Web Services
A web service is any service that is available over the Internet, uses a standardized XML messaging system, and is not tied to any one operating system or programming language.
There are several alternatives for XML messaging. For example, you could use XML Remote Procedure Calls (XML-RPC) or SOAP. Alternatively, you could just use HTTP GET/POST and pass arbitrary XML documents.

To summarize, a complete web service is , therefore, any service that:
• Is available over the Internet or private (intranet) networks
• Uses a standardized XML messaging system
• Is not tied to any one operating system or programming language
• Is self-describing via a common XML grammar
• Is discoverable via a simple find mechanism

There are currently many competing frameworks and proposals for web services. The three main contenders are Microsoft's .NET, IBM Web Services, and Sun Open Net Environment (ONE). All of the frameworks share a common set of technologies, mainly SOAP, WSDL, and UDDI.

Web Service Architecture
There are two ways to view the web service architecture. The first is to examine the individual roles of each web service actor , the second is to examine the emerging web service protocol stack.

1.Web Service Roles
There are three major roles within the web service architecture:
• Service provider
This is the provider of the web service. The service provider implements the service and makes it available on the Internet.
• Service requestor
This is any consumer of the web service. The requestor utilizes an existing web service by opening a network connection and sending an XML request.
• Service registry
This is a logically centralized directory of services. The registry provides a central place where developers can publish new services or find existing ones. It therefore serves as a centralized clearinghouse for companies and their services.

2.Web Service Protocol Stack
Currently there are four main layers
• Service transport
This layer is responsible for transporting messages between applications. Currently, this layer includes hypertext transfer protocol (HTTP), Simple Mail Transfer Protocol (SMTP), file transfer protocol (FTP), and newer protocols, such as Blocks Extensible Exchange Protocol (BEEP).
• XML messaging
This layer is responsible for encoding messages in a common XML format so that messages can be understood at either end. Currently, this layer includes XML-RPC and SOAP.
• Service description
This layer is responsible for describing the public interface to a specific web service. Currently, service description is handled via the Web Service Description Language(WSDL).
• Service discovery
This layer is responsible for centralizing services into a common registry, and
providing easy publish/find functionality. Currently, service discovery is handled via Universal Description, Discovery, and Integration (UDDI).
As web services evolve, additional layers may be added, and additional technologies may be added to each layer.