Phase 1 — Foundations
Start here. Build strong fundamentals before jumping to systems.
Beginner
How the Internet Works
IP, DNS, TCP/IP, HTTP/HTTPS basics. What happens when you type a URL.
TCP vs UDP
Reliable vs fast delivery. When to use which. Three-way handshake.
SQL vs NoSQL
Relational vs document/key-value/graph. ACID vs BASE. When to choose what.
Database Indexing
B-tree indexes, covering indexes, when indexes hurt. Query optimisation.
Caching Fundamentals
Why cache? Cache-aside, write-through, write-back. LRU, LFU. Redis basics.
OOP Principles (4 Pillars)
Encapsulation, Abstraction, Inheritance, Polymorphism with real examples.
SOLID Principles
SRP, OCP, LSP, ISP, DIP. The backbone of every LLD interview answer.
Phase 2 — Core HLD Building Blocks
The 10 concepts that appear in almost every system design interview.
Beginner–Intermediate
Phase 3 — LLD & Design Patterns
Object-Oriented design, all 23 GoF patterns, SOLID in practice.
Intermediate
Phase 4 — Intermediate System Design
Design real products end to end. Learn the tradeoffs.
Intermediate
Phase 5 — Advanced System Design
Distributed systems at scale. The problems that separate senior engineers.
Advanced
Phase 6 — Distributed Systems Deep Dive
Internals of databases, consensus, and large-scale infrastructure.
Advanced