I saw this article in ZapThink's ZapFlash called "The Lego Model of SOA". It's a great read for anyone looking for a nice metaphor to describe the benefits of SOA to a non-technical person. It made me think, though, about how to describe decision services in this context. Now one of the cool developments in Lego over the last few years has been the Mindstorms range of products - most recently Lego Mindstorms NXT. For those of you living under a rock Mindstorms is Lego's uber-cool robot design kit and you can find out more about it here.
Now the Mindstorms kit, like all Lego, consists of lots of pieces that can be assembled in different ways as well as some specialty bricks including one called "the intelligent brick". This one allows you to program behavior into the robot by setting it up, pushing it into the brick and then having the intelligent brick drive your robot in the ways you programmed. Essentially you sit down at your computer, build the logic you want to drive your robot and then download it into the "intelligent brick". You can keep changing the logic and updating the behavior of your robot without changing the rest of the robot. Equally you can add new pieces to your robot and then update the programming to take advantage of them.
Pushing the Lego metaphor to its very limit, it seems to me that if your SOA can be represented in terms of Lego then decision services are your "intelligent bricks". They plug in to other bricks (services) as normal but they allow for expertise and experience to be embedded rapidly into your SOA without changing any of the interfaces. You can sit down and apply your business expertise in the form of business rules or analytic insight by updating the behavior of your decision service. This helps prepare your SOA for ongoing change in the same way that the intelligent brick can have its behavior tweaked without having to change the rest of the robot.
It seemed like a fun analogy and, if you are interested, there is a longish article on SOA and rules here and I wrote a piece about how business rules can help you love change called business rules cafe that ebizQ ran once before.