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What is Grape?

Grape is a REST-like API micro-framework for Ruby. It is built to complement existing web application frameworks such as Rails and Sinatra by providing a simple DSL to easily provide APIs. It has built-in support for common conventions such as multiple formats, subdomain/prefix restriction, and versioning.

Project Tracking

Installation

Grape is available as a gem, to install it just install the gem:

gem install grape

If you're using Bundler, add the gem to Gemfile.

gem 'grape'

Basic Usage

Grape APIs are Rack applications that are created by subclassing Grape::API. Below is a simple example showing some of the more common features of Grape in the context of recreating parts of the Twitter API.

class Twitter::API < Grape::API
  version 'v1', :using => :header, :vendor => 'twitter'

  helpers do
    def current_user
      @current_user ||= User.authorize!(env)
    end

    def authenticate!
      error!('401 Unauthorized', 401) unless current_user
    end
  end

  resource :statuses do
    get :public_timeline do
      Tweet.limit(20)
    end

    get :home_timeline do
      authenticate!
      current_user.home_timeline
    end

    get '/show/:id' do
      Tweet.find(params[:id])
    end

    post :update do
      authenticate!
      Tweet.create(
        :user => current_user,
        :text => params[:status]
      )
    end
  end

  resource :account do
    before { authenticate! }

    get '/private' do
      "Congratulations, you found the secret!"
    end
  end

end

Optionally, you can define requirements for your named route parameters using regular expressions. The route will match only if all requirements are met.

get '/show/:id', :requirements => { :id => /[0-9]*/ } do
  Tweet.find(params[:id])
end

Mounting

The above sample creates a Rack application that can be run from a rackup config.ru file with rackup:

run Twitter::API

And would respond to the following routes:

GET  /statuses/public_timeline(.json)
GET  /statuses/home_timeline(.json)
GET  /statuses/show/:id(.json)
POST /statuses/update(.json)

In a Rails application, modify config/routes:

mount Twitter::API => "/"

You can mount multiple API implementations inside another one.

class Twitter::API < Grape::API
  mount Twitter::APIv1
  mount Twitter::APIv2
end

Versioning

There are three strategies in which clients can reach your API's endpoints: :header, :path and :param. The default strategy is :header.

version 'v1', :using => :header

Using this versioning strategy, clients should pass the desired version in the HTTP Accept head.

curl -H Accept=application/vnd.twitter-v1+json http://localhost:9292/statuses/public_timeline

By default, the first matching version is used when no Accept header is supplied. This behavior is similar to routing in Rails. To circumvent this default behavior, one could use the :strict option. When this option is set to true, a 404 Not found error is returned when no correct Accept header is supplied.

Path

version 'v1', :using => :path

Using this versioning strategy, clients should pass the desired version in the URL.

curl -H http://localhost:9292/v1/statuses/public_timeline

Serialization takes place automatically.

Param

version 'v1', :using => :param

Using this versioning strategy, clients should pass the desired version as a request parameter, either in the URL query string or in the request body.

curl -H http://localhost:9292/events?apiver=v1

The default name for the query parameter is 'apiver' but can be specified using the :parameter option.

version 'v1', :using => :param, :parameter => "v"

curl -H http://localhost:9292/events?v=v1

Parameters

Parameters are available through the params hash object. This includes GET and POST parameters, along with any named parameters you specify in your route strings.

get do
    Article.order(params[:sort_by])
end

Parameters are also populated from the request body on POST and PUT for JSON and XML content-types.

