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XML.com: What Is Service-Oriented Architecture

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Published on XML.com http://www.xml.com/pub/a/ws/2003/09/30/soa.html See this if you're having trouble printing code examples

What Is Service-Oriented Architecture
By Hao He September 30, 2003

"Things should be made as simple as possible, but no simpler." -- Albert Einstein

Introduction
Einstein made that famousstatement many decades ago, and it's still relevant today for building superior software systems. Unfortunately, as anyone who has been in the IT industry for long can point out, far too many software systems have failed Einstein's test. Some are made too simple to carry out the duties they are supposed to perform. Others are made too complex, and the costs of building and maintaining them have rocketed,not to mention the nearly impossible tasks of integrating different systems together. It seems that reaching the right level of simplicity is more like a dream than reality. Where have we gone wrong?

Loose Coupling
We don't have to look far to find the problems. As we build more and more software systems, we see similar situations and patterns appearing. Naturally, we want to reuse thefunctionality of existing systems rather than building them from scratch. A real dependency is a state of affairs in which one system depends on the functionality provided by another. If the world only contained real dependencies, Einstein's test would have been satisfied long time ago. The problem is that we also create artificial dependencies along with real dependencies. If you travel overseas onbusiness, you know that you must bring power adapters along with you or your life will be miserable. The real dependency is that you need power; the artificial dependency is that your plug must fit into the local outlet. Looking at all the varying sizes and shapes of those plugs from different countries, you would notice that some of them are small and compact while many others are big and bulky. Thelesson here is that we cannot remove artificial dependencies, but we can reduce them. If the artificial dependencies among systems have been reduced, ideally, to their minimum, we have achieved loose coupling. In that sense, Einstein was just talking about was loose coupling. We might rework his famous principle thus: "Artificial dependencies should be reduced to the minimum but real dependenciesshould not be altered." Related Reading

SOA Defined and Explained

Web Services Essentials Distributed Applications with XML-RPC, SOAP, UDDI & WSDL By Ethan Cerami

http://www.xml.com/lpt/a/1292

18/02/2012

XML.com: What Is Service-Oriented Architecture

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Now we are able to define a Service Oriented Architecture (SOA). SOA is an architectural style whose goal is toachieve loose coupling among interacting software agents. A service is a unit of work done by a service provider to achieve desired end results for a service consumer. Both provider and consumer are roles played by software agents on behalf of their owners. This sounds a bit too abstract, but SOA is actually everywhere. Let's look at an example of SOA which is likely to be found in your livingroom. Take a CD for instance. If you want to play it, you put your CD into a CD player and the player plays it for you. The CD player offers a CD playing service. Which is nice because you can replace one CD player with another. You can play the same CD on a portable player or on your expensive stereo. They both offer the same CD playing service, but the quality of service is different. The idea ofSOA departs significantly from that of object oriented programming, which strongly suggests that you should bind data and its processing together. So, in object oriented programming style, every CD would come with its own player and they are not supposed to be separated. This sounds odd, but it's the way we have built many software systems. The results of a service are usually the change of state...
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