Web Forms and Ruby on Rails - 15.18 Exposing Web Services on Your Web Site

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Problem

You want to offer SOAP and XML-RPC web services from your web application.

Solution

Rails comes with a built-in web service generator that makes it easy to expose a controllers actions as web services. You don't have to spend time writing WSDL files or even really know how SOAP and XML-RPC work.

Here's a simple example. First, follow the directions in Recipe 15.16 to create a database table named items, and to generate a model for that table. Don't generate a controller.

Now, run this from the command line:

  ./script/generate web_service Item add edit fetch
        create  app/apis/
        exists  app/controllers/
        exists  test/functional/
        create  app/apis/item_api.rb
        create  app/controllers/ item_controller.rb
        create  test/functional/ item_api_test.rb

This creates an item controller that supports three actions: add, edit, and fetch. But instead of web application actions with .rhtml views, these are web service actions that you access with SOAP or XML-RPC.

A Ruby method doesn't care about the data types of the objects it accepts as arguments, or the data type of its return value. But a SOAP or XML-RPC web service method does care. To expose a Ruby method through a SOAP or XML-RPC interface, we need to define type information for its signature. Open up the file app/apis/ item_api.rb and edit it to look like this:

  class ItemApi < ActionWebService::API::Base
    api_method :add, :expects => [:string, :string], :returns => [:int]
    api_method :edit, :expects => [:int, :string, :string], :returns => [:bool]
    api_method :fetch, :expects => [:int], :returns => [Item]
  end

Now we need to implement the actual web service interface. Open app/controllers/ item_controller.rb and edit it to look like this:

  class ItemController < ApplicationController
    wsdl_service_name 'Item'

    def add(name, value)
      Item.create(:name => name, :value => value).id
    end

    def edit(id, name, value)
      Item.find(id).update_attributes(:name => name, :value => value)
    end

    def fetch(id)
      Item.find(id)
    end
  end

Discussion

The item controller now implements SOAP and XML-RPC web services for the items table. This controller can live alongside an items controller that implements a traditional web interface.*

The URL to the XML-RPC API is http://www.yourserver.com/item/api, and the URL to the SOAP API is http://www.yourserver.com/item/service.wsdl. To test these services, here's a short Ruby script that calls the web service methods through a SOAP client:

  require 'soap/wsdlDriver' 

  wsdl = http://localhost:3000/item/service.wsdl
  item_server = SOAP::WSDLDriverFactory.new(wsdl).create_rpc_driver

  item_id = item_server.add('foo', 'bar')

  if item_server.edit(item_id, 'John', 'Doe')
    puts 'Hey, it worked!'
  else
    puts 'Back to the drawing board...'
  end
  # Hey, it worked!

  item = item_server.fetch(item_id)
  item.class     # => SOAP::Mapping::Object
  item.name      # => "John"
  item.value     # => "Doe"

Here's the XML-RPC equivalent:

  require 'xmlrpc/client'
  item_server = XMLRPC::Client.new2('http://localhost:3000/item/api')

  item_id = item_server.call('Add', 'foo', "bar")
  if item_server.call('Edit', item_id, 'John', 'Doe')
    puts 'Hey, it worked!'
  else
    puts 'Back to the drawing board...'
  end
  # Hey, it worked!

  item = item_server.call('Fetch', item_id)
  # => {"name"=>"John", "id"=>2, "value"=>"Doe"}
  item.class                   # => Hash

See Also

1. Matt Biddulph's article "REST on Rails" describes how to create REST-style web services on top of Rails (http://www.xml.com/pub/a/2005/11/02/rest-on-rails.html)
   
2. Recipe 16.3, "Writing an XML-RPC Client," and Recipe 16.4, "Writing a SOAP Client"
   
3. Recipe 16.5, "Writing a SOAP Server," shows a nonRails implementation of SOAP web services

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