The Request:

curl -d '{"some_key": "some_value"}' 'http://localhost:9292/json_endpoint' -H Content-Type:application/json -v

The Grape Endpoint:

post '/json_endpoint' do
    params[:some_key]
end

Headers

Headers are available through the env hash object.

get do
    error! 'Unauthorized', 401 unless env['HTTP_SECRET_PASSWORD'] == 'swordfish'
    ...
end

Helpers

You can define helper methods that your endpoints can use with the helpers macro by either giving a block or a module:

module MyHelpers
  def say_hello(user)
    "hey there #{user.name}"
  end
end

class API < Grape::API
  # define helpers with a block
  helpers do
    def current_user
      User.find(params[:user_id])
    end
  end

  # or mix in a module
  helpers MyHelpers

  get '/hello' do
    # helpers available in your endpoint and filters
    say_hello(current_user)
  end
end

Cookies

You can set, get and delete your cookies very simply using cookies method:

class API < Grape::API
  get '/counter' do
    cookies[:counter] ||= 0
    cookies[:counter] += 1
    { :counter => cookies[:counter] }
  end

  delete '/counter' do
    { :result => cookies.delete(:counter) }
  end
end

To set more than value use hash-based syntax:

cookies[:counter] = {
    :value => 0,
    :expires => Time.tomorrow,
    :domain => '.example.com',
    :path => '/'
}
cookies[:counter][:value] +=1

Redirect

You can redirect to a new url

redirect "/new_url"

use permanent redirect

redirect "/new_url", :permanent => true

Raising Errors

You can raise errors explicitly.

error!("Access Denied", 401)

You can also return JSON formatted objects explicitly by raising error! and passing a hash instead of a message.

error!({ "error" => "unexpected error", "detail" => "missing widget" }, 500)

Exception Handling

Grape can be told to rescue all exceptions and instead return them in text or json formats.

class Twitter::API < Grape::API
  rescue_from :all
end

You can also rescue specific exceptions.

class Twitter::API < Grape::API
  rescue_from ArgumentError, NotImplementedError
end

The error format can be specified using error_format. Available formats are :json and :txt (default).

class Twitter::API < Grape::API
  error_format :json
end

You can rescue all exceptions with a code block. The rack_response wrapper automatically sets the default error code and content-type.

class Twitter::API < Grape::API
  rescue_from :all do |e|
    rack_response({ :message => "rescued from #{e.class.name}" })
  end
end

You can also rescue specific exceptions with a code block and handle the Rack response at the lowest level.

class Twitter::API < Grape::API
  rescue_from :all do |e|
    Rack::Response.new([ e.message ], 500, { "Content-type" => "text/error" }).finish
  end
end

Or rescue specific exceptions.

class Twitter::API < Grape::API
  rescue_from ArgumentError do |e|
    Rack::Response.new([ "ArgumentError: #{e.message}" ], 500)
  end
  rescue_from NotImplementedError do |e|
    Rack::Response.new([ "NotImplementedError: #{e.message}" ], 500)
  end
end

Logging

Grape::API provides a logger method which by default will return an instance of the Logger class from Ruby's standard library.

To log messages from within an endpoint, you need to define a helper to make the logger available in the endpoint context:

class API < Grape::API
  helpers do
    def logger
      API.logger
    end
  end
  get '/hello' do
    logger.info "someone said hello"
    "hey there"
  end
end

You can also set your own logger:

class MyLogger
  def warning(message)
    puts "this is a warning: #{message}"
  end
end

class API < Grape::API
  logger MyLogger.new
  helpers do
    def logger
      API.logger
    end
  end
  get '/hello' do
    logger.warning "someone said hello"
    "hey there"
  end
end

Content-Types

By default, Grape supports XML, JSON, Atom, RSS, and text content-types. Your API can declare additional types to support. Response format is determined by the request's extension or Accept header.

class Twitter::API < Grape::API
  content_type :xls, "application/vnd.ms-excel"
end

You can also set the default format. The order for choosing the format is the following.

  • Use the file extension, if specified. If the file is .json, choose the JSON format.
  • Use the format, if specified by the format option.
  • Attempt to find an acceptable format from the Accept header.
  • Use the default format, if specified by the default_format option.
  • Default to :txt otherwise.
class Twitter::API < Grape::API
  format :json
  default_format :json
end

You can override the content-type by setting the Content-Type header.

class API < Grape::API
  get '/script' do
    content_type "application/javascript"
    "var x = 1;"
  end
end

Writing Tests

You can test a Grape API with RSpec by making HTTP requests and examining the response.

