DEFINING WEB SERVICES WITH WSDL

One of the biggest changes to enterprises in the past year has been the
effort to create new service-based systems. A service-based system 
isn't
necessarily new technology, per se, but rather a new perspective on
existing technology. As commerce and business processes have become 
more
electronic, more enterprise applications than ever are being 
integrated. These
applications are now viewed as providing particular services rather 
than
as just applications. Using Web technologies, they can be delivered via
the Internet. The Web Services Description Language (WSDL) is an XML
grammar that allows you to describe the Web-based services that are 
provided
by your applications.

WHAT IS A WEB SERVICE?

A Web service is usually a business process that can be automated using
an application. A service is made up of various operations. For 
example,
an organization might define a billing system Web service. This service
is comprised of operations, such as creating an invoice or submitting a
payment. The service is accessed using an HTTP gateway or Web server. 
The
Web server uses the familiar request/response HTTP model. Each request 
is
sent as an HTTP GET request and can be packaged as a Simple Object 
Access
Protocol (SOAP) request. The SOAP protocol defines a way to access
remote objects using the HTTP Web server protocol. Each message is 
itself an
XML document that includes the request or response being sent to or 
from
the Web service.

DESCRIBING WEB SERVICES

Now that we have a fundamental concept of a Web service as a collection
of operations accessible via a Web server, we can examine how these
services are defined and described. Each Web service is described in a 
WSDL
document. The document is made up of component definitions that 
describe
different aspects of the Web service.

There are six elements contained by the definitions element that 
provide
information about the Web service. These elements are types, message,
portType, binding, port, and service. Each may contain a document 
element
that provides documentation of the behavior defined by the element.

The types element defines data types that may be used in the message
being exchanged. Each type can be defined using existing XSD (XML 
Schema
Definition) data types, such as string, date, and short. Each type may 
be
defined in simple terms using primitive definitions (like string, date, 
and
short) or they may be complex types, which are composed of multiple
subtype definitions. For example, a name type might be defined as a 
string;
however, an Account type may be defined as a combination of
CustomerRecord, AddressRecord, AccountHistory, and AccountNumber. Each 
of these types
may then be composed of more subtypes.

The message element contains the abstract data that is exchanged in the
electronic conversation. Each message is made up of part elements that
describe individual segments of data that are defined by the types. 
While
the types elements define the characteristic of various types that may 
be
used in the exchange, the message element defines the actual parts that
make up the particular Web service.

The portType element defines the particular collection of operations
provided by the Web service. Each portType element is composed of one 
or
more operation elements. Each operation is a single transaction or 
process
that can be performed by the Web service. These can be thought of as 
the
methods that are exposed by the Web service. The input and output
elements, which make up the operation elements, define which messages 
are input
data to the operation and which messages are output data from the
operation.

The binding element describes how the various operation parameters
(defined by the input and output elements) will be encoded for 
transport. Each
operation and its associated input and output parameters must be
individually bound to a particular protocol, such as SOAP.

The service element describes the top-level view of the Web service
defined by the WSDL document. Each service element is comprised of 
multiple
port elements, which associate a particular binding with a physical
address (such as a Web server or e-mail address).

SUMMARY

Exposing interfaces to enterprise business processes is a cornerstone
for business-to-business electronic commerce. The WSDL is used to 
describe
how businesses and organizations interface with one another's systems
using Web services.

Brian Schaffner is a senior consultant for DMR Consulting, a Fujitsu
company. He provides architecture, design, and development support for 
DMR's
Telcom360 group.

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