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Chapter 6. Web Services 157

6.2 Web services

Web services is a technology that allows applications to communicate with each

other across a network, based in XML-formatted requests and with standard

communications protocols.

Web services are self-contained, self-describing

modular applications that can be accessed over the network:

򐂰 Self-contained

On the client side, the only requirement is a programming language with XML

and HTTP client support. On the server side, an HTTP and SOAP server are

required.

򐂰 Self-describing

Client and server communicate each other using established request and

response messages. The definition of the message format travels with the

messages.

򐂰 Modular

Web services can interact together to form an aggregate, more complex Web

service.

Web services are, essentially a way to implement a service-oriented architecture,

but not the only one. Web services allows:

򐂰 The interaction of applications of different architectures

򐂰 Changes in structure and implementation of the services using loose coupling

services

򐂰 A faster adaptation to any business condition or customer need change

򐂰 The creation of service interfaces for existing or legacy software applications

without changing them.

Note: For more information about SOA, consult the following references:

򐂰 WebSphere Version 6 Web Services Handbook: Development and

Deployment. SG24-6461-00.

򐂰 WebSphere Version 5.1 Application Developer 5.1.1 Web Services

Handbook. SG24-6891-00.

򐂰 See the Web site:

http://www.ibm.com/developerworks/webservices

158 WCTME: Application Development and Case Study

Web services are based on standards to guarantee interoperability with different

platforms. The core Web services standards are:

򐂰

Simple Object Access Protocol (SOAP) is an XML-based standard for the

exchange of structured information in a decentralized, distributed

environment. It is composed of three parts:

–An

envelope specifying who and how to deal with the message, and the

message itself.

– A set of encoding rules

–A

remote procedure call (RPC) representation.

SOAP runs over existing Internet infrastructure.

򐂰

Web Services Description Language (WSDL) is an XML-based open

specification that describes Web services interfaces over the network. It

provides a simple way to service providers to describe the format of the

requests for the services their offer, regardless of their implementation.

Service providers can specify the following Web services characteristics

using WSDL:

– Name and address information

– How to access the Web services operations (protocol and encoding style)

– Type of information (operations, parameters and data types that conform

the Web Service interface)

WSDL documents allow you to expose applications as network-accessible

services on the Internet.

򐂰

Universal, Description, Discovery and Integration (UDDI) is an open,

platform-independent standard that allow to publish and discover information

about Web Services over the Internet.

򐂰

Web Services Inspection Language (WSIL) is an XML-based specification that

describes the way to locate available Web Services descriptions on a Web

site. The WSIL specification complements UDDI facilitating the discovery of

services that are available on Web sites that may not be listed yet in a UDDI

registry.

򐂰 XML is the foundation standard of Web services.

The Java standards and APIs that are relevant to Web Services are:

򐂰 JSR 101 - Java APIs for XML based RPC (JAX-RPC)

򐂰 JSR 109 - Implementing Enterprise Web Services

򐂰 JSR 31 - XML Data Binding Specification (JAXB)

򐂰 JSR 67 - Java APIs for XML Messaging 1.0 (JAXM)

򐂰 JSR 93 - Java API for XML Registries 1.0 (JAXR)

Chapter 6. Web Services 159

򐂰 JSR 110 - Java APIs for WSDL (WSDL4J)

򐂰 JSR 172 - J2ME Web Services Specification

򐂰 JSR 173 - Streaming API for XML

򐂰 JSR 181 - Web Services Metadata for the Java Platform

򐂰 JSR 208 - Java Business Integration (JBI)

򐂰 JSR 222 - Java Architecture for XML Binding (JAXB) 2.0

򐂰 JSR 224 - Java API for XML-Based RPC (JAX-RPC) 2.0

򐂰 JSR 921 - Implementing Enterprise Web Services 1.1

Figure 6-2 shows the relationship between SOAP, WSDL and UDDI in Web

Services architecture:

򐂰 A service provider makes available a Web service and can be accessed by a

service requestor using HTTP or SOAP.

򐂰 A WSDL document contains the description of the Web service.

򐂰 The WSDL document is referenced by the service broker using UDDI.

򐂰 The service requestor can discover Web services using UDDI.

Figure 6-2 Web services components and standards

Service Broker

Service

Provider

Publish

Request/Response

(SOAP)

WSDL

Document

UDDI

Service

Requestor

SOAP

Discover

SOAP

HTTP

HTTP