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What is System Design?

System Design (HLD - High Level Design) focuses on the architecture of software systems. While LLD deals with classes and code, HLD deals with:
  • Components and how they interact
  • Scalability and handling growth
  • Reliability and fault tolerance
  • Data storage and retrieval patterns
  • Trade-offs between different approaches
Interview Reality: System design interviews are often the deciding factor for senior roles. A strong system design performance can make up for weaker coding rounds!

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🎯 Learning Path

Choose your path based on your experience level:
LevelFocus AreasTime
Junior/Mid (L3-L4)Fundamentals β†’ Building Blocks β†’ Easy Cases2-3 weeks
Senior (L5)+ Deep Dives β†’ Medium Cases β†’ Trade-offs4-6 weeks
Staff+ (L6+)+ Consensus β†’ Global Architecture β†’ All Cases6-8 weeks

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Start Here (Interview Prep)

πŸ“‹ Interview Guide

Start here! RESHADED framework and strategies

❓ Question Bank

30+ questions by difficulty with hints

🎭 Mock Interviews

Realistic scenarios with evaluation criteria

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Course Structure

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πŸ“š Foundations (Everyone)

Fundamentals

Scalability, Latency, Throughput, CAP Theorem

Building Blocks

Load Balancers, Caches, Message Queues, CDN

Networking

DNS, TCP/UDP, HTTP, WebSockets, gRPC

Databases

SQL vs NoSQL, Sharding, Replication, Indexing

API Design

REST, GraphQL, Rate Limiting, Versioning

Estimations Mastery

Back-of-envelope calculations with practice problems

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πŸ”§ Core Patterns (Senior+)

Distributed Systems

Consistency, Consensus, Transactions

Microservices

Patterns, Service Mesh, Communication

Scalability Patterns

Stateless Design, Multi-level Caching, Sharding

Reliability Patterns

Circuit Breakers, Retries, Bulkheads

Event-Driven Architecture

Event Sourcing, CQRS, Saga Pattern

Trade-off Framework

Systematic approach to design decisions

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πŸš€ Staff+ Deep Dives

For Staff/Principal Interviews: These topics separate senior from staff-level candidates. Interviewers expect deep technical knowledge and ability to make architectural decisions.

Consensus Algorithms

Raft, Paxos, Leader Election (with code)

Global Architecture

Multi-region, Disaster Recovery, Geo-distribution

Data Modeling

Event Sourcing, CQRS, Schema Evolution

Real-Time Systems

WebSockets, SSE, Push Architecture

Search Systems

Elasticsearch, Inverted Index, Ranking

Time-Series Data

Metrics, IoT, Financial data patterns

CDN & Edge Computing

Edge functions, Caching strategies

Data Pipelines

Batch vs Stream, Lambda/Kappa

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πŸ“– Quick References

Patterns Reference

Quick lookup for all design patterns

Observability

Metrics, Logs, Traces, SLIs/SLOs

Security Patterns

JWT, OAuth, RBAC, mTLS

Advanced Concepts

Linearizability, Vector Clocks

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πŸ’Ό Case Studies (Practice These!)

URL Shortener

🟒 Easy - Start here

Rate Limiter

🟒 Easy - Algorithms

Notification System

🟑 Medium - Multi-channel

Twitter/X

🟑 Medium - Feed design

WhatsApp

🟑 Medium - Real-time

E-Commerce

🟑 Medium - Transactions

Payment System

πŸ”΄ Hard - Financial

Netflix

πŸ”΄ Hard - Video streaming

Spotify

πŸ”΄ Hard - Audio streaming

Uber

πŸ”΄ Hard - Geo-matching

Google Maps

πŸ”΄ Hard - Navigation

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Interview Framework (RESHADED)

1

R - Requirements (5 min)

Ask about users, scale, features, and constraints. β€œAre we optimizing for reads or writes?”
2

E - Estimation (5 min)

Calculate QPS, storage, bandwidth. Show your math clearly.
3

S - Storage Schema (5 min)

Define data models, choose SQL vs NoSQL with justification.
4

H - High-Level Design (10 min)

Draw main components and data flow. Start simple.
5

A - API Design (5 min)

Define key endpoints, request/response formats.
6

D - Deep Dive (10 min)

Detail specific components, discuss trade-offs.
7

E - Edge Cases (3 min)

Handle failures, race conditions, hot spots.
8

D - Discuss (2 min)

Summarize trade-offs, mention what you’d add given more time.
Pro Tip: Check out the full Interview Guide for the complete RESHADED framework, cheatsheets, and common mistakes to avoid.

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Top Interview Problems by Level

ProblemLevelKey ConceptsPriority
URL ShortenerL3-L4Hashing, DB, CacheMust Know
Rate LimiterL3-L4Token Bucket, RedisMust Know
Twitter FeedL4-L5Fan-out, Timeline, CacheMust Know
WhatsAppL4-L5WebSockets, Message QueueMust Know
E-CommerceL4-L5Inventory, TransactionsMust Know
NotificationL4-L5Push/Pull, TemplatesRecommended
SpotifyL5Streaming, RecommendationsRecommended
YouTubeL5-L6Video Processing, CDNRecommended
UberL5-L6Location, MatchingKnow Basics
Google DocsL6+CRDT, Real-time collabStaff+

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Quick Reference: Key Numbers

Latency Numbers

  • L1 cache: 0.5 ns
  • RAM: 100 ns
  • SSD: 100 ΞΌs
  • HDD: 10 ms
  • Same DC: 0.5 ms
  • Cross-continent: 150 ms

Scale Numbers

  • Seconds/day: ~100,000
  • 1M DAU β†’ ~12 QPS
  • 100M DAU β†’ ~1,200 QPS
  • 99.99% = 52 min downtime/year
AvailabilityDowntime/YearUse Case
99% (2 nines)3.65 daysInternal tools
99.9% (3 nines)8.76 hoursStandard SaaS
99.99% (4 nines)52.6 minutesCritical services
99.999% (5 nines)5.26 minutesPayment systems

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What Makes This Course Different

πŸ’» Working Code

Every pattern includes Python + JavaScript implementations you can run

🎯 Interview Focused

Organized by what interviewers actually ask at each level

βš–οΈ Trade-off Framework

Systematic approach to design decisions, not just memorized answers

πŸ”„ Real Scenarios

Mock interview scripts with expected follow-ups
Start with fundamentals! Many candidates jump to case studies without understanding the building blocks. This leads to surface-level answers. Master the concepts first.