Presents your  
XML AND WEB SERVICES E-NEWSLETTER for November 3, 2003 
 
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WHY XML IS IMPORTANT TO WEB SERVICES   
  
Web services is often described in terms of its component technologies. 
The use of SOAP, UDDI, WSDL, XML, and HTTP all comprise the pieces of the 
Web services architecture and each one contributes important 
functionality. Here are some of the things that XML technology
brings to the Web services platform.   
  
STANDARDIZATION   
  
As with all of the Web services technology, standardization is key to 
making XML successful. Using XML provides a standard format for data being 
transmitted to and from Web services applications. The W3C manages the 
XML standard and publishes it for global adoption by XML technology 
providers, ensuring the compatibility of XML products.   
  
Beyond the XML engines and products are the XML grammars. Each XML 
grammar represents a standard approach for formatting data within a
particular problem space. By agreeing on and using XML grammars, you
can standardize the data format used to communicate between applications.   
  
INTERFACE CONTRACT   
  
When programming with interfaces, programmers often refer to them as 
contracts. Programmatically, an interface provides the structure for 
programming without exposing the implementation. XML has a similar feature
when you use DTDs and schemas. Both DTDs and schemas describe the structure 
and rules for creating XML documents. A set of these rules organized
into one or more related DTDs or schemas is called an XML grammar.   
  
Employing an XML grammar not only helps standardize the communication 
between applications, but it also provides developers with an interface 
contract. In other words, the developers don't need to know the 
implementation (i.e., an actual XML document) to create an application
that understands the document's structure. The DTD or schema describes the
structure and can be used as the interface contract for development. Any change 
to the DTD alters the contract.   
  
SIMPLIFICATION   
  
Another key aspect of XML is its simple nature. I'm not implying that 
all XML implementations are a cakewalk; rather, that the concept of XML and 
the general approach is simple. For example, XML is based on text 
documents. This simplifies the process of opening up an XML document 
and viewing the contents.   
  
It's also important that XML be simple to help simplify the Web services
model. Web services is inherently more complex than XML alone. Now, if 
XML were as complex as CORBA, and if all of the other Web services 
technologies were equally complex, then Web services technology would be
too cumbersome. But by using simple building blocks, such as XML, SOAP, and
so on, Web services achieves a tolerable level of complexity without
creating unmanageable applications.   
  
ENCAPSULATION   

One of my favorite features for using XML, particularly for Web services
development, is encapsulation. Encapsulation is essentially the concept 
of taking one or more chunks of data and grouping them together as a 
single object rather than as several independent objects. While this is 
similar to aggregation, the important difference is that encapsulation 
generally implies that there are no external dependencies.   

You can create XML documents with external dependencies; however, in a 
transactional Web services model, it makes more sense to take advantage of 
encapsulation. By designing XML solutions that package dependencies, you
can often eliminate problems in transactions. Forcing a transaction to 
locate external data and reconcile it with XML data can be detrimental, 
particularly if the external data is apt to change. Encapsulating the data 
into the XML grammar makes it available for the transaction with the 
correct data.   

Brian Schaffner is an associate director for Fujitsu Consulting. He 
provides architecture, design, and development support for Fujitsu's 
Technology Consulting practice.

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