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| XML: Leading the March to Web Services (cont.)
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The Final Step: Invoking Remote Components Using SOAP
Taken altogether, XML schema, XML documents, and object serialization to XML, the final step toward Web services is rather obvious. If you could only get companies to agree on a schema, then one machine could take advantage of code running on any other machine accessible via the Web by sending an HTTP-encoded, XML-based message to a "listener" on the receiver's Web server. The listener would read the message, pass any enclosed message data to the appropriate program or component, and then encode the response in XML and return that to the calling machine, which would parse the return message and extract the response. The message format would need to be simple and flexible, as well as easy to create, parse, and understand; yet robust enough to be useful for arbitrary messages. Of course, that step was taken, and the result is the Simple Object Access Protocol (SOAP). You can find reams of information explaining the SOAP "wrappers" elsewhere, but you don't need to know the details to appreciate the fact that XML is the enabling technology.
| If you could only get companies to agree on a schema, then one machine could take advantage of code running on any other machine accessible via the Web by sending an HTTP-encoded, XML-based message to a "listener" on the receiver's Web server. |
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You can invoke the methods of remote components by sending a SOAP message and parsing the return message, but you must first know the location, the method names, parameter types, and return types of the methods you want to invoke. But how can you find available Web services that match your needs? And once located, how can you discover their method names and parameter types?
XML comes to the rescue again. Two other emerging XML-based standards, Web Service Description Language (WSDL) and Universal Description, Discovery and Integration (UDDI), provide Web service discovery and description services. UDDI is a specification for building XML-document based repositories for finding Web services. Companies store descriptions of their Web services in a repository, and potential customers find the Web services by searching the repository for specific business types, service types, or keywords. You can write a program to search the repository for the location of Web services that match your needs by searching the Web service descriptions available in the UDDI repository. Thus UDDI repositories solve the first problem—location. After finding a Web service, a WSDL document describes the public interfaces to the service—the method names, parameter types, and the return types.
| It has been interesting to see how, with few exceptions (and in contrast to the usual foot-dragging on standards), companies throughout the world stampeded toward agreement on SOAP with unprecedented speed. |
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SOAP obviously fills an immediate need. It has been interesting to see how, with few exceptions (and in contrast to the usual foot-dragging on standards), companies throughout the world stampeded toward agreement on SOAP with unprecedented speed. SOAP and XML—and thus Web services—have been adopted and put into production more quickly than any other computer technology—even more quickly than HTML and Java. And the stampede is just beginning. Over the next few years, although the term "Web services" will probably fade into the background as the use of XML wrappers (such as SOAP and XML-RPC) for calling remote procedures become a standard part of interaction between systems, the technology itself will not.
As you can see, XML is the real hero behind Web services. Without XML, Web services wouldn't even exist. It's amazing that the simple ideas behind XML—tagging content with names and arranging it hierarchically—have led to such rapid and resounding change in the risk-averse business community. And Web services are only one facet of XML's evolution.
A. Russell Jones can be reached at rjones@devx.com.
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Introduction
Step 3: Achieving Firewall Transparency
Step 2: The Need for Data Types and Schema
The Final Step: Invoking Remote Components Using SOAP
