Today I learned about a program that generates the sounds that help people navigate as they exit long tunnels when an emergency such as a fire has destroyed the visibility.

The World’s Loudest Lisp Program to the Rescue

This describes the challenges of building a software system that has to work unmonitored once deployed, for years, as well as withstanding a fairly rugged environment where, for example, the installedc hardware will be periodically sprayed with a high-pressure hose as the walls get cleaned. Overall the system is soft real-time, but has to cope with component failure, network partitions, consensus, and all the usual distributed systems challenges, while be guaranteed to work when needed.

The developers built the system in Common Lisp, which wouldn’t be the normal go-to choice for an embedded system. But their argument was that they could better handle complex and changing requirements by retaining a high level of abstraction, and that development was overall far faster than using C. Modern Common Lisp compilers are so efficient that there’s no significant performance hit at deployment. They made use of complicated components like planners (for which Lisp is an ideal choice), and built a set of macros to wrap-up the handling of industrial control and robust communications.

It’s a great read.