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Level101 201 301 401

system design · 101

Foundations

Master system design interviews with scalable architecture patterns, distributed systems, and real-world design challenges.

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Course curriculum

OSI Networking Model 4

Foundations of the OSI Model

Layers 1–4: Physical, Data Link, Network, Transport

Layers 5–7: Session, Presentation, Application

Interview Q&A (Staff/Principal)

TCP & UDP 4

TCP Foundations

UDP Foundations

TCP vs UDP Deep Comparison & Advanced Internals

Interview Q&A (Staff/Principal)

DNS 5

DNS Foundations

DNS Architecture & Hierarchy

DNS Query Resolution & Caching

DNS in System Design

Real‑World DNS Architecture

HTTP & HTTPS 4

Foundations of HTTP/HTTPS

HTTP Architecture, Methods & Headers

Advanced HTTP Concepts

Interview Q&A (Staff/Principal)

REST APIs 4

REST Foundations

REST Architecture, Caching & Security

Advanced REST & Staff/Principal Interview Q&A

REST Revision Sheet

RPC (Remote Procedure Call) 4

Foundations of RPC

RPC Architecture & Internals

Advanced RPC Concepts & Comparisons

System Design & Interview Q&A

gRPC 4

gRPC Foundations

Architecture, HTTP/2, Protobuf & Streaming

Advanced Concepts & Interview Q&A

Real-World Examples & Code Walkthroughs

GraphQL 4

GraphQL Foundations

Architecture, Execution Engine & Query Planning

Advanced Topics & Interview Q&A (Senior/Staff)

Real-World Examples & Code Walkthroughs

WebSockets 4

Foundations of WebSockets

Architecture & Scaling

Advanced Concepts & Comparisons

Interview Q&A (Staff/Principal)

Message Queues & Pub/Sub 4

Foundations of Asynchronous Messaging

Broker Internals and Delivery Semantics

Idempotency, the Outbox Pattern, and Broker Comparisons

Interview Q&A (Staff/Principal)

Load Balancers 4

Foundations & Fundamentals

Architecture & Internals

Advanced Topics & Comparisons

Interview Q&A (Staff/Principal)

Reverse Proxy 3

Foundations of Reverse Proxy

Architecture & Advanced Concepts

Interview Q&A (Staff/Principal)

API Gateway 4

Foundations of API Gateways

Architecture & Internals

Advanced Concepts & Comparisons

Interview Q&A (Staff/Principal)

Rate Limiting 4

Foundations of Rate Limiting

Where and How to Enforce Rate Limits

Distributed Rate Limiting at Scale

Interview Q&A (Staff/Principal)

Caching 4

Foundations of Caching

Cache Architecture & Internals

Invalidation, Stampedes & Consistency

Interview Q&A (Staff/Principal)

CDN 4

Foundations of CDN

Architecture, Caching & Edge Compute

Interview Q&A (Staff/Principal)

CDN Revision Sheet

Databases: SQL vs NoSQL 4

The Relational Model and the Four NoSQL Families

Modeling by Access Pattern, Transactions, and Scaling

Polyglot Persistence, Choosing by Access Pattern, and CAP in the Wild

SQL vs NoSQL Interview Questions

ACID Transactions 4

Foundations of ACID Transactions

ACID Internals & Implementation

Advanced Concepts & Tradeoffs

System Design & Interview Q&A

CAP Theorem 4

Foundations of CAP Theorem

Consistency Models Deep Dive

CAP in Practice — Real Systems & Trade-offs

Interview Q&A (Staff/Principal)

User scenarios 5

What Happens When You Enter a URL?

Place an Order — Why Aren't You Charged Twice?

What Happens When You Send a Chat Message?

What Happens When You Press Play on a Video?

What Keeps the Site Up During a Flash Sale?

CDN — Quick Revision Sheet

The night-before-the-interview page. If you've read Parts 1-3, this is your fast refresh — the corner-shop chain in one screen.

What is a CDN?

A globally distributed network of cache servers (PoPs) that serves content from the location nearest each user — beating the speed-of-light latency floor by shortening the distance, not the signal. Think coffee shop on every corner instead of one roastery across the ocean.

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