In 1972 David L. Parnas published a classic paper entitled On the Criteria To Be Used in Decomposing Systems into Modules. It appeared in the December issue of the Communications of the ACM, Volume 15, Number 12.

In this paper, Parnas compared two different strategies for decomposing and separating the logic in a simple algorithm. The paper is fascinating reading, and I strongly urge you to study it. His conclusion, in part, is as follows:

“We have tried to demonstrate by these examples that it is almost always incorrect to begin the decomposition of a system into modules on the basis of a flowchart. We propose instead that one begins with a list of difficult design decisions or design decisions which are likely to change. Each module is then designed to hide such a decision from the others.”

I added the emphasis in the second to last sentence. Parnas’ conclusion was that modules should be separated based, at lease in part, on the way that they might change.

Two years later, Edsger Dijkstra wrote another classic paper entitled On the role of scientific thought. in which he introduced the term: The Separation of Concerns.

The 1970s and 1980s were a fertile time for principles of software architecture. Structured Programming and Design were all the rage. During that time the notions of Coupling and Cohesion were introduced by Larry Constantine, and amplified by Tom DeMarco, Meilir Page-Jones and many others.

In the late 1990s I tried to consolidate these notions into a principle, which I called: The Single Responsibility Principle. (I have this vague feeling that I stole the name of this principle from Bertrand Meyer, but I have not been able to confirm that.)

The Single Responsibility Principle (SRP) states that each software module should have one and only one reason to change. This sounds good, and seems to align with Parnas’ formulation. However it begs the question: What defines a reason to change?

Some folks have wondered whether a bug-fix qualifies as a reason to change. Others have wondered whether refactorings are reasons to change. These questions can be answered by pointing out the coupling between the term “reason to change” and “responsibility”.

Certainly the code is not responsible for bug fixes or refactoring. Those things are the responsibility of the programmer, not of the program. But if that is the case, what is the program responsible for? Or, perhaps a better question is: who is the program responsible to? Better yet: who must the design of the program respond to?

Imagine a typical business organization. There is a CEO at the top. Reporting to that CEO are the C-level executives: the CFO, COO, and CTO among others. The CFO is responsible for controlling the finances of the company. The COO is responsible for managing the operations of the company. And the CTO is responsible for the technology infrastructure and development within the company.

Now consider this bit of Java code:

public class Employee {
  public Money calculatePay();
  public void save();
  public String reportHours();
}

• The calculatePay method implements the algorithms that determine how much a particular employee should be paid, based on that employee’s contract, status, hours worked, etc.
• The ‘save’ method stores the data managed by the Employee object onto the enterprise database.
• The reportHours method returns a string which is appended to a report that auditors use to ensure that employees are working the appropriate number of hours and are being paid the appropriate compensation.

Now, which of those C-Level executives reporting to the CEO is responsible for specifying the behavior of the calculatePay method? Which of them would be fired[1] by the CEO if that method were catastrophically mis-specified? Clearly the answer is the CFO. Specifying the pay of employees is a financial responsibility. If all the employees were paid double for a year because someone in the CFOs organization mis-specified the rules for calculating pay, the CFO would likely be fired.

A different C-Level executive is responsible for specifying the format and content of the string returned from the reportHours method. That executive manages the auditors and reviewers, and that’s an operations responsibility. So if there were a catastrophic mis-specification of that report, the COO would be fired.

Finally, it should be obvious which of the C-Level executives would be fired if there were a catastrophic mis-specification of the save method. If the enterprise database were to be corrupted by such a horrific mis-specification, the CTO would likely be fired.

So it stands to reason that when changes are made to the algorithm within the calculatePay method, the request for those changes will originate from the organization headed by the CFO. Similarly it will be the COO’s organization that will request changes to the reportHours method, and the CTOs organization that will request changes to the save method.

And this gets to the crux of the Single Responsibility Principle. This principle is about people.

When you write a software module, you want to make sure that when changes are requested, those changes can only originate from a single person, or rather, a single tightly coupled group of people representing a single narrowly defined business function. You want to isolate your modules from the complexities of the organization as a whole, and design your systems such that each module is responsible (responds to) the needs of just that one business function.

Why? Because we don’t want to get the COO fired because we made a change requested by the CTO. Nothing terrifies our customers and managers more that discovering that a program malfunctioned in a way that was, from their point of view, completely unrelated to the changes they requested. If you change the calculatePay method, and inadvertently break the reportHours method; then the COO will start demanding that you never change the calculatePay method again.

Imagine you took your car to a mechanic in order to fix a broken electric window. He calls you the next day saying it’s all fixed. When you pick up your car, you find the window works fine; but the car won’t start. It’s not likely you will return to that mechanic because he’s clearly an idiot.

That’s how customers and managers feel when we break things they care about that they did not ask us to change.

This is the reason we do not put SQL in JSPs. This is the reason we do not generate HTML in the modules that compute results. This is the reason that business rules should not know the database schema. This is the reason we separate concerns.

Another wording for the Single Responsibility Principle is:

Gather together the things that change for the same reasons. Separate those things that change for different reasons.

If you think about this you’ll realize that this is just another way to define cohesion and coupling. We want to increase the cohesion between things that change for the same reasons, and we want to decrease the coupling between those things that change for different reasons.

However, as you think about this principle, remember that the reasons for change are people. It is people who request changes. And you don’t want to confuse those people, or yourself, by mixing together the code that many different people care about for different reasons.

[1] I am indulging in a bit of hyperbole here. C-level executives don’t usually get fired for small mis-specifications. Still, it’s not outside the realm of possibility, and it does emphasize that the organizations reporting to these executives care about different concerns.