The Web Services Layer Cake

Web services introduce a host of new acronyms and terms. To understand them, it is useful to divide the process of developing Web services into five layers: representing data, transmitting data, describing Web services, discovering Web services, and hooking existing COM applications and components to Web services. The following paragraphs identify the solutions Microsoft uses for each of these layers.

Data representation. EXtensible Markup Language (XML) data representations are used for Web service data exchanges. Developers describe their data using documents called schemas. Microsoft currently supports the XML Data Reduced (XDR) format for schemas, but will migrate to the XML Schema Definition (XSD) standard when that has been finalized.

Message protocols. Clients use Web services by exchanging request messages and responses. Both IBM and Microsoft are promoting the Simple Object Access Protocol (SOAP). SOAP is a standard for exchanging these messages encoded using XML over lower-level Internet transport protocols, such as the Hypertext Transport Protocol (HTTP) and the Simple Mail Transport Protocol (SMTP). Much like a letter, a SOAP message consists of an envelope that contains a header with the address of the message recipient and a variety of delivery options (e.g., encryption information) and a body with the data of the message.