API styleguide

This styleguide recommends best practices for API development.

Instance variables

Please do not use instance variables, there is no need for them (we don't need to access them as we do in Rails views), local variables are fine.


Always use an Entity to present the endpoint's payload.

Methods and parameters description

Every method must be described using the Grape DSL (see https://gitlab.com/gitlab-org/gitlab/blob/master/lib/api/environments.rb for a good example):

  • desc for the method summary. You should pass it a block for additional details such as:

    • The GitLab version when the endpoint was added. If it is behind a feature flag, mention that instead: This feature is gated by the :feature_flag_symbol feature flag.
    • If the endpoint is deprecated, and if so, when will it be removed
  • params for the method params. This acts as description, validation, and coercion of the parameters

A good example is as follows:

desc 'Get all broadcast messages' do
  detail 'This feature was introduced in GitLab 8.12.'
  success Entities::BroadcastMessage
params do
  optional :page,     type: Integer, desc: 'Current page number'
  optional :per_page, type: Integer, desc: 'Number of messages per page'
get do
  messages = BroadcastMessage.all

  present paginate(messages), with: Entities::BroadcastMessage

Declared params

Grape allows you to access only the parameters that have been declared by your params block. It filters out the params that have been passed, but are not allowed.


Exclude params from parent namespaces

By default declared(params)includes parameters that were defined in all parent namespaces.


In most cases you will want to exclude params from the parent namespaces:

declared(params, include_parent_namespaces: false)

When to use declared(params)

You should always use declared(params) when you pass the params hash as arguments to a method call.

For instance:

# bad
User.create(params) # imagine the user submitted `admin=1`... :)

# good
User.create(declared(params, include_parent_namespaces: false).to_h)

Note: declared(params) return a Hashie::Mash object, on which you will have to call .to_h.

But we can use params[key] directly when we access single elements.

For instance:

# good
Model.create(foo: params[:foo])

Using HTTP status helpers

For non-200 HTTP responses, use the provided helpers in lib/api/helpers.rb to ensure correct behaviour (not_found!, no_content! etc.). These will throw inside Grape and abort the execution of your endpoint.

For DELETE requests, you should also generally use the destroy_conditionally! helper which by default returns a 204 No Content response on success, or a 412 Precondition Failed response if the given If-Unmodified-Since header is out of range. This helper calls #destroy on the passed resource, but you can also implement a custom deletion method by passing a block.

Using API path helpers in GitLab Rails codebase

Because we support installing GitLab under a relative URL, one must take this into account when using API path helpers generated by Grape. Any such API path helper usage must be in wrapped into the expose_path helper call.

For instance:

- endpoint = expose_path(api_v4_projects_issues_related_merge_requests_path(id: @project.id, issue_iid: @issue.iid))

Internal API

The internal API is documented for internal use. Please keep it up to date so we know what endpoints different components are making use of.


When writing tests for new API endpoints, consider using a schema fixture located in /spec/fixtures/api/schemas. You can expect a response to match a given schema:

expect(response).to match_response_schema('merge_requests')