🚀 Introduction to System Design: LLD and SOLID Principles

🚀 Introduction to System Design: LLD and SOLID Principles

In the context of system design, Low-Level Design (LLD) serves as the foundational layer that translates high-level architectural decisions into detailed class structures, interfaces, and method definitions. While High-Level Design (HLD) outlines the overall system architecture, components, and technology stack, LLD focuses on the internal logic and object interactions that enable the system to function effectively. A critical component of effective LLD is the application of SOLID principles, which provide a structured approach to building maintainable, scalable, and extensible software systems.

This blog dives deep into why LLD matters, what SOLID principles are, and how they help create clean, maintainable, and scalable systems—with practical code examples.

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🧩 Why Low-Level Design (LLD) Matters

LLD is all about breaking a problem into classes and interactions while maintaining principles like reusability, extensibility, and testability. It:

• Bridges the gap between HLD and code

• Helps in better planning of class responsibilities

• Makes systems modular and easier to debug

• Supports clean architecture and future scalability

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🔍 What are SOLID Principles?

SOLID is an acronym for five key design principles in object-oriented programming that promote better LLD. Introduced by Robert C. Martin (Uncle Bob), they aim to make code:

• More readable

• Easier to maintain

• Flexible to change

SOLID stands for:

1. S – Single Responsibility Principle (SRP)

2. O – Open/Closed Principle (OCP)

3. L – Liskov Substitution Principle (LSP)

4. I – Interface Segregation Principle (ISP)

5. D – Dependency Inversion Principle (DIP)

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🧱 Deep Dive into SOLID with Examples

1. ✅ Single Responsibility Principle (SRP)

Definition: A class should have one, and only one, reason to change. Example: A class should either manage user data or send emails—not both.

❌ Bad Example:

class User {
public:
    void saveUser() {
        // Save user to DB
    }
    void sendEmail() {
        // Send confirmation email
    }
};

✅ Good Example:

class User {
public:
    void saveUser() {
        // Save user to DB
    }
};

class EmailService {
public:
    void sendEmail() {
        // Send confirmation email
    }
};

Each class now has a single responsibility, improving maintainability.

2. 🧯 Open/Closed Principle (OCP)

Definition: Software entities should be open for extension but closed for modification.

Example: Add new discount types without touching old code.

❌ Bad Example:

class DiscountCalculator {
public:
    int getDiscount(string type) {
        if (type == "regular") return 10;
        if (type == "vip") return 20;
        return 0;
    }
};

✅ Good Example:

lass Discount {
public:
    virtual int getDiscount() = 0;
};

class RegularDiscount : public Discount {
public:
    int getDiscount() override { return 10; }
};

class VIPDiscount : public Discount {
public:
    int getDiscount() override { return 20; }
};

Add new customer types by extending, not modifying existing logic.

3. 🧬 Liskov Substitution Principle (LSP)

Definition: Subtypes must be substitutable for their base types without altering the correctness of the program.

Example: A Bird base class should not assume all birds can fly.

❌ Bad Example:

class Bird {
public:
    virtual void fly() { cout << "Flying" << endl; }
};

class Ostrich : public Bird {
public:
    void fly() override { throw; } // Ostrich can't fly
}

✅ Good Example:

class Bird {
public:
    virtual void makeSound() { cout << "Chirp" << endl; }
};

class FlyingBird : public Bird {
public:
    virtual void fly() { cout << "Flying" << endl; }
};

class Sparrow : public FlyingBird {};
class Ostrich : public Bird {};

Child classes behave correctly when used in place of their base class.

4. 🧩 Interface Segregation Principle (ISP)

Definition: Clients should not be forced to depend on interfaces they do not use.

Example: Not every machine can fax

❌ Bad Example:

class Machine {
public:
    virtual void print() = 0;
    virtual void scan() = 0;
    virtual void fax() = 0;
};

class BasicPrinter : public Machine {
public:
    void print() override {}
    void scan() override {}
    void fax() override { throw; } // Doesn’t support fax
};

✅ Good Example:

class Printer {
public:
    virtual void print() = 0;
};

class Scanner {
public:
    virtual void scan() = 0;
};

class SimplePrinter : public Printer {
public:
    void print() override {}
};

Separate interfaces prevent unnecessary method implementations. Only implement what’s needed. No empty or broken methods.

5. 🔄 Dependency Inversion Principle (DIP)

Definition: High-level modules should not depend on low-level modules. Both should depend on abstractions.

Example: Let the app depend on Database abstraction, not directly on MySQL.

❌ Bad Example:

class MySQL {
public:
    void connect() { cout << "Connected to MySQL" << endl; }
};

class App {
    MySQL db;
public:
    void run() {
        db.connect(); // Tightly coupled
    }
};

✅ Good Example:

class Database {
public:
    virtual void connect() = 0;
};

class MySQL : public Database {
public:
    void connect() override { cout << "Connected to MySQL" << endl; }
};

class App {
    Database* db;
public:
    App(Database* db) : db(db) {}
    void run() {
        db->connect();
    }
};

You can easily switch to PostgreSQL or mock the DB for testing.

🌟 Advantages of SOLID Principles

Applying SOLID principles offers several benefits:

• ✅ Maintainability: Code becomes easier to modify without unintended side effects.

• ✅ Scalability: New features can be added with minimal changes to existing code.

• ✅ Testability: Modular design improves unit testing capabilities.

• ✅ Readability: Clear structure and responsibility make the code easier to understand.

• ✅ Reusability: Decoupled components are easier to reuse across different projects.

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Understanding and applying the SOLID principles is a foundational step toward writing clean, scalable, and maintainable software. These principles help in reducing code complexity, enhancing reusability, and ensuring long-term success in system design, especially at the Low-Level Design (LLD) stage.

This blog presents the core understanding in a simplified, practical way with easy-to-follow C++ examples

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Further Reading & References

• SOLID Principles by Uncle Bob (Robert C. Martin)

• SOLID Principles by GFG