XML-RPC and XML-RPC Server Classes

CodeIgniter's XML-RPC classes permit you to send requests to another server, or set up your own XML-RPC server to receive requests.

What is XML-RPC?

Quite simply it is a way for two computers to communicate over the internet using XML. One computer, which we will call the client, sends an XML-RPC request to another computer, which we will call the server. Once the server receives and processes the request it will send back a response to the client.

For example, using the MetaWeblog API, an XML-RPC Client (usually a desktop publishing tool) will send a request to an XML-RPC Server running on your site. This request might be a new weblog entry being sent for publication, or it could be a request for an existing entry for editing. When the XML-RPC Server receives this request it will examine it to determine which class/method should be called to process the request. Once processed, the server will then send back a response message.

For detailed specifications, you can visit the XML-RPC site.

Initializing the Class

Like most other classes in CodeIgniter, the XML-RPC and XML-RPCS classes are initialized in your controller using the $this->load->library function:

To load the XML-RPC class you will use:

$this->load->library('xmlrpc');

Once loaded, the xml-rpc library object will be available using: $this->xmlrpc

To load the XML-RPC Server class you will use:


$this->load->library('xmlrpc');

$this->load->library('xmlrpcs');

Once loaded, the xml-rpcs library object will be available using: $this->xmlrpcs

Note: When using the XML-RPC Server class you must load BOTH the XML-RPC class and the XML-RPC Server class.

Sending XML-RPC Requests

To send a request to an XML-RPC server you must specify the following information:

The URL of the server
The method on the server you wish to call
The request data (explained below).

Here is a basic example that sends a simple Weblogs.com ping to the Ping-o-Matic

$this->load->library('xmlrpc');



$this->xmlrpc->server('http://rpc.pingomatic.com/', 80);

$this->xmlrpc->method('weblogUpdates.ping');




$request = array('My Photoblog', 'http://www.my-site.com/photoblog/');

$this->xmlrpc->request($request);



if ( ! $this->xmlrpc->send_request())

{

    echo $this->xmlrpc->display_error();

}

Explanation

The above code initializes the XML-RPC class, sets the server URL and method to be called (weblogUpdates.ping). The request (in this case, the title and URL of your site) is placed into an array for transportation, and compiled using the request() function. Lastly, the full request is sent. If the send_request() method returns false we will display the error message sent back from the XML-RPC Server.

Anatomy of a Request

An XML-RPC request is simply the data you are sending to the XML-RPC server. Each piece of data in a request is referred to as a request parameter. The above example has two parameters: The URL and title of your site. When the XML-RPC server receives your request, it will look for parameters it requires.

Request parameters must be placed into an array for transportation, and each parameter can be one of seven data types (strings, numbers, dates, etc.). If your parameters are something other than strings you will have to include the data type in the request array.

Here is an example of a simple array with three parameters:

$request = array('John', 'Doe', 'www.some-site.com');

$this->xmlrpc->request($request);

If you use data types other than strings, or if you have several different data types, you will place each parameter into its own array, with the data type in the second position:


$request = array (

                   array('John', 'string'),

                   array('Doe', 'string'),

                   array(FALSE, 'boolean'),

                   array(12345, 'int')

                 );


$this->xmlrpc->request($request);

The Data Types section below has a full list of data types.

Creating an XML-RPC Server

An XML-RPC Server acts as a traffic cop of sorts, waiting for incoming requests and redirecting them to the appropriate functions for processing.

To create your own XML-RPC server involves initializing the XML-RPC Server class in your controller where you expect the incoming request to appear, then setting up an array with mapping instructions so that incoming requests can be sent to the appropriate class and method for processing.

Here is an example to illustrate:


$this->load->library('xmlrpc');

$this->load->library('xmlrpcs');



$config['functions]['new_post'] = array('function' => 'My_blog.new_entry'),

$config['functions]['update_post'] = array('function' => 'My_blog.update_entry');

$config['object'] = $this;



$this->xmlrpcs->initialize($config);

$this->xmlrpcs->serve();

The above example contains an array specifying two method requests that the Server allows. The allowed methods are on the left side of the array. When either of those are received, they will be mapped to the class and method on the right.

The 'object' key is a special key that you pass an instantiated class object with, which is necessary when the method you are mapping to is not part of the CodeIgniter super object.

In other words, if an XML-RPC Client sends a request for the new_post method, your server will load the My_blog class and call the new_entry function. If the request is for the update_post method, your server will load the My_blog class and call the update_entry function.

The function names in the above example are arbitrary. You'll decide what they should be called on your server, or if you are using standardized APIs, like the Blogger or MetaWeblog API, you'll use their function names.

Processing Server Requests

When the XML-RPC Server receives a request and loads the class/method for processing, it will pass an object to that method containing the data sent by the client.

