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Web Services

Overview

The term 'Web Services' refers to any service that can be shared by and used as components of distributed Web-based applications. The W3C's Web Services Architecture Board defines a Web Service as a "software application identified by a URI (Universal Resource Identifier), whose interfaces and bindings are capable of being defined, described and discovered by XML artifacts, and supports direct interactions with other software applications using XML based messages via internet-based protocols."

The concept of services is not new: RMI, COM, and CORBA are all service-oriented technologies. However, applications based on these technologies require them to be written using that particular technology, often from a particular vendor. This requirement typically hinders widespread acceptance of an application on the Web. To solve this problem, Web services are defined to share the following properties that make them easily accessible from heterogeneous environments:

Web services are accessed over the Web.
Web services describe themselves using an XML-based description language.
Web services communicate with clients (both end-user applications or other Web services) through XML messages that are transmitted by standard Internet protocols, such as HTTP.

Web Services Standards

Three main ingredients of Web Service are:

Simple Object Access Protocol (SOAP) : An XML based message protocol pluggable with transport protocols such as HTTP, HTTP(s), FTP and SMTP.
Web Services Description Language (WSDL) : The standard format for describing a web service. Expressed in XML, a WSDL definition describes how to access a web service and what operations it will perform.
Universal Description, Discovery and Integration (UDDI) : A directory model for web services. UDDI is a specification for maintaining standardized directories of information about web services, recording their capabilities, location and requirements in a universally recognized format.

The complete flow

The diagram below gives an overview of basic Web Services Architecture

A Service Provider implements a Web Service and describes it using WSDL.
The Service Provide publishes this WSDL file to a UDDI Repository.
A Web Service client (Consumer) looks up the WSDL from the UDDI Repository.
The Web Service client (Consumer) binds to a specific endpoint defined in the WSDL file.
The Web Service client (Consumer) starts communicating with the Service Provider using SOAP.

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