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

# Manage feature flags

Starting from GitLab 9.3 we support feature flags for features in GitLab via
[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)

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

GitLab does not yet support the use of feature flags in deployed user applications.
You can follow the progress on that [in the issue on our issue tracker](https://gitlab.com/gitlab-org/gitlab-ee/issues/779).

## 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
filter for example logs and errors based on actors too. Futhermore, this allows
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)
```

### 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.

If you need to test the feature flag in a different state, you need to stub it with:

```ruby
stub_feature_flags(my_feature_flag: false)
```