A Philosophy of Software Design

by John Ousterhout

A common mistake when modifying code is to put detailed information about the change in the commit message for the source code repository, but then not to document it in the code. Although commit messages can be browsed in the future by scanning the repository’s log, a developer who needs the information is unlikely to think of scanning the repository log. Even if they do scan the log, it will be tedious to find the right log message. When writing a commit message, ask yourself whether developers will need to use that information in the future. If so, then document this information in the code. An example is a commit message describing a subtle problem that motivated a code change. If this isn’t documented in the code, then a developer might come along later and undo the change without realizing that they have re-created a bug. If you want to include a copy of this information in the commit message as well, that’s fine, but the most important thing is to get it in the code. This illustrates the principle of placing documentation in the place where developers are most likely to see it; the commit log is rarely that place.