- When to use feature flags
- Feature flags in GitLab development
- Risk of a broken default branch
- Types of feature flags
- Feature flag definition and validation
- Create a new feature flag
- List all the feature flags
- Toggle a feature flag
- Delete a feature flag
- Develop with a feature flag
- Changelog
- Feature flags in tests
- Controlling Sidekiq worker behavior with feature flags
Feature flags in the development of GitLab
Design documents:
- (Latest) Feature Flags usage in GitLab development and operations
- Development Feature Flags Architecture
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
andbeta
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 totrue
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.
Constraints
-
default_enabled
: Should be set tofalse
in most cases, and only enabled to resolve temporary scalability issues or help debug production issues. - Maximum Lifespan: 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 totrue
. - 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
andundefined
). - 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 |
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 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.
--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 withhide_
,remove_
, ordisallow_
.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
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
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
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
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
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 totrue
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.
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
orother
.
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.
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.
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
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.