Tutorials System Design Tutorial
SOLID Principles in LLD — Complete Guide
SOLID Principles in LLD — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of System Design Tutorial on Toolliyo Academy.
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System Design Tutorial · Lesson 71 of 100
SOLID Principles in LLD
Basics ✓ → Scale ✓ → Interview
Interview · 3 — Case studies · ~10 min · Module 8: Low-Level Design
What is this?
SOLID are five design guidelines (SRP, OCP, LSP, ISP, DIP) that keep object-oriented code easier to change safely.
Why should you care?
ShopNest OrderService becomes unmaintainable when one class handles pricing, email, and inventory side effects.
See it live — copy this example
Sketch the architecture on paper. These lessons focus on concepts and trade-offs.
SRP: PricingService separate from OrderService
DIP: OrderService depends on IPaymentClient, not PayPalSdk
OCP: add UPI payer without editing every caller — new implRun Example »
This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.
What happened?
- Small focused types and abstractions at boundaries let ShopNest swap providers and test in isolation.
- Do not invent interfaces for every line.
Practice next
- Split a fat ShopNest service by responsibility.
- Introduce an interface at an external boundary.
- Add a second payment impl behind it.
- Extract EmailSender from OrderService.
- Unit-test with a fake IPaymentClient.
Remember
SOLID guides maintainable LLD. Abstract at boundaries. Keep classes focused.
Payment interface
ShopNest swaps a PSP behind IPaymentClient.
Outcome: Checkout code barely changes.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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