A Philosophy of Software Design

by John Ousterhout

Here are a few indications that two pieces of code are related: They share information; for example, both pieces of code might depend on the syntax of a particular type of document. They are used together: anyone using one of the pieces of code is likely to use the other as well. This form of relationship is only compelling if it is bidirectional. As a counter-example, a disk block cache will almost always involve a hash table, but hash tables can be used in many situations that don’t involve block caches; thus, these modules should be separate. They overlap conceptually, in that there is a simple higher-level category that includes both of the pieces of code. For example, searching for a substring and case conversion both fall under the category of string manipulation; flow control and reliable delivery both fall under the category of network communication. It is hard to understand one of the pieces of code without looking at the other.