Chapter 9 .

Creating Web Services :

Web services promises to be the next generation of software development. In essence, a web service is a means of interfacing to web or enterprise applications that allows you to integrate these with other enterprise applications, including those from different vendors and different platforms, using XML as the means of exchanging data.

There is a great deal of hype about web services. In the past there have been a number of initiatives to provide such vendor-and-platform agnostic integration technology, but none of these has been totally succesful. A good example is that of getting an enterprise applicaction running on Windows to communicate with an enterprise application running on Unix. Sure, it's been possible by various means but typically with a mechanism which is not equally well supported by both environments. That has changed with the advent of web services SOAP and XML provide an integration glue that everyone can finally agree on.

The basic mechanism of Web services is to use XML to transport its across different applications using the standard Web HTTP protocol (Hyper Text Transfer Protocol ). Specifically SOAP (Simple Object Access Protocol) is used, which is a lightweight XML based RPC (Remote Procedure Call) over HTTP. The protocol has three parts :

1. An envelope (SOAP envelope) that defines a framework for describing what is in a message and how to process it.

2. A set of encoding rules for expressing instances of application defined-dataypes.

3. A set of rules for representing remote procedure calls and responses.

Web Services Standards :

Now lets go through some evolving standards used in in Web Services, which are SOAP, WSDL (Web Services Description Language), UDDI (Universal Description Discovery and Integration), ebXML which is promoted by OASIS (Organization for the Advancement of Structured Information Standards) and UN/CEFACT (United Nations Centre for Trade Facilitation and Electronic Business). We will cover only SOAP and WSDL in this tutorial.

WSDL (Web Services Description Language) describes the interface of a network service similar to IDL (Interface Definition Language) in CORBA (Common Object Request Broker Architecture), specifying what messages an endpoint (i.e. a service instance which processes client requests and returns responses) will receive and send. WSDL is itself an XML metadata format. (In the terms of similar mechanisms like RMI or CORBA, a service is the interface or set of methods that a client will invoke across the wire).

SOAP (Simple Object Access Protocol) as discussed earlier describes the format of the data that is transmitted over the wire.

Both these standards are still evolving under the eye of the W3C, but these standards are regarded as the building block of the next revolution in distributed computing and the IT industry in general. Web services promise to do for machine-to-machine communication what the Web has done for human-to-human and machine-to-human communication.

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