The SOLID Principles
Use the SOLID principles as the foundation of good software design. They ensure your code remains understandable, testable, and easy to change—even as projects grow.
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What really slows teams isn’t complexity—it’s when the parts of code that change most get tangled together. As systems evolve, adding a new feature shouldn’t require rewriting half of the existing code. But over time, quick fixes and shortcuts blur the lines between modules. Logic that used to be separate starts to overlap, making every change feel riskier than it needs to be.
The SOLID principles were created to prevent that slow collapse. They describe how to organize code so it stays easy to understand, modify, and extend. Each principle tackles a different source of pain, but together they guide us toward systems that can grow without falling apart.
Let’s break them down in simple terms.
Single Responsibility Principle (SRP)
Every piece of code should have one clear purpose.
When a module attempts to do too many things—handle input, run logic, save data, and log results—it becomes fragile. Change one part, and something unrelated breaks. The fix is to separate responsibilities. One piece handles coordination. Another contains the logic. Another deals with persistence. Each has its own focus—and its own reason to change.
That separation makes code easier to read, test, and evolve.
Open/Closed Principle (OCP)
Code should be open for extension, but closed for modification.
When requirements change, we should be able to add new behavior without having to rewrite existing functionality. That means designing systems that can grow by adding, not editing. For example, if we need a new logging format or payment method, we add it as a new module instead of patching existing code. Stable parts stay untouched; new parts plug in cleanly.
This keeps change local and protects what’s already tested and trusted.
Liskov Substitution Principle (LSP)
Things that play the same role should behave similarly.
If a piece of code expects a “data store,” any implementation—memory, file, or database—should act the same from the outside. Different internals, same behavior. That consistency lets us swap pieces freely. We can refactor, test, or replace dependencies without breaking the rest of the system.
LSP is what makes abstractions dependable, rather than decorative.
Interface Segregation Principle (ISP)
Don’t depend on things you don’t use.
Oversized interfaces and utility modules often drag in unnecessary dependencies. We import one function and get ten we never needed. The solution is smaller, focused contracts. Each module should do one job and do it well. When code is specific, it’s easier to test, reason about, and replace when needed.
ISP keeps systems light and composable.
Dependency Inversion Principle (DIP)
Depend on abstractions, not details.
High-level code shouldn’t care how its dependencies work—only what they do. A service depends on a “cache” or “repository,” not on Redis or PostgreSQL directly. That separation makes code flexible. We can test with fakes, swap implementations, or scale out without touching the core logic.
Dependency inversion turns rigid systems into adaptable ones.
Bringing it all together
Taken together, the five SOLID principles form a way of thinking about design—how to build systems that stay clean as they grow.
SRP keeps modules focused.
OCP and DIP make systems extendable.
LSP ensures consistent, interchangeable parts.
ISP keeps dependencies clean and light.
These ideas form the foundation for what comes next.
These are the tools and structures you can actually build with. Patterns like Factory, Decorator, and Strategy aren’t just theoretical—they’re practical ways to apply these principles in real code.