XML Web Services

XML web services - the buildling blocks of distributed computing on the Internet
- exposing the functionality on the web through SOAP
- describing the interfaces in a WSDL document
- registered in UDDI

SOAP (Simple Object Access Protocol)
- SOAP is a specification that defines the XML format for messages
- SOAP spec also describes how to represent program data as XML and how to use SOAP to do Remote Procedure Calls. These optional parts of the spec are used to implement RPC-style applications where a SOAP message containing a callable function, and the parameters to pass to the function, is sent from the client, and the server returns a message with the results of the executed function.
- SOAP also supports document style applications where the SOAP message is just a wrapper around an XML document.
- The last optional part of the SOAP spec defines what an HTTP message that contains a SOAP message looks like. This HTTP binding is important because HTTP is supported by almost all current OS's.

WSDL (Web Services Description Language)
- WSDL file is an XML document that describes a set of SOAP messages and how the messages are exchanged.
- WSDL specifies what a request message must contain and what the response message will look like in unambiguous notation.
- WSDL also defines where the service is available and what communications protocol is used to talk to the service.

UDDL (Universal Discovery Description and Integration)
- UDDL is the yellow pages of Web services.
- A UDDI directory entry is an XML file that describes a business and the services it offers.
- There are three parts to an entry in the UDDI directory:
-- The "white pages" describe the company offering the service: name, address, contacts, etc.
-- The "yellow pages" include industrial categories based on standard taxonomies such as the North American Industry Classification System and the Standard Industrial Classification.
-- The "green pages" describe the interface to the service in enough detail for someone to write an application to use the Web service.

-- Microsoft