External Programming Interfaces
This article, by Eamonn McManus, is a nice little summary of API design principles. It contains a bit of motherhood and apple pie---nobody would ever set out to make an API difficult to learn or hard to use, for example---but the specifics are good (particularly the discussion of why interfaces are often the wrong thing to use). The article contains a link to this essay at the NetBeans site, which talks about some of the same ideas in more detail.
Together, the articles got me thinking: why don't we ever talk about or document a module's external programming interface (which I hereby dub "XPI")? This is the classes, methods, and system calls that the module depends on; more particularly, it is their semantics. Design by contract allows a piece of code to specify what it provides; allowing that code to specify what it requires using similar pre- and post-conditions would be a big help in managing the asynchronous evolution of libraries that makes postmodern programming so hard.