Pragmatic SOLID Principles: Writing Clean Systems Without Overengineering

Pragmatic SOLID Principles: Writing Clean Systems Without Overengineering

1. The Overengineering Trap

When developers first learn the SOLID principles, they often fall into the trap of dogmatic over-abstraction:

• Creating 15 interfaces for a single 20-line utility function.
• Abstracting database repositories before validating the business domain.
• Introducing Factory-Provider-Strategy patterns where a simple switch statement suffices.

Pragmatic SOLID = High Cohesion + Low Coupling + Explicit Boundaries
Overengineering = Endless Indirection + Premature Abstraction + Readability Loss

2. Pragmatic Breakdown of the 5 Principles

1. Single Responsibility Principle (SRP)

• Misconception: "A class must only do one single tiny thing."
• Pragmatic Rule: A module should be responsible to one, and only one, actor or business domain reason to change.

2. Open/Closed Principle (OCP)

• Misconception: "Never touch existing code; always subclass or add interfaces."
• Pragmatic Rule: Design extension points (like plugin handlers or strategy maps) around volatile business rules while keeping stable logic direct and readable.

3. Liskov Substitution Principle (LSP)

• Subtypes must be substitutable for their base types without breaking invariants or throwing unexpected runtime errors.

4. Interface Segregation Principle (ISP)

• Clients should not be forced to depend upon interfaces that contain methods they do not use. Prefer small, focused interfaces.

5. Dependency Inversion Principle (DIP)

• High-level policy should not depend on low-level details; both should depend on abstractions. Keep your domain logic free from direct database or vendor SDK dependencies.

3. Clean vs Overengineered Example

// Pragmatic & Clean
export interface NotificationService {
  send(userId: string, message: string): Promise<void>;
}

export class EmailNotificationService implements NotificationService {
  async send(userId: string, message: string) {
    // Direct and readable implementation
  }
}

4. Key Takeaways

• Defer abstractions until you observe real duplication or dynamic business variations (Rule of Three).
• Value clarity and testability over theoretical purity.