Guidelines for implementing Enterprise Edition features

Write the code and the tests.: As with any code, EE features should have good test coverage to prevent regressions.
Write documentation.: Add documentation to the doc/ directory. Describe the feature and include screenshots, if applicable.
Submit a MR to the www-gitlab-com project.: Add the new feature to the [EE features list][ee-features-list].

Act as CE when unlicensed

Since the implementation of GitLab CE features to work with unlicensed EE instance GitLab Enterprise Edition should work like GitLab Community Edition when no license is active. So EE features always should be guarded by project.feature_available? or group.feature_available? (or License.feature_available? if it is a system-wide feature).

CE specs should remain untouched as much as possible and extra specs should be added for EE. Licensed features can be stubbed using the spec helper stub_licensed_features in EE::LicenseHelpers.

Separation of EE code

We want a single code base eventually, but before we reach the goal, we still need to merge changes from GitLab CE to EE. To help us get there, we should make sure that we no longer edit CE files in place in order to implement EE features.

Instead, all EE codes should be put inside the ee/ top-level directory, and tests should be put inside spec/ee/. We don't use ee/spec for now due to technical limitation. The rest of codes should be as close as to the CE files.

EE-only features

If the feature being developed is not present in any form in CE, we don't need to put the codes under EE namespace. For example, an EE model could go into: ee/app/models/awesome.rb using Awesome as the class name. This is applied not only to models. Here's a list of other examples:

ee/app/controllers/foos_controller.rb
ee/app/finders/foos_finder.rb
ee/app/helpers/foos_helper.rb
ee/app/mailers/foos_mailer.rb
ee/app/models/foo.rb
ee/app/policies/foo_policy.rb
ee/app/serializers/foo_entity.rb
ee/app/serializers/foo_serializer.rb
ee/app/services/foo/create_service.rb
ee/app/validators/foo_attr_validator.rb
ee/app/workers/foo_worker.rb

EE features based on CE features

For features that build on existing CE features, write a module in the EE namespace and prepend it in the CE class. This makes conflicts less likely to happen during CE to EE merges because only one line is added to the CE class - the prepend line.

Since the module would require an EE namespace, the file should also be put in an ee/ sub-directory. For example, we want to extend the user model in EE, so we have a module called ::EE::User put inside ee/app/models/ee/user.rb.

This is also not just applied to models. Here's a list of other examples:

ee/app/controllers/ee/foos_controller.rb
ee/app/finders/ee/foos_finder.rb
ee/app/helpers/ee/foos_helper.rb
ee/app/mailers/ee/foos_mailer.rb
ee/app/models/ee/foo.rb
ee/app/policies/ee/foo_policy.rb
ee/app/serializers/ee/foo_entity.rb
ee/app/serializers/ee/foo_serializer.rb
ee/app/services/ee/foo/create_service.rb
ee/app/validators/ee/foo_attr_validator.rb
ee/app/workers/ee/foo_worker.rb

Overriding CE methods

To override a method present in the CE codebase, use prepend. It lets you override a method in a class with a method from a module, while still having access the class's implementation with super.

There are a few gotchas with it:

you should always extend ::Gitlab::Utils::Override and use override to guard the "overrider" method to ensure that if the method gets renamed in CE, the EE override won't be silently forgotten.
when the "overrider" would add a line in the middle of the CE implementation, you should refactor the CE method and split it in smaller methods. Or create a "hook" method that is empty in CE, and with the EE-specific implementation in EE.
when the original implementation contains a guard clause (e.g. return unless condition), we cannot easily extend the behaviour by overriding the method, because we can't know when the overridden method (i.e. calling super in the overriding method) would want to stop early. In this case, we shouldn't just override it, but update the original method to make it call the other method we want to extend, like a template method pattern. For example, given this base:
```
class Base
  def execute
    return unless enabled?

    # ...
    # ...
  end
end
```
Instead of just overriding Base#execute, we should update it and extract the behaviour into another method:
```
class Base
  def execute
    return unless enabled?

    do_something
  end

  private

  def do_something
    # ...
    # ...
  end
end
```
Then we're free to override that do_something without worrying about the guards:
```
module EE::Base
  extend ::Gitlab::Utils::Override

  override :do_something
  def do_something
    # Follow the above pattern to call super and extend it
  end
end
```
This would require updating CE first, or make sure this is back ported to CE.

When prepending, place them in the ee/ specific sub-directory, and wrap class or module in module EE to avoid naming conflicts.

For example to override the CE implementation of ApplicationController#after_sign_out_path_for:

def after_sign_out_path_for(resource)
  current_application_settings.after_sign_out_path.presence || new_user_session_path
end

Instead of modifying the method in place, you should add prepend to the existing file:

class ApplicationController < ActionController::Base
  prepend EE::ApplicationController
  # ...

  def after_sign_out_path_for(resource)
    current_application_settings.after_sign_out_path.presence || new_user_session_path
  end

  # ...
end

And create a new file in the ee/ sub-directory with the altered implementation:

module EE
  module ApplicationController
    extend ::Gitlab::Utils::Override

    override :after_sign_out_path_for
    def after_sign_out_path_for(resource)
      if Gitlab::Geo.secondary?
        Gitlab::Geo.primary_node.oauth_logout_url(@geo_logout_state)
      else
        super
      end
    end
  end
end

Use self-descriptive wrapper methods

When it's not possible/logical to modify the implementation of a method. Wrap it in a self-descriptive method and use that method.

