I am currently working on the project YouTubeLayer. The prototype worked fine initially, but when I tried to move it to a working production system and scale it, everything started breaking. It felt like playing Jenga — touch one block, and the whole tower comes crashing down.
That’s when I stumbled upon something that completely changed the way I thought about software design — the SOLID principles.
These five principles aren’t just fancy theory; they’re a way of writing code that’s easy to understand, extend, and maintain. And trust me — once you start applying them, your code (and your sanity) will thank you.
Let’s go through them one by one, in the simplest way possible 👇
🧩 S — Single Responsibility Principle (SRP)
Definition:
A class should have only one reason to change — in other words, it should do only one thing.
In plain English:
Don’t make a “God class” that does everything — logging, sending emails, calculating bills, and making coffee.
Example:
I once worked on a UserManager class that handled both user authentication and sending email notifications. Every time we updated the notification logic — like changing email templates or adding a new notification type — the authentication system mysteriously broke. 😅
We eventually split it into two separate classes — AuthManager for authentication and NotificationManager for sending emails. After that, changes became smooth and predictable. It was such a relief!
Why it matters:
It makes debugging, testing, and updating code simpler and safer.
🚪 O — Open/Closed Principle (OCP)
Definition:
Classes should be open for extension but closed for modification.
In plain English:
You should be able to add new features without touching existing code.
Example:
This reminds me of a time when I had to add subscription payments to my platform. Initially, all the payment logic lived inside one giant if-else block — handling credit cards, UPI, and wallets all in one place. When I tried adding a new option like PayPal, I had to dive back into that messy code and risk breaking existing functionality.
Later, I refactored the system by introducing a base Payment interface and creating separate classes like CreditCardPayment, UPIPayment, and PayPalPayment. After that, adding a new payment mode was as simple as creating a new class — no need to touch the old code. It felt like magic ✨
Why it matters:
It keeps old features safe while allowing new features to be added easily.
🔄 L — Liskov Substitution Principle (LSP)
Definition:
Subclasses should be replaceable with their parent classes without breaking the application.
In plain English:
If a subclass can’t perform the same role as its parent, it shouldn’t be a subclass.
Example:
This happened when I designed a Payment interface for my subscription system with a processSubscription() method. I had different payment classes like CreditCardPayment, UPIPayment, and CashOnDelivery.
At first, I thought they could all just extend Payment, but then I realized — Cash on Delivery couldn’t actually process a subscription renewal automatically. When I passed a CashOnDelivery object where a regular Payment object was expected, the system broke because it couldn’t handle renewals properly.
Lesson learned — not every subclass should substitute its parent. Inheritance (or implementation) should make logical sense for the behavior it represents.
Why it matters:
It ensures consistency and prevents unexpected behavior when using inheritance or interfaces.
🧱 I — Interface Segregation Principle (ISP)
Definition:
Clients shouldn’t be forced to depend on interfaces they don’t use.
In plain English:
Don’t create one huge interface — break it down into smaller, more specific ones.
Example:
This reminds me of a project where we had a Notifier interface with methods like sendEmail(), sendSMS(), and sendPushNotification().
But not every notification service supported all of these. For example, EmailNotifier didn’t need sendSMS() or sendPushNotification.
So I refactored it into smaller interfaces — EmailNotifier, SMSNotifier, and PushNotifier. That way, each service only implemented what it actually used.
Why it matters:
It keeps code modular and prevents classes from carrying unnecessary baggage.
⚙️ D — Dependency Inversion Principle (DIP)
Definition:
Depend on abstractions, not on concrete implementations.
In plain English:
High-level modules (like your business logic) shouldn’t depend on low-level modules (like database classes). Both should depend on an interface or abstraction.
Example:
When I built a logging feature, I initially called a FileLogger class directly. Later, when we switched to DatabaseLogger, I had to change everything. The fix? Use a Logger interface, and just swap implementations.
Why it matters:
It makes your code flexible and easy to maintain when changing technologies or components.
🧠 Wrapping It All Up
The SOLID principles are like a set of guardrails — they don’t restrict your creativity; they just keep you from falling off the cliff.
Here’s a quick recap:
| Principle | Stands For | Key Idea |
|---|---|---|
| S | Single Responsibility | One class = one job |
| O | Open/Closed | Extend without modifying |
| L | Liskov Substitution | Subclasses should behave like their parents |
| I | Interface Segregation | No unnecessary interface methods |
| D | Dependency Inversion | Depend on abstractions, not concretes |
When I look back, I realize the biggest jump in my programming maturity came when I stopped focusing on just writing code and started focusing on designing it well.
So next time you’re building something — pause for a moment and ask,
“Am I being SOLID about this?”
📚 References
- The S.O.L.I.D Principles in Pictures — Medium (Backticks & Tildes)
- S.O.L.I.D: The First Five Principles of Object-Oriented Design — DigitalOcean
- SOLID Principles for Better Software Design — Ashutosh Krishna Blog
- Understanding SOLID Principles in JavaScript (2024) — Amir Mustafa
- SOLID Principles in One Video 🔥 | Clean Code & OOP Design Simplified for Developers
- SOLID Design Principles | Complete Guide with Code Examples
- SOLID Design Principles | part 2
Top comments (2)
Love how you connected real-world scaling pain with SOLID. The simple explanations and concrete examples make each principle click, and really show how design, not just code, is what keeps systems sane and maintainable.
Thanks buddy, glad to know you see it useful.