technically: - Separate the UI from the application and business logic and make the interface to the latter clean and well documented. - Either use a currentl

technically: - Separate the UI from the application and business logic and make the interface to the latter clean and well documented. - Either use a currently very widely deployed ecosystem (aka Java) which will have a fair chance of evolving or being emulated etc over the course of 100 years, or use a self-bootstrapping environment with a simple core like forth or scheme/lisp etc, where everything you need is self-contained. But really what makes a system still useful in 100 years or further is that it solves a problem so well, so reliably, etc, or just well enough if it's a huge and complex problem, that there's no incentive, or the investment would be prohibitive, to replace it with newer technology. Think CICS.