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What Is an XML Web Service?

XML Web service

XML Web services are the fundamental building blocks in the move to distributed computing on the Internet. Open standards and the focus on communication and collaboration among people and applications have created an environment where XML Web services are becoming the platform for application integration. Applications are constructed using multiple XML Web services from various sources that work together regardless of where they reside or how they were implemented.


There are probably as many definitions of XML Web Service as there are companies building them, but almost all definitions have these things in common:

XML Web Services expose useful functionality to Web users through a standard Web protocol. In most cases, the protocol used is SOAP.

XML Web services provide a way to describe their interfaces in enough detail to allow a user to build a client application to talk to them. This description is usually provided in an XML document called a Web Services Description Language (WSDL) document.

XML Web services are registered so that potential users can find them easily. This is done with Universal Discovery Description and Integration (UDDI).

Let's see why we should care about XML Web services.


One of the primary advantages of the XML Web services architecture is that it allows programs written in different languages on different platforms to communicate with each other in a standards-based way. Those of you who have been around the industry a while are now saying, "Wait a minute! Didn't I hear those same promises from CORBA and before that DCE? How is this any different?" The first difference is that SOAP is significantly less complex than earlier approaches, so the barrier to entry for a standards-compliant SOAP implementation is significantly lower. Paul Kulchenko maintains a list of SOAP implementations at: http://www.soapware.org/directory/4/implementations which at last count contained 79 entries. You'll find SOAP implementations from most of the big software companies, as you would expect, but you will also find many implementations that are built and maintained by a single developer. The other significant advantage that XML Web services have over previous efforts is that they work with standard Web protocols—XML, HTTP and TCP/IP. A significant number of companies already have a Web infrastructure, and people with knowledge and experience in maintaining it, so again, the cost of entry for XML Web services is significantly less than previous technologies.

We've defined an XML Web service as a software service exposed on the Web through SOAP, described with a WSDL file and registered in UDDI.

The next logical question is. "What can I do with XML Web services?"

The first XML Web services tended to be information sources that you could easily incorporate into applications—stock quotes, weather forecasts, sports scores etc. It's easy to imagine a whole class of applications that could be built to analyze and aggregate the information you care about and present it to you in a variety of ways; for example, you might have a Microsoft® Excel spreadsheet that summarizes your whole financial picture—stocks, 401K, bank accounts, loans, etc. If this information is available through XML Web services, Excel can update it continuously. Some of this information will be free and some might require a subscription to the service. Most of this information is available now on the Web, but XML Web services will make programmatic access to it easier and more reliable.


Exposing existing applications as XML Web services will allow users to build new, more powerful applications that use XML Web services as building blocks. For example, a user might develop a purchasing application to automatically obtain price information from a variety of vendors, allow the user to select a vendor, submit the order and then track the shipment until it is received. The vendor application, in addition to exposing its services on the Web, might in turn use XML Web services to check the customer's credit, charge the customer's account and set up the shipment with a shipping company.


In the future, some of the most interesting XML Web services will support applications that use the Web to do things that can't be done today. For example, one of the services that XML Web Services would make possible is a calendar service. If your dentist and mechanic exposed their calendars through this XML Web service, you could schedule appointments with them on line or they could schedule appointments for cleaning and routine maintenance directly in your calendar if you like. With a little imagination, you can envision hundreds of applications that can be built once you have the ability to program the Web.

WebServices - UDDI

UDDI

Universal Discovery Description and Integration is the yellow pages of Web services. As with traditional yellow pages, you can search for a company that offers the services you need, read about the service offered and contact someone for more information. You can, of course, offer a Web service without registering it in UDDI, just as you can open a business in your basement and rely on word-of-mouth advertising but if you want to reach a significant market, you need UDDI so your customers can find you.

A UDDI directory entry is an XML file that describes a business and the services it offers. There are three parts to an entry in the UDDI directory. The "white pages" describe the company offering the service: name, address, contacts, etc. The "yellow pages" include industrial categories based on standard taxonomies such as the North American Industry Classification System and the Standard Industrial Classification. The "green pages" describe the interface to the service in enough detail for someone to write an application to use the Web service. The way services are defined is through a UDDI document called a Type Model or tModel. In many cases, the tModel contains a WSDL file that describes a SOAP interface to an XML Web service, but the tModel is flexible enough to describe almost any kind of service.

The UDDI directory also includes several ways to search for the services you need to build your applications. For example, you can search for providers of a service in a specified geographic location or for business of a specified type. The UDDI directory will then supply information, contacts, links, and technical data to allow you to evaluate which services meet your requirements. UDDI allows you to find businesses you might want to obtain Web services from. What if you already know whom you want to do business with but you don't know what services are offered? The WS-Inspection specification allows you to browse through a collection of XML Web services offered on a specific server to find which ones might meet your needs.

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