Web Services

XML-RPC is a protocol that uses XML messages to perform Remote Procedure Calls. Requests are encoded in XML and sent via HTTP POST; XML responses are embedded in the body of the HTTP response.

More succinctly, XML-RPC = HTTP + XML + Remote Procedure Calls.

Because XML-RPC is platform independent, diverse applications can communicate with one another. For example, a Java client can speak XML-RPC to a Perl server.

To get a quick sense of XML-RPC, here is a sample XML-RPC request to a weather service (with the HTTP Headers omitted):

<?xml version="1.0″ encoding="ISO-8859-1″?>

<methodCall>

<methodName>weather.getWeather</methodName>

<params>

<param><value>10016</value></param>

</params>

</methodCall>

The request consists of a simple element, which specifies the method name (getWeather) and any method parameters (zip code).

Here is a sample XML-RPC response from the weather service:

<?xml version="1.0″ encoding="ISO-8859-1″?>

<methodResponse>

<params>

<param>

<value><int>65</int></value>

</param>

</params>

</methodResponse>

The response consists of a single element, which specifies the return value (the current temperature). In this case, the return value is specified as an integer.

In many ways, XML-RPC is much simpler than SOAP, and therefore represents the easiest way to get started with Web services.

The official XML-RPC specification is available at XML-RPC.com. Dozens of XML-RPC implementations are available in Perl, Python, Java, and Ruby. See the XML-RPC home page for a complete list of implementations.

Yes and no. The SOAP and WSDL standards are relatively new, but the idea of sending messages between programs over a network has been around for decades. Past efforts to create network-oriented distributed object frameworks like CORBA, DCOM, and Java RMI haven't gone to waste. While creating these tools, the computer science and application development communities discovered and learned important information about how to

build widely distributed systems. In the words of Tim Bray, co-editor of the XML 1.0 spec:

SOAP has the advantages that it's simpler and easier to implement than any existing alternative, and makes better use of the pervasive Web

infrastructure. The effect is that you can pull a system together using SOAP in weeks, not quarters. Obviously, the alternatives offer richer feature sets, particularly in the area of transaction semantics, security, and so on, but at a dramatically higher cost. SOAP will not sweep all before it; but it will be very widely deployed across the Internet and the intranets of this world.

People have been using Remote Procedure Calls (RPC) for some time now, and they long ago discovered how to send such calls over HTTP.

So, what is really new about Web services? The answer is XML.

XML lies at the core of Web services, and provides a common language for describing Remote Procedure Calls, Web services, and Web service directories.

Prior to XML, one could share data among different applications, but XML makes this so much easier to do. In the same vein, one can share services and code without Web services, but XML makes it easier to do these as well.

By standardizing on XML, different applications can more easily talk to one another, and this makes software a whole lot more interesting.

Today's variety of computer platforms, operating systems, programming languages, and development environments shows no signs of diminishing. Heterogeneity is the norm in the IT world, has always been so, and will remain so for the foreseeable future. Getting diverse systems to communicate and cooperate is an ongoing challenge for many organizations. Past efforts to create an overall framework for network-oriented program integration, such as DCOM and CORBA, are complex, proprietary, and expensive to implement. Reducing the cost and complexity of integration projects is a driving force behind Web Services standardization and adoption. While current Web Service systems have fewer features

than a system like CORBA, today's Web Services offer more features than developers actually need. Fortunately, the largest software vendors in the world, including- Microsoft, IBM, Sun, and Oracle-are cooperating on the core Web Service standards. 4th Dimension's Web Services features enable developers to quickly and easily publish and subscribe to Web Services.