System Design: The Complete Guide (HLD, LLD & Interview Prep)
Learn system design from scratch to senior level — the complete free guide covering fundamentals, high-level design (HLD), low-level design (LLD), design patterns, and real interview case studies. Beginner-friendly explanations with senior-grade depth.
Welcome — this is the page I wish I'd had when I started. Whether you've never heard the term "system design" or you're prepping for a staff-engineer interview, this guide takes you the whole way: from "what even is a server?" to designing systems that serve millions of users and writing clean object-oriented code under interview pressure. Everything is explained in plain English first, with real analogies, and then goes as deep as you want. Bookmark it — you won't need another site.
First, the big picture: HLD vs LLD
System design splits into two halves, and interviews test both:
| HLD — High-Level Design | LLD — Low-Level Design | |
|---|---|---|
| Question | "Design Instagram for 100M users" | "Design a parking lot in code" |
| About | Architecture: servers, databases, caches, queues | Classes, objects, design patterns, clean code |
| You draw | Boxes and arrows (components) | Class diagrams (UML) |
New to the difference? Start with HLD vs LLD explained. Then follow the path below.
🟢 Start Here (never done this before? begin here)
The absolute basics, explained like you're five. No prior knowledge assumed.
- What is System Design? — and why every engineer needs it
- How the Web Works — what actually happens when you type a URL
- Servers, Databases & APIs — the basic building blocks in plain words
- Latency, Throughput & Availability — the vocabulary engineers use
- Scaling from Zero to Millions of Users — the story that ties it all together
🔵 Fundamentals (the building blocks)
Every concept you'll reuse in every design. Learn these once, apply them forever.
- How to Approach Any System Design Interview — the 6-step framework 🟠
- Scalability: Vertical vs Horizontal
- Load Balancing · Reverse Proxy vs Load Balancer
- Caching Strategies · CDNs Explained
- SQL vs NoSQL · Database Indexing · ACID & Isolation Levels
- Replication · Sharding & Partitioning
- CAP Theorem · Consistency Models · Consistent Hashing
- Message Queues & Event-Driven Architecture
- API Design: REST vs gRPC vs GraphQL · Real-Time: WebSockets, Long Polling & SSE
- Rate Limiting Algorithms · Bloom Filters
- Blob & Object Storage (S3) · Resilience: Retries, Timeouts & Circuit Breakers
- Distributed Transactions: 2PC vs Saga · Microservices Architecture
- Back-of-the-Envelope Estimation · Latency Numbers Every Engineer Should Know
- Consensus: Raft & Paxos 🔴 · Storage Engines: LSM-Trees vs B-Trees 🔴
🟠 High-Level Design: Design Real Systems
Apply the fundamentals to the exact questions asked at Google, Amazon, and every top company.
- Design a URL Shortener (TinyURL) — the perfect first design
- Design a Rate Limiter · Design Search Autocomplete
- Design a News Feed (Twitter/Facebook) · Design a Chat System (WhatsApp)
- Design a Notification System · Design a Web Crawler
- Design a Distributed Cache · Design a Key-Value Store (DynamoDB)
- Design YouTube · Design Uber
- Design a Payment System · Design Dropbox · Design Ticketmaster
🟠 Low-Level Design: Object-Oriented Design & Patterns
The code-level design and "machine coding" rounds — huge for SDE-2 and senior roles. This builds directly on object-oriented programming.
Foundations: How to Approach an LLD Interview · OOD Basics · SOLID Principles · UML Class Diagrams
Design Patterns (start with the patterns overview): Singleton · Factory · Builder · Strategy · Observer · Decorator · Adapter · State · and more.
Machine-coding problems: Parking Lot · Elevator · Vending Machine · BookMyShow · Splitwise · LRU Cache · Tic-Tac-Toe · ATM and more.
📌 Quick references (keep these open during interviews)
- System Design Cheat Sheet — every concept on one page
- Design Patterns Cheat Sheet
- The 6-Week System Design Study Plan
Where to start (my honest advice)
Here's what I tell everyone who asks me how to learn this. Don't try to memorise designs — it never works, and interviewers can tell instantly. Instead, learn the fundamentals deeply (the boring stuff: caching, databases, queues), because every "impossible" design question is just those Lego bricks snapped together in a new shape. Start with the 0-to-millions story if you're new, or the 6-step framework if you have an interview soon. Then do two designs by hand — the URL shortener for HLD and the parking lot for LLD — actually drawing them out, out loud. Do that, and you'll walk into the room reasoning from a model instead of reciting from memory. That's the whole game. Take your time, enjoy it — this stuff is genuinely fun once it clicks.
Related on GyaanPost
- Learn Java from Scratch — the language most of these examples use
- OOP Interview Questions — the foundation for LLD
- Java Multithreading — concurrency shows up everywhere in system design
- DSA Interview Questions — the other half of technical interviews