directly (example: ability to use the feature) or indirectly (examples: ability to take advantage of background jobs, performance improvements, or database migration updates).

Database migrations are always accessible by an end-user indirectly, as self-managed customers need to be aware of database changes before upgrading. For this reason, they should have a changelog entry.
Any change behind a feature flag disabled by default should not have a changelog entry.
Any change behind a feature flag that is enabled by default should have a changelog entry.

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

flowchart LR
  FDOFF(Flag is currently<br>'default: off')
  FDON(Flag is currently<br>'default: on')
  CDO{Change to<br>'default: on'}
  ACF(added / changed / fixed / '...')
  RF{Remove flag}
  RF2{Remove flag}
  RC(removed / changed)
  OTHER(other)

  FDOFF -->CDO-->ACF
  FDOFF -->RF
  RF-->|Keep new code?| ACF
  RF-->|Keep old code?| OTHER

  FDON -->RF2
  RF2-->|Keep old code?| RC
  RF2-->|Keep new code?| OTHER

The changelog for a feature flag should describe the feature and not the flag, unless a default on feature flag is removed keeping the new code (other in the flowchart above).
A feature flag can also be used for rolling out a bug fix or a maintenance work. In this scenario, the changelog must be related to it, for example; fixed or other.

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 include automated tests for all code affected by a feature flag, both when enabled and disabled to ensure the feature works properly. If automated tests are not included for both states, the functionality associated with the untested code path should be manually tested before deployment to production.

When using the testing environment, all feature flags are enabled by default. Flags can be disabled by default in the spec/spec_helper.rb file. Add a comment inline to explain why the flag needs to be disabled. You can also attach the issue URL for reference if possible.

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

A common pattern of testing both paths looks like:

it 'ci_live_trace works' do
  # tests assuming ci_live_trace is enabled in tests by default
  Feature.enabled?(:ci_live_trace) # => true
end

context 'when ci_live_trace is disabled' do
  before do
    stub_feature_flags(ci_live_trace: false)
  end

  it 'ci_live_trace does not work' do
    Feature.enabled?(:ci_live_trace) # => false
  end
end

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:

You can enable an override for a specified actor to be enabled.
You can disable (remove) an override for a specified actor, falling back to the default state.
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

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.

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 #e2e-run-staging or #e2e-run-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.

Controlling Sidekiq worker behavior with feature flags

Feature flags with worker type can be used to control the behavior of a Sidekiq worker.

Deferring Sidekiq jobs

When disabled, feature flags with the format of run_sidekiq_jobs_{WorkerName} delay the execution of the worker by scheduling the job at a later time. This feature flag is enabled by default for all workers. Deferring jobs can be useful during an incident where contentious behavior from worker instances are saturating infrastructure resources (such as database and database connection pool). The implementation can be found at SkipJobs Sidekiq server middleware.

NOTE: Jobs are deferred indefinitely as long as the feature flag is disabled. It is important to remove the feature flag after the worker is deemed safe to continue processing.

When set to false, 100% of the jobs are deferred. When you want processing to resume, you can use a percentage of time rollout. For example:

# not running any jobs, deferring all 100% of the jobs
/chatops run feature set run_sidekiq_jobs_SlowRunningWorker false

# only running 10% of the jobs, deferring 90% of the jobs
/chatops run feature set run_sidekiq_jobs_SlowRunningWorker 10

# running 50% of the jobs, deferring 50% of the jobs
/chatops run feature set run_sidekiq_jobs_SlowRunningWorker 50

# back to running all jobs normally
/chatops run feature delete run_sidekiq_jobs_SlowRunningWorker

Dropping Sidekiq jobs

Instead of deferring jobs, jobs can be entirely dropped by enabling the feature flag drop_sidekiq_jobs_{WorkerName}. Use this feature flag when you are certain the jobs are safe to be dropped, i.e. the jobs do not need to be processed in the future.

# drop all the jobs
/chatops run feature set drop_sidekiq_jobs_SlowRunningWorker true

# process jobs normally
/chatops run feature delete drop_sidekiq_jobs_SlowRunningWorker

NOTE: Dropping feature flag (drop_sidekiq_jobs_{WorkerName}) takes precedence over deferring feature flag (run_sidekiq_jobs_{WorkerName}), i.e. when drop_sidekiq_jobs is enabled and run_sidekiq_jobs is disabled, jobs are entirely dropped.

Feature flags in the development of GitLab

When to use feature flags

Do not use feature flags for long lived settings

Feature flags in GitLab development

Risk of a broken default branch

Types of feature flags

gitlab_com_derisk type

Constraints

Usage

wip type

Constraints

Usage

beta type

Constraints

Usage

ops type

Constraints

Usage

experiment type

Constraints

worker type

(Deprecated) development type

Feature flag definition and validation

Create a new feature flag

Naming new flags

Risk of a broken master (main) branch

Optionally add a .patch file for automated removal of feature flags

List all the feature flags

Toggle a feature flag

Delete a feature flag

Develop with a feature flag

Backend

Recursion detection

Frontend

Feature actors

Mixing actor types

Instance actor

Current request actor

Use actors for verifying in production

Enable additional objects as actors

Feature flags for licensed features

Feature groups

Controlling feature flags locally

On rails console

On your browser

Logging

Changelog

Feature flags in tests

have_pushed_frontend_feature_flags

stub_feature_flags vs Feature.enable*

Stubbing actor

Controlling feature flags engine in tests

stub_feature_flags: true (default and preferred)

stub_feature_flags: false

End-to-end (QA) tests

Controlling Sidekiq worker behavior with feature flags

Deferring Sidekiq jobs

Dropping Sidekiq jobs

`gitlab_com_derisk` type

`wip` type

`beta` type

`ops` type

`experiment` type

`worker` type

(Deprecated) `development` type

Optionally add a `.patch` file for automated removal of feature flags

`have_pushed_frontend_feature_flags`

`stub_feature_flags` vs `Feature.enable*`

`stub_feature_flags: true` (default and preferred)

`stub_feature_flags: false`