Single Responsibility Principle

Also Known As

TL;DR

Explanation

The S in SOLID, SRP states that every module, class, or function should be responsible for one part of the system's functionality. "One reason to change" means: if two different types of requirements changes would require editing the same class, that class probably has too many responsibilities. SRP makes code easier to understand, test, and modify because changes to one concern don't accidentally break another.

Common Misconception

Why It Matters

Common Mistakes

• Confusing SRP with "one method per class" — it means one reason to change, not one action.
• Putting database queries, business logic, and HTTP response building all inside a controller.
• Splitting too aggressively — a 5-line helper class for every trivial operation creates its own maintenance burden.
• Ignoring SRP during prototyping and never refactoring once the code "works".

Code Examples

class User {
    public function save(): void { /* DB logic */ }
    public function sendWelcomeEmail(): void { /* SMTP logic */ }
    public function toJson(): string { /* serialisation */ }
    public function validatePassword(string $pw): bool { /* validation */ }
    // Four reasons to change
}

class OrderController {
    public function store(Request $r, OrderService $orders) {
        $order = $orders->create($r->validated());
        return response()->json($order);
    }
}
// OrderService handles business logic, dispatches events/jobs

References

• ↗ https://en.wikipedia.org/wiki/Single-responsibility_principle

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