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What is WSDL?

Aug 26, 2007
Remember the good old days, when the internet consisted of nothing more than a few simple web sites formatted in basic HTML? Web design was so easy. You could just slap some text into a file, insert some HTML tags into the document to tell the web browser how the text should appear and what the structure of the page should be, and presto, the job was finished.

As time went on, web design became arduous. Businesses, especially ecommerce businesses that depended upon their internet storefronts to make a living, began to present web designers with near impossible projects that were too difficult to complete with basic HTML. The data being used and the designs that were being implemented for websites required more versatile programming specifications.

Another culprit that made site design so difficult was the advent of the wireless revolution. Wireless devices often have web browsers that work very differently from the ones installed on an ordinary desktop PC. Many of these browsers are not fully compatible with many elements of the HTML programming language. As a result, people surfing the net on their wireless gadgets can not view many websites that are created using only basic HTML.

To solve the dilemma, languages and specifications that could be adapted across any platform were created. The most important new language introduced was XML. XML was a language that defined data without telling the web browser how to display it, as opposed to HTML, which forced data to be displayed a certain way. This enabled data to be viewed on virtually any system because an XML file was a simple text file that could be understood and displayed on any browser.

Now that you understand how and why web design has changed, you are now ready for a brief introduction to the main topic of this article, WSDL (Web Services Description Language). It is written in XML, and it describes the location and methods of a web service. The definition of a web service as it pertains to this discussion is a software system designed to support machine-to-machine interaction over a network.

A WSDL document is a relatively simple XML document that generally consists of four main elements. The first and most important of these elements is the portType element. PortType describes the web service, the messages that can be used and the operations that can be performed. The next element is called message. Message defines the data elements and is akin to the parameters that are used in a function when a function is called. The third element is the types element. Types defines the data types utilized by the web service; the main WSDL data type is XML Schema. The final important element is called binding. It defines the communication protocol for the service.

Since portType is the most important because it defines the operations of the web service in question, we will list the four different types of operations that can be defined by the portType element. The first operation type is one-way, meaning the operation can receive a message but will not return a response. The second type is request-response, which means the operation can receive a message and return a response. Next is solicit-response, which sends a message and waits for a response, and the fourth type of operation is called notification, which simply sends a message and does not wait for a response. Other than portType, the other important WSDL element that is worth describing here is the binding element. It defines the communication protocol to be used. An example would be SOAP, which is an XML-based protocol that enables applications to exchange information using HTTP.

A simple example of a situation in which WSDL might be used would be for a web site that provides real-time stock quotes. When the user inputs the ticker symbol for the stock for which they want the current market price, the web service that supplies the current price would be called to action by a WSDL file that would initiate a solicit-response operation so that the web service will return a response to the message. The response would be either the current quoted price, or perhaps an error message stating that the ticker symbol entered is not valid.

WSDL is an extremely important specification. For those of you who design web sites for a living, especially if you create interactive sites that are connected to web services, you need to understand how to implement these types of files. With so many computers and other devices no longer being fully compatible with HTML, it is important to use an XML-based language such as WSDL to insure that your web site functions properly across all web browsers and platforms.
About the Author
Jim Pretin is the owner of http://www.forms4free.com, a service that helps programmers make email forms.
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