Much of the work we do as architects involves dealing with the dreaded “legacy systems”. Of course legacy actually means the last system built, not one that is necessarily 5, 10, 20 or more years old. As soon as a system goes into production it is basically “legacy”. As soon as new features get added that legacy system gets harder to maintain and more difficult to understand; entropy (in the sense of an expression of disorder or randomness) sets in.
Apple have recently been in the news again for the wrong reasons because some of the latest iPod’s do not work with previous versions of Mac OSX. Users have been complaining that they are being forced to to upgrade to the latest version of OSX in order to get their shiny new iPods to work. To make matters worse however Apple do support the relatively ancient version of Windows XP. Apple have always taken a fairly hard line when it comes to legacy by not supporting backwards compatibility particularly well when their OS gets upgraded. The upside is the operating systems does not suffer from the “OS bloat” that Windows seems to (the last version of OSX actually had a smaller footprint than the previous version).
As architects it is difficult to focus both on maintaining legacy systems and also figuring out how to replace them. As Seth Godin says:“Driving with your eyes on the rearview mirror is difficult indeed”. At some point you need to figure out whether it is better to abandon the legacy system and replace it or soldier on supporting an ever harder to maintain system. There comes a point where the effort and cost in maintaining legacy is greater than that needed to replace the system entirely. I’m not aware of any formal methods that would help answer this particularly hard architectural decision but it’s one I think any architect should try and answer before embarking on a risky upgrade program that involves updating existing systems.