# API style guide

This style guide 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.

## Entities

Always use an [Entity](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/api/entities) to present the endpoint's payload.

## Documentation

API endpoints must come with [documentation](documentation/styleguide.md#api), unless it is internal or behind a feature flag.
The docs should be in the same merge request, or, if strictly necessary,
in a follow-up with the same milestone as the original merge request.

## Methods and parameters description

Every method must be described using the [Grape DSL](https://github.com/ruby-grape/grape#describing-methods)
(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 parameters. This acts as description,
  [validation, and coercion of the parameters](https://github.com/ruby-grape/grape#parameter-validation-and-coercion)

A good example is as follows:

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

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

## Declared parameters

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

– <https://github.com/ruby-grape/grape#declared>

### Exclude parameters from parent namespaces

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

– <https://github.com/ruby-grape/grape#include-parent-namespaces>

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

```ruby
declared(params, include_parent_namespaces: false)
```

### When to use `declared(params)`

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

For instance:

```ruby
# 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:

```ruby
# 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 behavior (`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](../install/relative_url.md), 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:

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

## Internal API

The [internal API](./internal_api.md) is documented for internal use. Please keep it up to date so we know what endpoints
different components are making use of.

## Avoiding N+1 problems

In order to avoid N+1 problems that are common when returning collections
of records in an API endpoint, we should use eager loading.

A standard way to do this within the API is for models to implement a
scope called `with_api_entity_associations` that will preload the
associations and data returned in the API. An example of this scope can
be seen in
[the `Issue` model](https://gitlab.com/gitlab-org/gitlab/blob/2fedc47b97837ea08c3016cf2fb773a0300a4a25/app%2Fmodels%2Fissue.rb#L62).

In situations where the same model has multiple entities in the API
(for instance, `UserBasic`, `User` and `UserPublic`) you should use your
discretion with applying this scope. It may be that you optimize for the
most basic entity, with successive entities building upon that scope.

The `with_api_entity_associations` scope will also [automatically preload
data](https://gitlab.com/gitlab-org/gitlab/blob/19f74903240e209736c7668132e6a5a735954e7c/app%2Fmodels%2Ftodo.rb#L34)
for `Todo` _targets_ when returned in the Todos API.

For more context and discussion about preloading see
[this merge request](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/25711)
which introduced the scope.

### Verifying with tests

When an API endpoint returns collections, always add a test to verify
that the API endpoint does not have an N+1 problem, now and in the future.
We can do this using [`ActiveRecord::QueryRecorder`](query_recorder.md).

Example:

```ruby
def make_api_request
  get api('/foo', personal_access_token: pat)
end

it 'avoids N+1 queries', :request_store do
  # Firstly, record how many PostgreSQL queries the endpoint will make
  # when it returns a single record
  create_record

  control = ActiveRecord::QueryRecorder.new { make_api_request }

  # Now create a second record and ensure that the API does not execute
  # any more queries than before
  create_record

  expect { make_api_request }.not_to exceed_query_limit(control)
end
```

## Testing

When writing tests for new API endpoints, consider using a schema [fixture](./testing_guide/best_practices.md#fixtures) located in `/spec/fixtures/api/schemas`. You can `expect` a response to match a given schema:

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

Also see [verifying N+1 performance](#verifying-with-tests) in tests.
