Master the Five Core SOLID Architecture Principles: SRP, OCP, LSP, ISP, DIP

1. Single Responsibility Principle (SRP)

Definition: A software module should have only one reason to change; a class or module must handle a single responsibility.

Example: In a simple e‑commerce system, product, order, supply‑chain, and logistics are separate modules that communicate via messages, preventing changes in one module from affecting others.

2. Open‑Closed Principle (OCP)

Definition: Software entities (classes, modules, functions) should be open for extension but closed for modification. New functionality is added by extending existing abstractions rather than altering existing code.

Core Idea: Develop extensions without modifying the original code.

Example: In an e‑commerce platform, component A is used by both the shopping‑cart and order modules. Upgrading A for the cart should not force changes in the order module; instead, extensions are created.

3. Liskov Substitution Principle (LSP)

Definition: Objects of a superclass must be replaceable with objects of a subclass without altering the desirable properties of the program.

Example: A Rectangle class provides setWidth() and setHeight(). A Square subclass overrides these methods to keep width and height equal, causing setWidth(2) to also set height to 2, breaking the parent class's contract.

Impact: Client code expecting independent width/height may behave incorrectly when a Square is substituted.

Another case: a base Account class requires withdrawal amounts ≤ balance, while a VIPAccount subclass allows overdraft, violating the base class's input contract.

4. Interface Segregation Principle (ISP)

Definition: Clients should not be forced to depend on interfaces they do not use. Large interfaces should be split into smaller, role‑specific ones.

Example: Module A provides inventory deduction, order status update, payment, fulfillment, and logistics. The ordering module depends on all of A, while the payment module only needs the payment function; forcing it to implement the other interfaces wastes resources.

5. Dependency Inversion Principle (DIP)

Definition: High‑level modules should not depend on low‑level modules; both should depend on abstractions. Abstractions must not depend on details; details depend on abstractions.

Example: In an e‑commerce transaction flow, order placement involves inventory deduction, order creation, and payment. These core actions are tightly coupled, while auxiliary functions like logistics or after‑sale services should remain independent and depend on abstractions rather than concrete implementations.