XML Reference Guide

Mar 14, 2003

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While enterprise web services will probably settle in on a standard based on SOAP or some variety of it, another important format is XML-RPC. XML-RPC is a less flexible type of message than a SOAP message, intended specifically for executing Remote Procedure Calls, or functions on a remote server.

First developed by Dave Winer of Radio Userland, XML-RPC is used for applications such as alerting Weblogs.com to changes in a weblog, or for the Internet Topic Exchange, to add a new message to a channel. In your own applications, you may find it convenient to use XML-RPC when all you want to do is tell the server to do something.

For example, I could use XML-RPC to tell Weblogs.com that I've updated my personal site by sending a message to http://rpc.weblogs.com/RPC2:

<?xml version="1.0"?>
<methodCall>
    <methodName>weblogUpdates.ping</methodName>
    <params>
        <param>
            <value>The Vanguard Science Fiction Report</value>
        </param>
        <param>
            <value>http://www.vanguardreport.com/phpnuke/index.php</value>
        </param>
    </params>
</methodCall>

This tells the server at rpc.weblogs.com to execute the method:

weblogUpdates.ping("The Vanguard Science Fiction Report", "http://www.vanguardreport.com/phpnuke/index.php")

I don't need to know (or care) what platform weblogs.com is running; the RPC server on the other end takes care of that for me.

When I send the message, I get back a response. That response could be a fault, or it could be a success message. In this case, if my page has changed, I'd get a response back of

<?xml version="1.0"?>
<methodResponse>
    <params>
        <param>
            <value>
                <struct>
                    <member>
                        <name>flerror</name>
                        <value>
                            <boolean>0</boolean>
                        </value>
                    </member>
                    <member>
                        <name>message</name>
                        <value>Thanks for the ping. We checked and found that the 
                               "The Vanguard Science Fiction Report" weblog has 
                               changed, so it will appear in &lt;a 
                               href="http://www.weblogs.com/changes.xml"&gt;changes.xml
                               &lt;/a&gt; next time it is updated.</value>
                    </member>
                </struct>
            </value>
        </param>
    </params>
</methodResponse>

Notice that the return message has both parameter names and, in the case of the boolean value, a data type. We could just as easily have sent the original request as

<?xml version="1.0"?>
<methodCall>
    <methodName>weblogUpdates.ping</methodName>
    <params>
        <param>
            <name>weblogname</name>
            <value><string>The Vanguard Science Fiction Report</string></value>
        </param>
        <param>
            <name>weblogurl</name>
            <value><string>http://www.vanguardreport.com/phpnuke/index.php</string></value>
        </param>
    </params>
</methodCall>

XML-RPC defines 7 different types: string, boolean, dateTime.iso8601, i4, int, double, and base64, with arrays and structs for record-type information.

In most cases, you'll use XML-RPC without directly constructing the messages. For example, I sent the above message with a Java implementation of XML-RPC from the Apache project (downloadable here). The class simply constructed the message and sent it:

import org.apache.xmlrpc.XmlRpcClient;
import java.util.Vector;

public class TestTopic {

   public static void main(String[] args) {
		
      try {
         XmlRpcClient xmlrpc = new XmlRpcClient ("http://rpc.weblogs.com/RPC2");
         Vector params = new Vector ();

         params.addElement ("The Vanguard Science Fiction Report");
         params.addElement ("http://www.vanguardreport.com/phpnuke/index.php");
  
         Object result = xmlrpc.execute ("weblogUpdates.ping", params);
         System.out.println(result.toString());
      } catch (Exception e) {
         e.printStackTrace();
      }
		
   }
}

It's important to understand that while XML-RPC is will suited to remote procedure calls, it does have several signficant limitations. There is no way to add additional XML data without turning it into a binary file, and there is no mechanism for describing the service for other developers, as we can do for a SOAP service using a WSDL file. Security is also lacking.

On the other hand, it's simple to use, and if these issues aren't a concern for you, XML-RPC can be a simple way to dip your toes into web services.

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