Why Use Web Services
Traditionally, Web applications enable interaction between an end user and a Web site,
while Web services are service-oriented and enable applicationto- application
communication over the Internet and easy accessibility to heterogeneous applications and
devices. The following are the major technical reasons for choosing Web services over Web
applications:
■ Web services can be invoked through XML-based RPC mechanisms across firewalls.
■ Web services provide a cross-platform, cross-language solution based on XML messaging.
■ Web services facilitate ease of application integration using a lightweight infrastructure
without affecting scalability.
■ Web services enable interoperability among heterogeneous applications.
Web Services Architecture and Its Core Building Blocks
The basic principles behind the Web services architecture are based on SOAand the
Internet protocols. It represents a composable application solution based on standards and
standards-based technologies. This ensures that the implementations of Web services
applications are compliant to standard specifications, thus enabling interoperability with
those compliant applications. Some of the key design requirements of the Web services
architecture are the following: ■ To provide a universal interface and a consistent solution
model to define the application as modular components, thus enabling them as exposable
services
■ To define a framework with a standards-based infrastructure model and protocols to
support services-based applications over the Internet
■ To address a variety of service delivery scenarios ranging from e-business (B2C),
business-to-business (B2B), peer-to-peer (P2P), and enterprise application integration
(EAI)-based application communication
■ To enable distributable modular applications as a centralized and decentralized
application environment that supports boundary-less application communication for
interenterprise and intra-enterprise application connectivity
■ To enable the publishing of services to one or more public or private directories, thus
enabling potential users to locate the published services using standard-based mechanisms
that are defined by standards organizations
■ To enable the invocation of those services when it is required, subject to authentication,
authorization, and other security measures
Web Services Description Language (WSDL)
The Web Services Description Language, or WDDL, is an XML schemabased specification for
describing Web services as a collection of operations and data input/output parameters as
messages. WSDL also defines the communication model with a binding mechanism to
attach any transport protocol, data format, or structure to an abstract message, operation,
or endpoint. Listing 3.2 shows a WSDL example that describes a Web service meant for
obtaining a price of a book using a GetBookPrice operation.
