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Top Ten FAQs for Web Servicesby Ethan Cerami, author of Web Services Essentials
Web services represent an important evolutionary step in building distributed applications. But, what exactly is a Web service? What is the Web service protocol stack? And, does the World Wide Web Consortium support any Web service standards?
Below are answers to the top ten most frequently asked questions (FAQs) about Web services. Together, they provide an overview of the Web services landscape as well as links to additional resources for more in-depth material.
1. What is a Web service?
Many people and companies have debated the exact definition of Web services. At a minimum, however, a Web service is any piece of software that makes itself available over the Internet and uses a standardized XML messaging system.
XML is used to encode all communications to a Web service. For example, a client invokes a Web service by sending an XML message, then waits for a corresponding XML response. Because all communication is in XML, Web services are not tied to any one operating system or programming language--Java can talk with Perl; Windows applications can talk with Unix applications.
Beyond this basic definition, a Web service may also have two additional (and desirable) properties:
Web services currently run a wide gamut from news syndication and stock-market data to weather reports and package-tracking systems. For a quick look at the range of Web services currently available, check out the XMethods directory of Web services.
2. What is new about Web services?
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.
3. I keep reading about Web services, but I have never actually seen one. Can you show me a real Web service in action?
If you want a more intuitive feel for Web services, try out the IBM Web Services Browser, available on the IBM Alphaworks site. The browser provides a series of Web services demonstrations. Behind the scenes, it ties together SOAP, WSDL, and UDDI to provide a simple plug-and-play interface for finding and invoking Web services. For example, you can find a stock-quote service, a traffic-report service, and a weather service. Each service is independent, and you can stack services like building blocks. You can, therefore, create a single page that displays multiple services--where the end result looks like a stripped-down version of my.yahoo or my.excite.
4. What is the Web service protocol stack?
The Web service protocol stack is an evolving set of protocols used to define, discover, and implement Web services. The core protocol stack consists of four layers:
Beyond the essentials of XML-RPC, SOAP, WSDL, and UDDI, the Web service protocol stack includes a whole zoo of newer, evolving protocols. These include WSFL (Web Services Flow Language), SOAP-DSIG (SOAP Security Extensions: Digital Signature), and USML (UDDI Search Markup Language). For an overview of these protocols, check out Pavel Kulchenko's article, Web Services Acronyms, Demystified, on XML.com.
Fortunately, you do not need to understand the full protocol stack to get started with Web services. Assuming you already know the basics of HTTP, it is best to start at the XML Messaging layer and work your way up.
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