Module 0 — Core Concepts

What is System Design?

The art of designing systems that are scalable, reliable, and maintainable. The foundation of building real-world applications.

10 min readFoundation

1The City Planning Analogy

💡 Simple Analogy
Think of designing a city. You need:
  • • Roads (networks) - how people travel between places
  • • Buildings (servers) - where work happens
  • • Water/electricity (data flow) - resources that need to reach everyone
  • • Traffic lights (load balancers) - managing flow efficiently
  • • Emergency services (failover) - handling problems

A city planner doesn't just build for today—they plan for 10x growth. System designers do the same.

System Design is the process of defining the architecture, components, modules, interfaces, and data flow of a system to satisfy specified requirements. It's about making decisions that affect scalability, performance, reliability, and maintainability.

2Why Does System Design Matter?

📈Handles Growth

A well-designed system can scale from 100 to 100 million users without complete rewrites.

💪Survives Failures

When servers crash (they will), good design keeps your app running.

⚡Stays Fast

Users expect sub-second responses. Design determines if you can deliver.

💰Controls Costs

Smart architecture choices can save millions in infrastructure costs.

Interview Reality

System design interviews test your ability to think at scale. Companies want engineers who can architect solutions, not just write code. It's the difference between a senior and junior engineer.

3The System Design Process

1
Clarify Requirements
What problem are we solving? What are the constraints? How many users? What features are must-haves vs nice-to-haves?
2
Define Scope
In a 45-min interview, you can't design everything. Focus on core components. State your assumptions clearly.
3
High-Level Design
Draw the big boxes: clients, servers, databases, caches. Show how data flows between them.
4
Deep Dive
Pick critical components and go deeper. Database schema? API design? How does feature X work exactly?
5
Identify Bottlenecks
Where will the system break at scale? What happens if this component fails? How do we handle it?
6
Scale & Optimize
Add caching, sharding, replication. Trade-offs are key—explain WHY you chose each solution.

4Key Concepts You'll Learn

Scalability
  • • Vertical vs Horizontal
  • • Sharding
  • • Replication
  • • Load Balancing
Reliability
  • • Redundancy
  • • Failover
  • • Health Checks
  • • Circuit Breakers
Performance
  • • Caching
  • • CDNs
  • • Database Indexing
  • • Async Processing
Data
  • • SQL vs NoSQL
  • • Consistency Models
  • • CAP Theorem
  • • ACID vs BASE
Communication
  • • REST, GraphQL, gRPC
  • • Message Queues
  • • WebSockets
  • • Pub/Sub
Architecture
  • • Microservices
  • • Event-Driven
  • • CQRS
  • • Saga Pattern

5System Design in Interviews

What Interviewers Look For

25%
Communication
Can you explain your thinking clearly? Do you ask good questions?
25%
Problem Solving
Do you break down complex problems? Handle ambiguity well?
25%
Technical Knowledge
Do you know the building blocks? Databases, caching, queues, etc.
25%
Trade-off Analysis
Every choice has pros/cons. Do you understand and explain them?

✓ Do This

  • • Ask clarifying questions first
  • • Think out loud
  • • Start with high-level, then dive deep
  • • Discuss trade-offs explicitly
  • • Use real numbers (QPS, storage, bandwidth)

✗ Avoid This

  • • Jumping straight to solution
  • • Over-engineering from the start
  • • Ignoring requirements
  • • Not explaining your reasoning
  • • Saying "it depends" without context

6Common System Design Questions

Design a URL shortener (like bit.ly)
Hashing, databases, redirectsEasy
Design Twitter/X feed
Fan-out, caching, timelinesMedium
Design WhatsApp
Real-time messaging, presence, deliveryMedium
Design YouTube
Video storage, CDN, transcodingHard
Design Google Search
Crawling, indexing, rankingHard

7Key Takeaways

1System design is about making architectural decisions that handle scale, failures, and growth.
2It's not about memorizing solutions—it's about understanding trade-offs.
3Always start with requirements and constraints before diving into solutions.
4Good designs are simple first, then add complexity only when needed.
5No perfect solution exists—every choice has trade-offs. Explain them.
6Practice by designing systems you use daily: Instagram, Uber, Spotify.