Software is digital, so how could it age? Parnas identified two different causes for software getting "older" when time passes:
- "Lack of Movement", i.e. the inability for the software to match evolving user or environment expectations. A computer environment evolves constantly; language and compiler changes, operating system and library changes. But this is also true for user expectations: in the 70' people expected a text interface, in the 90' a graphical interface and now they want a web interface accessible from any device or an App usable on their mobile devices;
- "Ignorant Surgery", i.e. the inability to correctly adapt the software as time passes. When somebody modifies the software, probably not the original developer(s), she/he won't probably understand its design and probably modify it as she/he understand the software. At this point understanding the software means understanding the original design as well as the introduced new design. After several of such modifications, nobody can understand the software and in some cases it should be rewritten from scratch.
Although this idea of Software Aging was proposed 22 years ago (and appeared 47 years ago, the famous "Software Crisis", according to Parnas), it is striking to see that it is still true. When I see it takes nearly one year to make a new release of the Debian system, I view it at software aging at work: the environment evolves in unexpected ways and each software should be adapted to match its new environment. And sometimes, applying those changes can be very difficult and lead to Ignorant Surgery, like the famous Debian OpenSSL bug.
How to avoid this software decay? Parnas gives some advises, I just summarizes them here:
- Design for change, i.e. apply usual Software Engineering principles like Information Hiding, abstraction, separation of concerns, proper use of object orientation, etc.
- Document! Document your design, your choices and document for the people who'll come after you work on the software.
- Review! Review the design, code or other software artifacts by other members of your team, by experts in the domain on which your software is applying, etc.
Even for actual software, above principles can be applied, retro-actively. Parnas gives some advises.
All current software are aging. Hopefully, 22 years ago, Parnas gave some very good advises on how to develop and maintain software to limit this phenomenon. Listen to him, apply them.