Protected Variation

Open-Closed Principle "Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification [Martin, p.99]"

Description of protected variation

Protected variation and Polymorphism Protected Variation and Polymorphism seem related. What is the difference between the two? Where would you apply one pattern over the other?

Polymorphism is a powerful technology that is very useful for handling Protected Variation. Protected Variation gives one reason "why" for a particular use of Polymorphism.

OCP

An example of an .Key Open-Closed Principle is... X can be opened to Y and always Z. And it's only open to Y if it Y needs to access something and has permission to. While Z can always access it because it has permission to. That look right?

I'd express it like this: if X uses Y in some way or other you don't want changes to Y to effect X, but you also want to be able to change Y in ways that don't change X.

Introduction

Keep information out of the grasp of components that could damage integrity. Introduce reader to protected variation -Open/Closed Principle and Information Hiding