In a new post to his site Frederick Vanbrabant tackles the integration operation segregation principle. While the term sounds intimidating, it's just a long way to say something you probably already do: refactor code into smaller testable chunks.

A few weeks ago I attended a DDDBelgium meetup where I was lucky to participate in a refactor workshop lead by Pim and Joop. After the incredible workshop Pim, Dries and me were discussing some code that we refactored earlier . Not so long in the conversation the words “Integration Operation Segregation Principle” casually got dropped by Pim.

Now I’m going, to be honest with you (as I was with them), I had no idea what the hell he was talking about. And maybe neither do you.

He starts with some simple code for a "calculator" class with a calculate method to handle the pricing of a rental car. He includes the test for the class/method as well, using a mock object and several expects calls to handle the method calls. The test ends up being larger than is probably good, so he looks into refactoring the original code to abstract out some of the functionality and make it more testable. In the process this also makes the code easier to follow and, while there is more of it, more maintainable and flexible in the end.