Writing Tests with Rack

Use rack-test and define your API as app.

require 'spec_helper'

describe Twitter::API do
  include Rack::Test::Methods

  def app
    Twitter::API
  end

  describe Twitter::API do
    describe "GET /api/v1/statuses" do
      it "returns an empty array of statuses" do
        get "/api/v1/statuses"
        last_response.status.should == 200
        JSON.parse(response.body).should == []
      end
    end
    describe "GET /api/v1/statuses/:id" do
      it "returns a status by id" do
        status = Status.create!
        get "/api/v1/statuses/#{status.id}"
        last_response.body.should == status.to_json
      end
    end
  end
end

Writing Tests with Rails

require 'spec_helper'

describe Twitter::API do
  describe "GET /api/v1/statuses" do
    it "returns an empty array of statuses" do
      get "/api/v1/statuses"
      response.status.should == 200
      JSON.parse(response.body).should == []
    end
  end
  describe "GET /api/v1/statuses/:id" do
    it "returns a status by id" do
      status = Status.create!
      get "/api/v1/statuses/#{status.id}"
      resonse.body.should == status.to_json
    end
  end
end

In Rails, HTTP request tests would go into the spec/request group. You may want your API code to go into app/api - you can match that layout under spec by adding the following in spec/spec_helper.rb.

RSpec.configure do |config|
  config.include RSpec::Rails::RequestExampleGroup, :type => :request, :example_group => {
    :file_path => /spec\/api/
  }
end

Reusable Responses with Entities

Entities are a reusable means for converting Ruby objects to API responses. Entities can be used to conditionally include fields, nest other entities, and build ever larger responses, using inheritance.

Defining Entities

Entities inherit from Grape::Entity, and define a simple DSL. Exposures can use runtime options to determine which fields should be visible, these options are available to :if, :unless, and :proc. The option keys :version and :collection will always be defined. The :version key is defined as api.version. The :collection key is boolean, and defined as true if the object presented is an array.

  • expose SYMBOLS
    • define a list of fields which will always be exposed
  • expose SYMBOLS, HASH
    • HASH keys include :if, :unless, :proc, :as, :using, :format_with, :documentation
      • :if and :unless accept hashes (passed during runtime) or procs (arguments are object and options)
  • expose SYMBOL, {:format_with => :formatter}
    • expose a value, formatting it first
    • :format_with can only be applied to one exposure at a time
  • expose SYMBOL, {:as => "alias"}
    • Expose a value, changing its hash key from SYMBOL to alias
    • :as can only be applied to one exposure at a time
  • expose SYMBOL BLOCK
    • block arguments are object and options
    • expose the value returned by the block
    • block can only be applied to one exposure at a time
module API
  module Entities
    class User < Grape::Entity
      expose :first_name, :last_name
      expose :field, :documentation => {:type => "string", :desc => "words go here"}
      expose :email, :if => {:type => :full}
      expose :user_type, user_id, :if => lambda{|user,options| user.confirmed?}
      expose(:name){|user,options| [user.first_name, user.last_name].join(' ')}
      expose :latest_status, :using => API::Status, :as => :status
    end
  end
end

module API
  module Entities
    class UserDetailed < API::Entities::User
      expose :account_id
    end
  end
end

Using Entities

Once an entity is defined, it can be used within endpoints, by calling #present. The #present method accepts two arguments, the object to be presented and the options associated with it. The options hash must always include :with, which defines the entity to expose.

If the entity includes documentation it can be included in an endpoint's description.

module API
  class Users < Grape::API
    version 'v1'

    desc 'User index', {
      :object_fields => API::Entities::User.documentation
    }
    get '/users' do
      @users = User.all
      type = current_user.admin? ? :full : :default
      present @users, with: API::Entities::User, :type => type
    end
  end
end

Caveats

Entities with duplicate exposure names and conditions will silently overwrite one another. In the following example, when object#check equals "foo", only afield will be exposed. However, when object#check equals "bar" both bfield and foo will be exposed.

