The SOLID principles have guided software design for decades, helping developers write cleaner, more maintainable, and adaptable code. But with modern architectures, AI-assisted coding, serverless platforms, and rapid-release cycles—are these principles still relevant?
Short answer: yes — but with caveats. Let’s break down what SOLID is, why it still matters, and how to apply it thoughtfully rather than dogmatically.
The SOLID acronym represents five fundamental object-oriented design principles.
A class or module should have one reason to change.
Encourages clear separation of responsibilities
Reduces complexity and coupling
Makes testing and modification easier
In practice, SRP is about cohesive behaviour, not necessarily fewer files. A module can do multiple tasks if they are part of the same responsibility.
Software entities should be open for extension, but closed for modification.
New behaviour is added without rewriting core logic
Achieved through abstraction, composition, or plug-in patterns
Supports long-term stability and reuse
In modern systems, OCP aligns closely with extensible APIs, event-driven designs, and plugin architectures.
Objects of a superclass should be replaceable with objects of a subclass without breaking behaviour.
Prevents fragile inheritance hierarchies
Encourages predictable, substitutable components
Reinforces correctness in polymorphic systems
Today, LSP is especially relevant when building SDKs, frameworks, and microservices contracts.
Clients should not be forced to depend on interfaces they do not use.
Avoids “fat” or overly broad interfaces
Reduces unnecessary dependencies
Improves modularity and autonomy
Today, ISP shows up in API design, microservices contracts, and modular front-end architectures.
Depend on abstractions, not concrete implementations.
Promotes loosely-coupled architectures
Enables testing and swapping implementations
Fits naturally with dependency injection frameworks
DIP remains foundational to clean architecture, hexagonal architecture, and testability.
SOLID remains valuable because it addresses timeless engineering goals:
Maintainability
Testability
Modularity
Flexibility
Long-term sustainability
However, the context has evolved:
Serverless and event-driven systems emphasise function boundaries instead of classes
Functional programming encourages composition over inheritance
AI code generation makes clarity and design discipline more important than ever
Microservices shift “design principles” from classes to service boundaries
In other words, SOLID is still relevant — but often applied at higher levels of abstraction than traditional OOP.
Over-engineering small systems
Creating too many layers or abstractions
Blindly following patterns without business value
Good engineering balances simplicity with flexibility.
This is critical:
SOLID is a set of guiding principles, not strict rules or mandates.
They help you think about design trade-offs, not enforce rigid structure.
Break them when simplicity wins
Adapt them to your architecture
Apply them pragmatically, not dogmatically
Great engineers know when not to apply a pattern.
The SOLID principles are still relevant — especially for maintainable, scalable, and testable systems.
Their modern application extends beyond OOP to APIs, services, and modular architectures.
They should be treated as principles, not hard rules — tools for reasoning, not constraints on creativity.
Used wisely, SOLID continues to support the same goal today as when it was introduced: building software that lasts.