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Document implicit feature flag checks

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Robert Speicher 2018-10-29 14:54:35 -05:00
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doc/development/rolling_out_changes_using_feature_flags.md
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@ -152,14 +152,31 @@ in RC1, followed by the feature flag being removed in RC2. This in turn means
the feature will be stable by the time we publish a stable package around the
22nd of the month.
## Undefined feature flags default to "on"
## Implicit feature flags
By default, the [`Project#feature_available?`][project-fa],
The [`Project#feature_available?`][project-fa],
[`Namespace#feature_available?`][namespace-fa] (EE), and
[`License.feature_available?`][license-fa] (EE) methods will check if the
specified feature is behind a feature flag. Unless the feature is explicitly
disabled or limited to a percentage of users, the feature flag check will
default to `true`.
[`License.feature_available?`][license-fa] (EE) methods all implicitly check for
a feature flag by the same name as the provided argument.
For example if a feature is license-gated, there's no need to add an additional
explicit feature flag check since the flag will be checked as part of the
`License.feature_available?` call. Similarly, there's no need to "clean up" a
feature flag once the feature has reached general availability.
You'd still want to use an explicit `Feature.enabled?` check if your new feature
isn't gated by a License or Plan.
[project-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/app/models/project_feature.rb#L63-68
[namespace-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/ee/app/models/ee/namespace.rb#L71-85
[license-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/ee/app/models/license.rb#L293-300
### Undefined feature flags default to "on"
An important side-effect of the [implicit feature
flags][#implicit-feature-flags] mentioned above is that unless the feature is
explicitly disabled or limited to a percentage of users, the feature flag check
will default to `true`.
As an example, if you were to ship the backend half of a feature behind a flag,
you'd want to explicitly disable that flag until the frontend half is also ready
@ -171,7 +188,3 @@ to be shipped. You can do this via ChatOps:
Note that you can do this at any time, even before the merge request using the
flag has been merged!
[project-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/app/models/project_feature.rb#L63-68
[namespace-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/ee/app/models/ee/namespace.rb#L71-85
[license-fa]: https://gitlab.com/gitlab-org/gitlab-ee/blob/4cc1c62918aa4c31750cb21dfb1a6c3492d71080/ee/app/models/license.rb#L293-300