module API
  module Entities
    class User < Grape::Entity
      expose :afield, :foo, :if => lambda{|object,options| object.check=="foo"}
      expose :bfield, :foo, :if => lambda{|object,options| object.check=="bar"}
    end
  end
end

This can be problematic, when you have mixed collections. Using #respond_to? is safer.

module API
  module Entities
    class User < Grape::Entity
      expose :afield, :if => lambda{|object,options| object.check=="foo"}
      expose :bfield, :if => lambda{|object,options| object.check=="bar"}
      expose :foo, :if => lambda{object,options| object.respond_to?(:foo)}
    end
  end
end

Describing and Inspecting an API

Grape lets you add a description to an API along with any other optional elements that can also be inspected at runtime. This can be useful for generating documentation. If the response requires documentation, consider using an entity.

class TwitterAPI < Grape::API

  version 'v1'

  desc "Retrieves the API version number."
  get "version" do
    api.version
  end

  desc "Reverses a string.", { :params =>
    { "s" => { :desc => "string to reverse", :type => "string" }}
  }
  get "reverse" do
    params[:s].reverse
  end
end

Grape then exposes arrays of API versions and compiled routes. Each route contains a route_prefix, route_version, route_namespace, route_method, route_path and route_params. The description and the optional hash that follows the API path may contain any number of keys and its values are also accessible via dynamically-generated route_[name] functions.

TwitterAPI::versions # yields [ 'v1', 'v2' ]
TwitterAPI::routes # yields an array of Grape::Route objects
TwitterAPI::routes[0].route_version # yields 'v1'
TwitterAPI::routes[0].route_description # yields [ { "s" => { :desc => "string to reverse", :type => "string" }} ]

Parameters can also be tagged to the method declaration itself.

class StringAPI < Grape::API
  get "split/:string", { :params => { "token" => "a token" }, :optional_params => { "limit" => "the limit" } } do
    params[:string].split(params[:token], (params[:limit] || 0))
  end
end

StringAPI::routes[0].route_params # yields a hash {"string" => "", "token" => "a token"}
StringAPI::routes[0].route_optional_params # yields a hash {"limit" => "the limit"}

It's possible to retrieve the information about the current route from within an API call with route.

class MyAPI < Grape::API
  desc "Returns a description of a parameter.", { :params => { "id" => "a required id" } }
  get "params/:id" do
    route.route_params[params[:id]] # returns "a required id"
  end
end

You can use this information to create a helper that will check if the request has all required parameters:

class MyAPI < Grape::API

  helpers do
    def validate_request!
      # skip validation if no parameter is declared
      return unless route.route_params
      route.route_params.each do |k, v|
        if !params.has_key? k
          error!("Missing field: #{k}", 400)
        end
      end
    end
  end

  before { validate_request! }

  desc "creates a new item resource", :params => { :name => 'name is a required parameter' }
  post :items do
    ...
  end
end

Anchoring

Grape by default anchors all request paths, which means that the request URL should match from start to end to match, otherwise a 404 Not Found is returned. However, this is sometimes not what you want, because it is not always known up front what can be expected from the call. This is because Rack-mount by default anchors requests to match from the start to the end, or not at all. Rails solves this problem by using a :anchor => false option in your routes. In Grape this option can be used as well when a method is defined.

For instance when you're API needs to get part of an URL, for instance:

class UrlAPI < Grape::API
  namespace :urls do
    get '/(*:url)', :anchor => false do
      some_data
    end
  end
end

This will match all paths starting with '/urls/'. There is one caveat though: the params[:url] parameter only holds the first part of the request url. Luckily this can be circumvented by using the described above syntax for path specification and using the PATH_INFO Rack environment variable, using env["PATH_INFO"]. This will hold everything that comes after the '/urls/' part.

Note on Patches/Pull Requests

  • Fork the project
  • Write tests for your new feature or a test that reproduces a bug
  • Implement your feature or make a bug fix
  • Do not mess with Rakefile, version or history
  • Commit, push and make a pull request. Bonus points for topical branches.

License

MIT License. See LICENSE for details.

Copyright (c) 2010-2012 Michael Bleigh and Intridea, Inc.