As an excursion in XP I am currently implementing a simple instant messaging application so that I can try out the mechanics of it’s practices. As I am alone on this I will not be able to take full advantage of some of the practices (it will be hard to pair-programming for one). The system metaphor is one of the practices that I really don’t see how it will affect development for me personally, for a team I can definitely see the uses. Nonetheless I will attempt to come up with a metaphor to see if it does me any good.
A system metaphor is an analogy that describes the system in a way that it can be discussed by all parties that have an interest in the system to be developed. This is by far the concept in XP that I have most trouble grasping, and from browsing through mailing lists and talking to people, I know I am not the only one. From what I understand the metaphor is not as explicit in the second edition of Beck’s book as it was in the first, but I’m going to try it out anyway.
The first thing that comes to mind is that of a telephone network. Especially the old school types, where you just lifted the receiver and got connected to an human operator. You’d then ask the operator to connect you to whomever you wanted to talk to.
So the act of sending a message to another user may be described as follows:
“A subscriber contacts an operator and asks to be connected to another subscriber. The operator connects the call between the subscribers, who then continues their conversation directly; without involving the operator.”
After getting some feedback on this from the extreme programming group I hesitated whether I should use this metaphor at all. Does this really add anything that thinking about “a client connects to a server and…” I mean, I think of this project as a simulation of an XP project, but I am still the only project member. I don’t think I can simulate the informal communication going on in an agile team at all. So, do I need a metaphor to describe something to myself in other words than clients, servers, and user databases?
I’m going to stick with the metaphor some more and see. One thing I can see already is that the system metaphor makes the application design come to life. When I think of it in terms of clients and servers, the image that pops into my head is that of a UML-like diagram. Something like a sketch on a white board. With the metaphor I get images of people doing things like making calls, working as an operator and connecting phone calls. It’s a more vivid image.
I’ll try to be aware of when this metaphor helps me, or hinders me.
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[...] For this session we elect to use CRC-cards (Class, Responsibility, and Collaboration). We walk through the scenario that we are about to implement and try to find the major classes involved. Where possible we try to use terminology from the system metaphor that we decided to use for describing the system. We write the name of those classes along with their responsibilities. Finally, we walk through one or two scenarios and try to see how the classes interact and write down the collaborators of the classes on the cards as well. [...]
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