Using the above example, if the new_post method is requested, the server will expect a class to exist with this prototype:

class My_blog extends Controller {



    function new_post($request)

    {

	

    }

}

The $request variable is an object compiled by the Server, which contains the data sent by the XML-RPC Client. Using this object you will have access to the request parameters enabling you to process the request. When you are done you will send a Response back to the Client.

Below is a real-world example, using the Blogger API. One of the methods in the Blogger API is getUserInfo(). Using this method, an XML-RPC Client can send the Server a username and password, in return the Server sends back information about that particular user (nickname, user ID, email address, etc.). Here is how the processing function might look:

class My_blog extends Controller {



    function getUserInfo($request)

    {


        $username = 'smitty';

        $password = 'secretsmittypass';



        $this->load->library('xmlrpc');

    

        $parameters = $request->output_parameters();

    

        if ($parameters['1'] != $username AND $parameters['2'] != $password)

        {

            return $this->xmlrpc->send_error_message('100', 'Invalid Access');

        }

    

        $response = array(array('nickname'  => array('Smitty','string'),

                                'userid'    => array('99','string'),

                                'url'       => array('http://yoursite.com','string'),

                                'email'     => array('jsmith@yoursite.com','string'),

                                'lastname'  => array('Smith','string'),

                                'firstname' => array('John','string')

                                ),

                         'struct');



        return $this->xmlrpc->send_response($response);

    }

}

Notes:

The output_parameters() function retrieves an indexed array corresponding to the request parameters sent by the client. In the above example, the output parameters will be the username and password.

If the username and password sent by the client were not valid, and error message is returned using send_error_message().

If the operation was successful, the client will be sent back a response array containing the user's info.

Formatting a Response

Similar to Requests, Responses must be formatted as an array. However, unlike requests, a response is an array that contains a single item. This item can be an array with several additional arrays, but there can be only one primary array index. In other words, the basic prototype is this:

$response = array('Response data', 'array');

Responses, however, usually contain multiple pieces of information. In order to accomplish this we must put the response into its own array so that the primary array continues to contain a single piece of data. Here's an example showing how this might be accomplished:


$response = array (

                   array(

                         'first_name' => array('John', 'string'),

                         'last_name' => array('Doe', 'string'),

                         'member_id' => array(123435, 'int'),

                         'todo_list' => array(array('clean house', 'call mom', 'water plants'), 'array'),

                        ),

                 'struct'

                 );

Notice that the above array is formatted as a struct. This is the most common data type for responses.

As with Requests, a response can be one of the seven data types listed in the Data Types section.

Sending an Error Response

If you need to send the client an error response you will use the following:

return $this->xmlrpc->send_error_message('123', 'Requested data not available');

The first parameter is the error number while the second parameter is the error message.

Creating Your Own Client and Server

To help you understand everything we've covered thus far, let's create a couple controllers that act as XML-RPC Client and Server. You'll use the Client to send a request to the Server and receive a response.

The Client

Using a text editor, create a controller called xmlrpc_client.php. In it, place this code and save it to your applications/controllers/ folder:

Note: In the above code we are using a "url helper". You can find more information in the Helpers Functions page.

The Server

Using a text editor, create a controller called xmlrpc_server.php. In it, place this code and save it to your applications/controllers/ folder:

Try it!

Now visit the your site using a URL similar to this:

www.your-site.com/index.php/xmlrpc_client/

You should now see the message you sent to the server, and its response back to you.

The client you created sends a message ("How's is going?") to the server, along with a request for the "Greetings" method. The Server receives the request and maps it to the "process" function, where a response is sent back.

XML-RPC Function Reference

$this->xmlrpc->server()

Sets the URL and port number of the server to which a request is to be sent:

$this->xmlrpc->server('http://www.sometimes.com/pings.php', 80);

$this->xmlrpc->timeout()

Set a time out period (in seconds) after which the request will be canceled:

$this->xmlrpc->timeout(6);

$this->xmlrpc->method()

Sets the method that will be requested from the XML-RPC server:

$this->xmlrpc->method('method');

Where method is the name of the method.

$this->xmlrpc->request()

Takes an array of data and builds request to be sent to XML-RPC server:

$request = array(array('My Photoblog', 'string'), 'http://www.yoursite.com/photoblog/');

$this->xmlrpc->request($request);

$this->xmlrpc->send_request()

The request sending function. Returns boolean TRUE or FALSE based on success for failure, enabling it to be used conditionally.

$this->xmlrpc->set_debug(TRUE);

Enables debugging, which will display a variety of information and error data helpful during development.

$this->xmlrpc->display_error()

Returns an error message as a string if your request failed for some reason.

echo $this->xmlrpc->display_error();

$this->xmlrpc->display_response()

Returns the response from the remote server once request is received. The response will typically be an associative array.

$this->xmlrpc->display_response();

$this->xmlrpc->send_error_message()

This function lets you send an error message from your server to the client. First parameter is the error number while the second parameter is the error message.