28 October 2010

When I left work yesterday I had to work through a turnstile. One of those big ones that only one person can go through at any one time, in either direction. So it turns both ways. It is secured so I have to swipe my access card to get in or out.

This got me thinking about how that system works. Let’s look at a highly coupled mechanism:

Since the turnstile allows either in or out the service the readers needs to know which reader does what. When a request arrives to allow someone access into the building our card reader service checks with the authorization system whether the person has access and if so sends a message to the turnstile to allow it to turn in the relevant direction. So here our card reader service needs to know how the turnstile works and which card reader is attached to which action. This is rather high behavioural coupling.

Now for something a bit less coupled. Let’s say our turnstile simply ‘unlocks’ allowing the turnstile to turn once in any direction. Now the card reader service simply has to ask whether the person has access to unlock the turnstile and send the unlock message. So our card reader service now does not care how the turnstile works. This is low behavioural coupling.

Either way can be implemented but I’d say the first is somewhat more complex to get done.