Architecture). The difference is important.

Eric Van der Vlist, author of XML Schema: The W3C’s Object-Oriented Descriptions for XML (O’Reilly, 2002), describes the differences between the two types of coupling with this analogy: In a tightly coupled system, you have direct control over the operation. For example, flipping a wall switch to turn on a light is a tightly coupled system. However, making a telephone call to order a book is a loosely coupled system. It could be tightly coupled only if you had access to the button controlling the printer that will print the book you order.

Tightly coupled systems
are usually fast and safe, and

the risk of transmission errors is very low. Loosely coupled systems, on the other hand, are usually more error-prone but also more flexible. The clerk you talk to on the phone may misunderstand the ISBN number of the book you want to order or make an error while entering it. But if you don’t remember the ISBN number, you can still tell the clerk that you want the latest book on the World Wide Web Consortium’s XML schema by a guy with a Dutch name from a publisher that puts pictures of animals on its book covers — and when you do that, you’ve got a good chance of being understood.

Tight coupling tends to make component maintenance

organizations that see
service-oriented archi-
tectures and event-driven
architectures as shortcuts
to agility should anticipate
significant roadblocks
to successful migration
unless they address un-
derlying information man-
agement issues such as
data quality, transparency
and semantic reconciliation.

SourCE: Gar TnEr In C., 2006

 

and reuse much more difficult, because a change in one component automatically means changes in others. Similarly, tight coupling makes extra work when an application has

to adapt to changing business requirements, because each modification to one application may force developers to make changes in other connected applications.

In general terms, a Web service is a type of SOA in which interfaces are based on standardized Internet protocols. In addition, except for binary data attachment, Web service messages must be in XML. Generally speaking, a Web service is little more than an SOA that uses Simple Object Access Protocol and the Web Services Description Language. However, an SOA doesn’t require the use of Web services as we understand them, and some types of Web services can be deployed without an SOA.