Single Responsibility Principle

The Single Responsibility Principle (SRP) is a follow-on/extension to Don’t Repeat Yourself (DRY). It basically says that and given module should be responsible for one part of a program’s functionality.

Or does it? That’s the common understanding, but if you go back to the author and the original text that’s not quite it. In Robert Martin’s clarifying blog post you find something a little bit different. Instead of being based on functionality, it’s based on reasons for change.

Gather together the things that change for the same reasons. Separate those things that change for different reasons. -- Uncle Bob Martin

Which, while related to function, isn’t really about function. Consider HTML. There are multiple ways to style an element. If you want purple text you could put the declaration in every element. That would work, but it wouldn’t be DRY. You could define it in a div, then put everything in the div, but that adds an unneeded element. Or, you could put your style(s) into a CSS file. Not only do you get DRY, but you also separate responsibility. You’ve split the “look” of the site from the functionality of the site. The CSS changes when you want the color or padding or some other style element to change, but, in general, the functionality, both Javascript and HTML, don’t. 

Conversely, if you need to change what a button does, or change the new validation to a form then you don’t need to change the style. In fact, two people could make the changes in parallel and not have a merge conflict. That’s always a nice thing.

It also means that when the business needs change you all of the things related to that change are together. Changing the way the system responds to say, a pedestrian, could be handled individually by all of the things that notice pedestrians, or, you could gather all of the pedestrian related decisions together, making it easier to find and understand the interactions.

Of course, as with all architectural decisions, the specific answer depends on the specific case, it’s definitely another way to think about how to break things up.