| Since the introduction of Web services, various definitions have surfaced. As a simple definition, I like this MSDN (Microsoft Developer Network) definition of a Web service. "A Web Service is a unit of application logic providing data and services to other applications. Applications access Web Services via ubiquitous Web protocols and data formats such as HTTP, XML and SOAP with no need to worry about how each Web Service is implemented." Even this simplistic definition captures a number of key points: -
Unit of application logic ” Web services, in a number of ways, are like software components, such as .NET components and COM components , because they represent a unit of exposed application logic. This also indicates that there should be an interface. As you will see later, that is where WSDL comes into play. -
Provide data and services ” This indicates that Web services make data available (using XML) and provide an interface to applications. -
Access through ubiquitous Web protocols ” The most ubiquitous protocol of the Web is HTTP (Hypertext Transfer Protocol), which itself is built on top of TCP/IP. Web services are built on top of HTTP. (As you will see further in this chapter, this statement doesn't indicate that Web services are limited to HTTP, but can utilize other protocols such as SMTP, S/MIME, and Messaging.) -
Ubiquitous data formats ” Web services use XML and SOAP (Simple Object Access Protocol, which itself is defined in XML) standards for data and metadata representation. -
No worries about how service is implemented ” This is different than a component. Typically, a component defined using a particular technology needs a common runtime to be accessed. For instance, COM components can be accessed by COM clients, .NET components can be accessed by .NET clients (or COM clients through interoperability), Java/EJB classes can be invoked through Java clients , and so on. However, because Web services are accessed through Web protocols (such as HTTP, which are platform- and language-independent protocols) and provide a standard interface through XML/SOAP (also platform- and language-independent formats), Web services can be defined (Web services creation) by any technology platform and later accessed (Web services consumption) by a component in any other technology platform. |
