XML-RPC: Bridging the Gap Between Perl and Java

In the early months of 2001, the "Interoperability" dream is finally coming true. We're moving away from proprietary binary protocols like DCOM or CORBA. Instead, we're using the "Universal Language" of the web: XML over HTTP. While some are pushing for the complex SOAP specification, many of us are sticking with the elegant simplicity of XML-RPC.

The Concept

XML-RPC allows you to call a function on a remote server as if it were local. The request is an XML document sent via HTTP POST, and the response is also XML.

The Java Server (using Apache XML-RPC)

On the server side, we can expose a simple Java class to the world.

public class Calculator {
    public int add(int i1, int i2) {
        return i1 + i2;
    }
}

// In your Servlet or Main class:
XmlRpcServer xmlrpc = new XmlRpcServer();
xmlrpc.addHandler("math", new Calculator());

The Perl Client (using Frontier::Client)

Perl is still the "glue of the internet," and its XML-RPC support is excellent.

use Frontier::Client;

my $server = Frontier::Client->new(url => 'http://java-server.example.com/RPC2');

my $result = $server->call('math.add', 5, 10);

print "The sum is: $result\n";

Under the Hood: The XML Payload

What’s actually going over the wire? It’s a readable (though verbose) XML document:

<?xml version="1.0"?>
<methodCall>
  <methodName>math.add</methodName>
  <params>
    <param><value><int>5</int></value></param>
    <param><value><int>10</int></value></param>
  </params>
</methodCall>

Why XML-RPC?

Unlike the heavy-weight enterprise solutions, XML-RPC has a tiny specification (it fits on one page!). It supports basic data types: strings, integers, booleans, dates, and even base64 for binary data. It doesn't have the overhead of WSDLs or complex namespaces. In a world where we're just trying to get different systems to share a bit of data, XML-RPC is a breath of fresh air.

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