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+# Guidelines for implementing Enterprise Edition feature
+
+- **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][ee-as-ce]
+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`.
+
+[ee-as-ce]: https://gitlab.com/gitlab-org/gitlab-ee/issues/2500
+
+## 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.
+
+[single code base]: https://gitlab.com/gitlab-org/gitlab-ee/issues/2952#note_41016454
+
+### 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 add a `raise NotImplementedError unless defined?(super)`
+  guard clause in 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](https://en.wikipedia.org/wiki/Template_method_pattern).
+  For example, given this base:
+  ``` ruby
+    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:
+  ``` ruby
+    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:
+  ``` ruby
+    module EE::Base
+      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`:
+
+```ruby
+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:
+
+```ruby
+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:
+
+```ruby
+module EE
+  class ApplicationController
+    def after_sign_out_path_for(resource)
+      raise NotImplementedError unless defined?(super)
+
+      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:
+
+```ruby
+def full_private_access?
+  admin?
+end
+```
+
+In EE, the implementation `ee/app/models/ee/users.rb` would be:
+
+```ruby
+def full_private_access?
+  raise NotImplementedError unless defined?(super)
+  super || auditor?
+end
+```
+
+In `lib/gitlab/visibility_level.rb` this method is used to return the
+allowed visibilty levels:
+
+```ruby
+def levels_for_user(user = nil)
+  if user.full_private_access?
+    [PRIVATE, INTERNAL, PUBLIC]
+  elsif # ...
+end
+```
+
+See [CE MR][ce-mr-full-private] and [EE MR][ee-mr-full-private] for
+full implementation details.
+
+[ce-mr-full-private]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12373
+[ee-mr-full-private]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2199
+
+### 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`:
+
+```ruby
+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:
+
+```ruby
+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`:
+
+```javascript
+    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.
+
+```haml
+- 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
+```scss
+.section-body {
+  .section-title {
+    background: $gl-header-color;
+  }
+
+  &.ee-section-body {
+    .section-title {
+      background: $gl-header-color-cyan;
+    }
+  }
+}
+```
+
+#### Good
+```scss
+.section-body {
+  .section-title {
+    background: $gl-header-color;
+  }
+}
+
+/* EE-specific styles */
+.section-body.ee-section-body {
+  .section-title {
+    background: $gl-header-color-cyan;
+  }
+}
+```