Web Services explained

First, defining Web Services using Web Services Description Language (WSDL) will be reviewed. That will be followed by SOAP, which provides means of sending messages.

Also see Web Services definition and Service-oriented architecture (SOA) definition.

 

Using the Web Services Description Language (WSDL)

The Web Services Description Language (WSDL) forms the basis for Web Services. The following figure illustrates the use of WSDL. At the left is a service provider. At the right is a service consumer. The steps involved in providing and consuming a service are:

  1. A service provider describes its service using WSDL. This definition is published to a directory of services. The directory could use Universal Description, Discovery, and Integration (UDDI). Other forms of directories can also be used.
  2. A service consumer issues one or more queries to the directory to locate a service and determine how to communicate with that service. 
  3. Part of the WSDL provided by the service provider is passed to the service consumer. This tells the service consumer what the requests and responses are for the service provider.
  4. The service consumer uses the WSDL to send a request to the service provider.
  5. The service provider provides the expected response to the service consumer.

More on Web Services Description Language (new window).

Web Services basics

Using Universal Description, Discovery, and Integration (UDDI)

The directory shown in the above figure could be a UDDI registry. The UDDI registry is intended to eventually serve as a means of "discovering" Web Services described using WSDL . The idea is that the UDDI registry can be searched in various ways to obtain contact information and the Web Services available for various organizations. How much "discovery" will be used in the early days of Web Services is open to discussion. Nevertheless, even without the discovery portion, the UDDI registry is a way to keep up-to-date on the Web Services your organization currently uses. More on Universal Description, Discovery, and Integration (new window). An alternative to UDDI is the ebXML Registry (new window).

Using SOAP

All the messages shown in the above figure are sent using SOAP. (SOAP at one time stood for Simple Object Access Protocol. Now, the letters in the acronym have no particular meaning .) SOAP essentially provides the envelope for sending the Web Services messages. SOAP generally uses HTTP , but other means of connection may be used. HTTP is the familiar connection we all use for the Internet. In fact, it is the pervasiveness of HTTP connections that will help drive the adoption of Web Services. More on SOAP and Messaging (new window).

The next figure provides more detail on the messages sent using Web Services. At the left of the figure is a fragment of the WSDL sent to the directory. It shows a CustomerInfoRequest that requires the customer's account to object information. Also shown is the CustomerInfoResponse that provides a series of items on customer including name, phone, and address items.

Web Services messaging

At the right of this figure is a fragment of the WSDL being sent to the service consumer. This is the same fragment sent to the directory by the service provider. The service consumer uses this WSDL to create the service request shown above the arrow connecting the service consumer to the service provider. Upon receiving the request, the service provider returns a message using the format described in the original WSDL. That message appears at the bottom of the figure.

Using XML with WSDL

WSDL uses XML to define messages. XML has a tagged message format. This is shown in the above figure. The tag <city> has the value of Burnsville. And </city> is the ending tag indicating the end of the value of city. Both the service provider and service consumer use these tags. In fact, the service provider could send the data shown at the bottom of this figure in any order. The service consumer uses the tags and not the order of the data to get the data values. More on the use of XML tags and a comparison of XML to using fixed record formats can be found in chapter 3 of Web Services and Service-Oriented Architectures: The Savvy Manager's Guide.

Simplified Web Services notation

For other figures on this site, a simplified notation will be used for Web Services. This is shown below. In the simplified notation, the directory is implicit in the wide rectangle labeled "Web Services" at the top of this figure. You could think of Web Services much like the bus in a PC in which you plug various circuit boards. Other middleware solutions appear similar and use the same "bus" concept. See Prior Service-Oriented Architecture specifications (new window).

Another important concept in service-oriented architectures is that any service provider could also be a service consumer. This is why the figure below shows only services at the bottom of the figure under the Web Services bus rather than a "service provider" and a "service consumer". 

Simplified notation

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Related books at Amazon.com


Understanding Web Services: XML, WSDL, SOAP, and UDDI (Independent Technology Guides)
by Eric Newcomer
Average Customer Review: 3.5 stars based on 25 reviews.
Customer Review: This title is very good for understanding basic WS technologies. But is older for now and some informations are outdated. Reprint with updated information (espec. UDDIv3) would be good.

J2EE Web Services: XML SOAP WSDL UDDI WS-I JAX-RPC JAXR SAAJ JAXP
by Richard Monson-Haefel
Average Customer Review: 4 stars based on 32 reviews.
Customer Review: Perfect book for a beginer. The book starts from basics to leads complex points in a balanced manner.

Building Web Services with Java: Making Sense of XML, SOAP, WSDL, and UDDI (2nd Edition) (Developer's Library)
by Steve Graham, Doug Davis, Simeon Simeonov, Glen Daniels, Peter Brittenham, Yuichi Nakamura, Paul Fremantle, Dieter Koenig, Claudia Zentner
Average Customer Review: 3.5 stars based on 35 reviews.
Customer Review: This book has helped me immensely in implementing some really intense production quality data interchange across systems using web services. This book will quickly help you understand the entire XML stack of technologies that you will need for Web Services. The authors have uniquely enabled the readers to develop an understanding of...

Programming Web Services with SOAP
by James Snell, Doug Tidwell, Pavel Kulchenko
Average Customer Review: 2 stars based on 14 reviews.
Customer Review: If you are new to SOAP and you want to get the overall picture, and you don't care for details, this is the book you need. If you need a reference guide, this is not the book you want. If you're looking for a book about SOAP on a particular platform (say Java), this is not the book you need.

Perspectives on Web Services: Applying SOAP, WSDL and UDDI to Real-World Projects (Springer Professional Computing)
by Olaf Zimmermann, Mark R. Tomlinson, Stefan Peuser
Average Customer Review: 4.5 stars based on 7 reviews.
Customer Review: This book excells in explaining the IBM Toolsets and their applicability in the Web Services and SOA area. Unfortunately they are for version 5 and a version 6.x addendum would be great. Having said that working the examples into version 6 format is good practice and not too much sweat. This book provides all the coverage you need if...

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