Because debugging often occupies three-quarters or more of development time, work done early to ease debugging can be a very good investment. A particularly effective way to ease debugging is to design for transparency and discoverability .
A software system is transparent when you can look at it and immediately understand what it is doing and how. It is discoverable when it has facilities for monitoring and display of internal state so that your program not only functions well but can be seen to function well.
Designing for these qualities will have implications throughout a project. At minimum, it implies that debugging options should not be minimal afterthoughts. Rather, they should be designed in from the beginning — from the point of view that the program should be able to both demonstrate its own correctness and communicate to future developers the original developer's mental model of the problem it solves.
For a program to demonstrate its own correctness, it needs to be using input and output formats sufficiently simple so that the proper relationship between valid input and correct output is easy to check.
The objective of designing for transparency and discoverability should also encourage simple interfaces that can easily be manipulated by other programs — in particular, test and monitoring harnesses and debugging scripts.
