Accessing Web Services From DHTML
As of December 2011, this topic has been archived. As a result, it is no longer actively maintained. For more information, see Archived Content. For information, recommendations, and guidance regarding the current version of Internet Explorer, see Internet Explorer Developer Center.
January 22, 2001
(Updated November 1, 2001)
One of the most exciting and interesting developments taking place on the Internet at the moment is the emergence of Web Services. For this column, I'll discuss how you can access these Web Services from DHTML by using the new WebService behavior.
Typically when we think of the Internet, we think of a browser displaying HTML pages that are retrieved from a server. However the connectivity of the Internet offers much greater potential if computers connected to the Internet can share information without the need for someone to use a browser. Web Services expose functionality as a service that another computer can call. Examples of such services include supplying stock quotes, weather information, current inventory levels, flight and traffic information, etc. The emerging standard for Web Services is the Simple Object Access Protocol (SOAP), which uses XML as the interchange format, allowing computers to exchange information regardless of operating system, database, or other software. I tend to think of accessing a Web Service as being much like calling a function in a programming language and receiving a response—except the function resides on another machine.
Web Services are at the heart of Microsoft's .NET initiative, and Microsoft is providing tools to assist in exposing and consuming Web Services. For more information and an in-depth discussion of Web Services, take a look at http://msdn.microsoft.com/webservices/.
So why would you wish to access a Web Service from DHTML? Let's imagine a Web page for an airline where customers can find out whether flights will arrive on time. Many airlines offer such Web pages, and they typically have fields that allow users to enter flight numbers and a submit button to submit the flight numbers to a server so fresh pages can be returned to users with the requested information. This model is a little inefficient and clunky in the way it works. In the ideal world, we don't really wish to create a new Web page; we just want to update a single area of the page for the results. This way the user is not taken to a new page and only the requested data need be communicated back to the browser.
Now let's imagine that the airline exposes a Web Service for current flight information, allowing information about flight delays to be shared easily with other computers. Using this model, we can now have an HTML page that uses the Web Service to retrieve the required flight information. When the user enters a flight number and clicks submit, a simple call to the Web Service on the server is made and the flight information is retrieved as part of an XML data packet without having to generate and send a complete HTML page to the browser. This is a simple example of the use of a Web Service from a browser. But take this a step further and think of how we might utilize Web Services in a corporate environment to get updates on inventory and pricing to build a powerful sales application with great interaction, all the while avoiding the pitfalls associated with Web page navigation.
A Web Service can be accessed from the browser with script using XML over http in a very easy process. The WebService behavior is an attached behavior, so it will work with Internet Explorer 5.0 or later. The WebService behavior is written as an HTML Component or HTC and uses the emerging SOAP standard to communicate with Web Services on the server.
For the purposes of demonstration, we'll use the math test Web Service exposed on a public server. This Web Service is extremely simple and offers methods to Add, Subtract, Multiply, and Divide integers. You can try these methods directly from the .asmx page that exposes the Web Service.
Simply enter two integers for the Add method and click on the link to invoke the method. A new browser window then opens to show the result as the XML data is returned.
Now let's take a look at a DHTML page that uses the Add method through the WebService behavior.
First take a look at the div element with the id of service toward the end of the HTML file. This div has the webservice.htc behavior attached to it. This behavior takes care of all the effort involved in calling the service so we don't have to worry about it. The onresult event handler is called by the behavior when the method being called returns a result. Next look at the init() script function that is executed when the page loads. This sets up the service we wish to call with the friendly name math. The Web Service itself is exposed through the mathservice.asmx file.
The button element in the file executes the call to the Add method that is exposed by the Web Service, offering the values of the two text boxes as parameters. The onmyresult() script function then handles the result and displays it on the page.
Note that the method call is asynchronous. This means that once the call to the method on the server has been made, any script on the page will continue to run and the returned result from the method will come later as a callback function. The alternative to asynchronous calls is a synchronous call that returns the result directly to the call to the function. This synchronous calling is the traditional technique used when calling functions in script local to the machine. You call a function and the function executes, then returns the result. However, such synchronous calling is inappropriate for use on the Internet because the Web Service is executing on a different machine than the calling script on the client machine. The reality is that the Web Service might not be available; to have the processing of the browser stall while waiting for the Web Service to return would create an unacceptable user experience.
Note that synchronous calls are available through the Web Service behavior, but asynchronous calls are recommended, as the browser will appear to hang while waiting for the result to return. It is, of course, easier to control the flow of a program if you do not have to worry about asynchronous behavior. Yet for developers, asynchronicity is a reality of programming on the Internet, where bandwidth and availability cannot be controlled. Therefore developers should design and code appropriately to cope with a slow or nonexistent response.
There are some restrictions on using the WebService behavior that should be explained. The WebService behavior is an .htc file built from HTML and script, and is therefore as secure as running script in an HTML page. It's not subject to the ActiveX control level of security associated with running binary native code. To ensure this level of security, both the .htc file and the Web Service must be on the same Internet domain as the HTML file that uses them. This restriction protects users from nefarious use of the service and the data by other domains. So the best way to experiment with this functionality is to host a Web Service with the WebService behavior and the HTML page together. The easiest way to do this is by downloading the .NET Framework.
Despite the necessary restriction, Web Services have the potential to be extremely useful for communicating with a server and avoiding navigation of pages in the process. The example above is extremely simplistic, as it calls a simple method that returns a single result. However, it is possible to call methods that have much more complex functionality, and that return more complex sets of data in XML, which can then be manipulated on the client using the XML DOM, before being bound to a table for display.
In this column I've barely started to uncover the potential that this technology offers, and I expect to write further on this topic as the Web Service model evolves and DHTML support for accessing services also develops.
David Massy occasionally wears sun glasses and pretends to be a dude, but when the dark glasses are removed, he works as a technical evangelist on Windows and Internet Explorer technologies, which means he talks to customers of Microsoft about how they might best use the technologies. Before becoming a technical evangelist, Dave worked on the Internet Explorer team as a program manager. Since Dave is originally from England, he is valiantly struggling to educate his American colleagues on how to correctly pronounce "tomato."