For example, in CE only an admin is allowed to access all private projects/groups, but in EE also an auditor has full private access. It would be incorrect to override the implementation of User#admin?, so instead add a method full_private_access? to app/models/users.rb. The implementation in CE will be:

def full_private_access?
  admin?
end

In EE, the implementation ee/app/models/ee/users.rb would be:

override :full_private_access?
def full_private_access?
  super || auditor?
end

In lib/gitlab/visibility_level.rb this method is used to return the allowed visibilty levels:

def levels_for_user(user = nil)
  if user.full_private_access?
    [PRIVATE, INTERNAL, PUBLIC]
  elsif # ...
end

See CE MR and EE MR for full implementation details.

Code in `app/controllers/`

In controllers, the most common type of conflict is with before_action that has a list of actions in CE but EE adds some actions to that list.

The same problem often occurs for params.require / params.permit calls.

Mitigations

Separate CE and EE actions/keywords. For instance for params.require in ProjectsController:

def project_params
  params.require(:project).permit(project_params_attributes)
end

# Always returns an array of symbols, created however best fits the use case.
# It _should_ be sorted alphabetically.
def project_params_attributes
  %i[
    description
    name
    path
  ]
end

In the EE::ProjectsController module:

def project_params_attributes
  super + project_params_attributes_ee
end

def project_params_attributes_ee
  %i[
    approvals_before_merge
    approver_group_ids
    approver_ids
    ...
  ]
end

Code in `app/models/`

EE-specific models should extend EE::Model.

For example, if EE has a specific Tanuki model, you would place it in ee/app/models/ee/tanuki.rb.

Code in `app/views/`

It's a very frequent problem that EE is adding some specific view code in a CE view. For instance the approval code in the project's settings page.

Mitigations

Blocks of code that are EE-specific should be moved to partials. This avoids conflicts with big chunks of HAML code that that are not fun to resolve when you add the indentation to the equation.

EE-specific views should be placed in ee/app/views/ee/, using extra sub-directories if appropriate.

Code in `lib/`

Place EE-specific logic in the top-level EE module namespace. Namespace the class beneath the EE module just as you would normally.

For example, if CE has LDAP classes in lib/gitlab/ldap/ then you would place EE-specific LDAP classes in ee/lib/ee/gitlab/ldap.

Code in `spec/`

When you're testing EE-only features, avoid adding examples to the existing CE specs. Also do no change existing CE examples, since they should remain working as-is when EE is running without a license.

Instead place EE specs in the spec/ee/spec folder.

JavaScript code in `assets/javascripts/`

To separate EE-specific JS-files we can also move the files into an ee folder.

For example there can be an app/assets/javascripts/protected_branches/protected_branches_bundle.js and an EE counterpart ee/app/assets/javascripts/protected_branches/protected_branches_bundle.js.

That way we can create a separate webpack bundle in webpack.config.js:

    protected_branches:    '~/protected_branches',
    ee_protected_branches: 'ee/protected_branches/protected_branches_bundle.js',

With the separate bundle in place, we can decide which bundle to load inside the view, using the page_specific_javascript_bundle_tag helper.

- content_for :page_specific_javascripts do
  = page_specific_javascript_bundle_tag('protected_branches')

SCSS code in `assets/stylesheets`

To separate EE-specific styles in SCSS files, if a component you're adding styles for is limited to only EE, it is better to have a separate SCSS file in appropriate directory within app/assets/stylesheets.

In some cases, this is not entirely possible or creating dedicated SCSS file is an overkill, e.g. a text style of some component is different for EE. In such cases, styles are usually kept in stylesheet that is common for both CE and EE, and it is wise to isolate such ruleset from rest of CE rules (along with adding comment describing the same) to avoid conflicts during CE to EE merge.

Bad

.section-body {
  .section-title {
    background: $gl-header-color;
  }

  &.ee-section-body {
    .section-title {
      background: $gl-header-color-cyan;
    }
  }
}

Good

.section-body {
  .section-title {
    background: $gl-header-color;
  }
}

/* EE-specific styles */
.section-body.ee-section-body {
  .section-title {
    background: $gl-header-color-cyan;
  }
}

gitlab-svgs

Conflicts in app/assets/images/icons.json or app/assets/images/icons.svg can be resolved simply by regenerating those assets with yarn run svg.

Guidelines for implementing Enterprise Edition features

Act as CE when unlicensed

Separation of EE code

EE-only features

EE features based on CE features

Overriding CE methods

Use self-descriptive wrapper methods

Code in app/controllers/

Code in app/models/

Code in app/views/

Code in lib/

Code in spec/

JavaScript code in assets/javascripts/

SCSS code in assets/stylesheets

Bad