When Cindy and I went to Australia, we spent some time in the rain forests on the Queensland coast. One of the natural wonders of this area are the huge strangler vines. They seed in the upper branches of a fig tree and gradually work their way down the tree until they root in the soil. Over many years they grow into fantastic and beautiful shapes, meanwhile strangling and killing the tree that was their host.
This metaphor struck me as a way of describing a way of doing a rewrite of an important system. Much of my career has involved rewrites of critical systems. You would think such a thing as easy - just make the new one do what the old one did. Yet they are always much more complex than they seem, and overflowing with risk. The big cut-over date looms, the pressure is on. While new features (there are always new features) are liked, old stuff has to remain. Even old bugs often need to be added to the rewritten system.
An alternative route is to gradually create a new system around the edges of the old, letting it grow slowly over several years until the old system is strangled. Doing this sounds hard, but increasingly I think it's one of those things that isn't tried enough. In particular I've noticed a couple of basic strategies that work well. The fundamental strategy is EventInterception, which can be used to gradually move functionality to the strangler and to enable AssetCapture.
My colleague Chris Stevenson was involved in a project that did this recently with a great deal of success. They published a first paper on this at XP 2004, and I'm hoping for more that describe more aspects of this project. They aren't yet at the point where the old application is strangled - but they've delivered valuable functionality to the business that gives the team the credibility to go further. And even if they stop now, they have a huge return on investment - which is more than many cut-over rewrites achieve.
The most important reason to consider a strangler application over a cut-over rewrite is reduced risk. A strangler can give value steadily and the frequent releases allow you to monitor its progress more carefully. Many people still don't consider a strangler since they think it will cost more - I'm not convinced about that. Since you can use shorter release cycles with a strangler you can avoid a lot of the unnecessary features that cut over rewrites often generate.
There's another important idea here - when designing a new application you should design it in such a way as to make it easier for it to be strangled in the future. Let's face it, all we are doing is writing tomorrow's legacy software today. By making it easy to be strangled in the future, you are enabling the graceful fading away of today's work.
Paul Hammant has a good summary of case studies using this approach.