Separation of Concerns

Also Known As

TL;DR

Explanation

Separation of Concerns (SoC) is a design principle advocating that a module, class, or function should address a single, well-bounded concern. Mixing concerns — e.g., SQL queries inside HTML templates, or business rules inside database models — produces code that is hard to test, reuse, and change in isolation. SoC underpins MVC architecture, layered architectures, and microservices. In PHP, enforcing SoC means keeping template rendering, business logic, and data access in separate layers with well-defined interfaces between them.

Common Misconception

Why It Matters

Common Mistakes

• Controllers that contain business logic, database queries, and HTML generation.
• Models that send emails, dispatch HTTP requests, or format output.
• Business logic in database stored procedures — makes it invisible to application-level tests.
• Not having a clear mental model of which layer owns which concern before writing code.

Code Examples

// Controller doing DB queries, business logic, AND formatting
class ProductController {
    public function show(int $id): string {
        $row = $this->db->query('SELECT * FROM products WHERE id = ?', [$id]);
        $price = $row['price'] * (1 - $row['discount']);
        return "<h1>{$row['name']}</h1><p>£{$price}</p>";
    }
}

// Controller: HTTP only
class ProductController {
    public function show(int $id): JsonResponse {
        return response()->json($this->productService->getDetails($id));
    }
}
// Service: business logic
// Repository: data access
// View/Resource: formatting

Tags

Related Terms

References

• https://en.wikipedia.org/wiki/Separation_of_concerns