SOLID is an acronym for five design principles that make software designs more understandable, flexible, and maintainable. Essential for Software Engineering.
The Five Principles
Single Responsibility Principle (SRP)
A class should have one, and only one, reason to change.
- Cohesion: Things that change together should stay together
- Each module/class/function should do one thing well
- Easier to understand, test, and maintain
- Reduces coupling between unrelated functionality
Benefits:
- Simpler debugging (fewer reasons for bugs)
- Easier testing (focused unit tests)
- Better reusability
Open-Closed Principle (OCP)
Software entities should be open for extension but closed for modification.
- Extend behaviour without modifying existing code
- Use composition and interfaces, not inheritance
- If existing code is now wrong, replace it with correct code (don’t hack around it)
- Avoid inheritance - it’s often a bad practice in modern development
Application:
- Plugin architectures
- Strategy pattern
- Dependency injection
Liskov Substitution Principle (LSP)
Objects of a superclass should be replaceable with objects of a subclass without breaking the application.
- Subclasses/implementations must be substitutable for their base type
- Contract-based programming
- Use small interfaces over deep inheritance hierarchies
- Prevents surprising behaviour when using polymorphism
Implications:
- Design by contract
- Interface-based design
- behavioural subtyping
Interface Segregation Principle (ISP)
Clients should not be forced to depend on interfaces they don’t use.
- Use small, focused interfaces
- Better than one large “god” interface
- Depend only on what you actually need
- Reduces coupling and improves flexibility
Benefits:
- Clearer dependencies
- Easier mocking for tests
- Better separation of concerns
- Common in Golang design (small interfaces)
Dependency Inversion Principle (DIP)
Depend on abstractions, not concretions.
- High-level modules shouldn’t depend on low-level modules
- Both should depend on abstractions (interfaces)
- Details should depend on abstractions, not the other way around
- Essential for testability
Enables:
- Dependency injection
- Inversion of Control (IoC)
- Mock testing
- Loose coupling
Additional Design Principles
Encapsulation
- Hide internal implementation details
- Expose minimal public interface
- Control access to internal state
- Prevents unintended coupling
Loose Coupling
- Minimize dependencies between components
- Use interfaces and abstractions
- Makes systems more maintainable
High Cohesion
- Related functionality stays together
- Clear, focused responsibilities
- Easier to understand and maintain
- Complements SRP
Common Anti-Patterns to Avoid
- God Objects: Violate SRP
- Tight Coupling: Violate DIP
- Fat Interfaces: Violate ISP
- Fragile Base Classes: Violate LSP, OCP
- Deep Inheritance: Prefer composition
Benefits of SOLID
When applied appropriately:
- More maintainable code
- Easier to test
- Better separation of concerns
- Reduced code duplication
- More flexible and extensible
- Easier to understand
Caveats
- Don’t apply dogmatically
- Sometimes simple is better than “pure”
- Balance with Software Engineering pragmatism
- Consider project context and team
- Avoid over-engineering