2.9 QA: A time to recap and reflect

A: Web services are totally and unabashedly XML-centric. All data transfers to and from a Web service are always in the form of XML documents. The input parameters to a Web service are thus structured and defined using XML, as is the output generated by a Web service. In addition, the I/O operations of a Web service, which are implemented in the form of a remote procedure call mechanism, are realized using SOAP, which happens to be an XML-based messaging scheme. The workings of a Web service are also described using XML, in this case WSDL, which is an XML derivative.

A: No, XML is definitely not a replacement for HTML. Though they share a common ancestry (namely, SGML), XML and HTML serve different purposes. HTML deals with the presentation of data for visual consumption. It does not address what the data means or attempt to categorize the data. This, on the other hand, is what XML is all about. Thus, XML per se, in marked contrast to HTML, does not concern itself about how data should be formatted. If you want the data contained within an XML document to be presented in formatted mode, then you need to use Cascading Stylesheets or XSL, so XML is certainly not a replacement for HTML. At a stretch you could contend, if you want to be generous, that they complement each other, but that is about as close as you can get. HTML is chalk and XML is cheese. That is how different they are.

A: XML is a flexible, no-holds-barred mechanism for describing data in a systematic manner. It can be used with any type of data, without exception, and is, moreover, platform, programming language, and application independent. Consequently, it is a powerful, standardized means for facilitating data interchange between incompatible systems ”provided, of course, that the data description is suitably described using a DTD or XML schema.

A: XML is certainly not a programming language, nor is it associated with any programming language. XML is very much a technology that is programming language and computer platform independent. It is a standard for creating application-specific markup languages.

A: Unlike HTML, XML does not come with its own set of markup tags, though it has a few, strictly optional, processing instructions, which appear within tags that are delimited with ? marks (e.g., < ?xml version=1"? > ). With XML, users define and create custom, application-specific tags that optimally describe the data that are being described. There are really no overly restrictive rules or caveats as to which tags can be created, what they can be called, how they can be structured, or how many different tags you can have ”other than some minor naming conventions (e.g., names cannot start with numeric digits) and a need to ensure that tags are correctly nested without hanging overlaps.

A: XML deals with data within the context of documents. Data are included in an XML document as strings of text. The data are bracketed by XML text markup, which sets out to describe these data. The basic building blocks of an XML document are called elements ”where an element is a specific unit of data along with the XML markup describing these data. An XML element is made up of a name and some content. The XML markup is in the form of tags. An XML element is delimited by two tags: a start tag and an end tag. The start tag is what gives the element its name .

A: To successfully process XML, you need a mutual understanding at both ends of the transaction. XML provides mechanisms to facilitate this mutual understanding. The two main schemes for this are called Document Type Definitions (DTDs) and XML schema. In some special cases it is possible to have DTD-less XML documents, provided the elements are structured in some type of self-explanatory manner.

A: Yes, there are at least two key means by which to present the data contained within an XML document in a visually compelling manner; these are Cascading Stylesheets (CSS) and eXtensible Style Language (XSL).

A: At a minimum there are XML authoring, XML conversion, and XML validation tools. XML authoring tools help one create valid XML documents from scratch, whereas XML converters will take an existing structured document (e.g., spreadsheet or word processing file) and automatically generate a corresponding XML document. XML validators carefully parse XML documents and determine if they are valid per XML conventions and check that they meet all the XML requirements.

A: WSDL is used to define the high-level functionality of a Web service, in terms of its external interface, and to describe how that Web service can be accessed over the Web. WSDL will specify how a Web service actually functions, particularly in terms of what it accepts as input and what it will return as its output. The WSDL description of a Web service will specify where the service can be found and how it should be invoked.