gitlab-org--gitlab-foss/doc/development/feature_flags.md

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# Manage feature flags
Starting from GitLab 9.3 we support feature flags for features in GitLab via
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[Flipper](https://github.com/jnunemaker/flipper/). You should use the `Feature`
class (defined in `lib/feature.rb`) in your code to get, set and list feature
flags.
During runtime you can set the values for the gates via the
[features API](../api/features.md) (accessible to admins only).
## Feature groups
Starting from GitLab 9.4 we support feature groups via
[Flipper groups](https://github.com/jnunemaker/flipper/blob/v0.10.2/docs/Gates.md#2-group).
Feature groups must be defined statically in `lib/feature.rb` (in the
`.register_feature_groups` method), but their implementation can obviously be
dynamic (querying the DB etc.).
Once defined in `lib/feature.rb`, you will be able to activate a
feature for a given feature group via the [`feature_group` param of the features API](../api/features.md#set-or-create-a-feature)
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For GitLab.com, team members have access to feature flags through chatops. Only
percentage gates are supported at this time. Setting a feature to be used 50% of
the time, you should execute `/chatops run feature set my_feature_flag 50`.
## Feature flags for user applications
This document only covers feature flags used in the development of GitLab
itself. Feature flags in deployed user applications can be found at
[Feature Flags](https://docs.gitlab.com/ee/user/project/operations/feature_flags.html)
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## Developing with feature flags
In general, it's better to have a group- or user-based gate, and you should prefer
it over the use of percentage gates. This would make debugging easier, as you
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filter for example logs and errors based on actors too. Furthermore, this allows
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for enabling for the `gitlab-org` group first, while the rest of the users
aren't impacted.
```ruby
# Good
Feature.enabled?(:feature_flag, project)
# Avoid, if possible
Feature.enabled?(:feature_flag)
```
To use feature gates based on actors, the model needs to respond to
`flipper_id`. For example, to enable for the Foo model:
```ruby
class Foo < ActiveRecord::Base
include FeatureGate
end
```
Features that are developed and are intended to be merged behind a feature flag
should not include a changelog entry. The entry should be added in the merge
request removing the feature flags.
In the rare case that you need the feature flag to be on automatically, use
`default_enabled: true` when checking:
```ruby
Feature.enabled?(:feature_flag, project, default_enabled: true)
```
For more information about rolling out changes using feature flags, refer to the
[Rolling out changes using feature flags](rolling_out_changes_using_feature_flags.md)
guide.
### Frontend
For frontend code you can use the method `push_frontend_feature_flag`, which is
available to all controllers that inherit from `ApplicationController`. Using
this method you can expose the state of a feature flag as follows:
```ruby
before_action do
push_frontend_feature_flag(:vim_bindings)
end
def index
# ...
end
def edit
# ...
end
```
You can then check for the state of the feature flag in JavaScript as follows:
```javascript
if ( gon.features.vimBindings ) {
// ...
}
```
The name of the feature flag in JavaScript will always be camelCased, meaning
that checking for `gon.features.vim_bindings` would not work.
### Specs
In the test environment `Feature.enabled?` is stubbed to always respond to `true`,
so we make sure behavior under feature flag doesn't go untested in some non-specific
contexts.
Whenever a feature flag is present, make sure to test _both_ states of the
feature flag. You can stub a feature flag as follows:
```ruby
stub_feature_flags(my_feature_flag: false)
```
## Enabling a feature flag (in development)
In the rails console (`rails c`), enter the following command to enable your feature flag
```ruby
Feature.enable(:feature_flag_name)
```
## Enabling a feature flag (in production)
Check how to [roll out changes using feature flags](rolling_out_changes_using_feature_flags.md).