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System Design Interview Questions

All 38 lessons in curriculum order, grouped by track. Each lesson pairs deep-dive questions with interactive diagrams and a quiz, and ends with a hands-on challenge.

Foundations & First Principles

What Is System Design? A Beginner's Introduction

Start here. What system design actually is, the vocabulary you need before anything else makes sense, and the four things an interviewer is quietly grading while you talk.

14 min · 4 questions · beginner

Back-of-the-Envelope Estimation for System Design

The arithmetic that turns a vague brief into a concrete architecture. Learn to size QPS, storage and bandwidth in under two minutes - and to know instantly whether a design is plausible.

16 min · 4 questions · beginner

Scalability, Availability and Reliability Explained

The three properties every interviewer probes, made precise. How availability maths actually composes, why reliability is not the same thing, and how to spot a single point of failure in any diagram.

15 min · 3 questions · beginner

Networking & API Design

How the Internet Works: DNS, TCP, TLS and HTTP

Everything that happens between a user typing a URL and bytes arriving - and why each hop is a place where latency and failure live.

16 min · 3 questions · beginner

API Design: REST vs gRPC vs GraphQL

The API is the contract every other decision has to live with. How to choose a protocol, design endpoints that survive five years of change, and get pagination and idempotency right.

17 min · 3 questions · intermediate

Real-Time Communication: Polling, SSE and WebSockets

Four ways to push data to a client, the scaling cost of each, and how to handle the hard part - a million long-lived connections that all reconnect at once.

15 min · 2 questions · intermediate

Databases & Storage

Database Indexes and Query Performance

Why one query returns in 2 ms and an identical-looking one takes 40 seconds. Indexes from first principles, and the storage-engine choice that decides whether your database prefers reads or writes.

16 min · 3 questions · intermediate

SQL vs NoSQL: Choosing the Right Database

Stop asking 'SQL or NoSQL'. Ask what your access patterns are, then let the answer pick the store - with a defensible reason you can say in one sentence.

15 min · 2 questions · intermediate

ACID Transactions and Isolation Levels

The concurrency bugs that only appear in production. What each isolation level actually prevents, why the default is weaker than you think, and how to pick locking strategy.

16 min · 2 questions · advanced

Database Replication and Sharding

How one database becomes many. Replication for read scale and availability, sharding for write scale, and the operational traps in both.

17 min · 2 questions · advanced

Caching & Content Delivery

Caching Strategies and Eviction Policies

The single highest-leverage component in most systems, and the one with the most ways to go subtly wrong. Patterns, eviction, invalidation and the failure modes that cause outages.

17 min · 3 questions · intermediate

CDNs and Edge Caching

The cheapest performance improvement available to any global product. How CDNs work, the HTTP headers that control them, and how to invalidate content across hundreds of locations.

14 min · 2 questions · intermediate

Scaling & Load Balancing

Horizontal vs Vertical Scaling and Stateless Design

How a single server becomes a fleet. Why statelessness is the precondition for everything, and the ordered sequence of moves that takes a system from one box to millions of users.

14 min · 1 question · beginner

Load Balancing Algorithms and Health Checking

Which algorithm to pick and why it matters more than you think. Plus the operational half nobody prepares for: health checks, connection draining and retry amplification.

15 min · 2 questions · intermediate

Consistent Hashing Explained

Why hash(key) % N breaks catastrophically when N changes, and the ring that fixes it. The algorithm behind Cassandra, DynamoDB, Memcached clients and every sharded cache.

14 min · 1 question · advanced

Distributed Systems Theory

CAP Theorem and PACELC, Properly Explained

The most misquoted theorem in the industry. What CAP really says, why the interesting trade-off happens when there is no partition, and how to classify real systems.

14 min · 1 question · advanced

Consistency Models and Replication Strategies

The spectrum between 'always correct' and 'always fast'. Which guarantee each model gives, how quorums work, and how to resolve conflicts when two writers disagree.

16 min · 1 question · expert

Consensus, Leader Election and Distributed Locks

How a group of unreliable machines agrees on anything. Raft in plain language, why split brain is the danger, and why most distributed locks are quietly broken.

15 min · 2 questions · expert

Distributed Transactions, Sagas and Exactly-Once

What to do when one operation must span two systems. Why 2PC is usually the wrong answer, how sagas work, and why exactly-once delivery is impossible but exactly-once processing is not.

16 min · 2 questions · expert

Messaging, Queues & Streams

Message Queues vs Event Streams

Queues and logs look similar and behave completely differently. Which to reach for, how backpressure actually works, and what to do with messages that will never succeed.

15 min · 1 question · intermediate

Kafka Architecture and Delivery Semantics

How Kafka actually stores and delivers data, why partitions are the unit of both parallelism and ordering, and the operational realities - rebalances, lag and hot partitions.

15 min · 1 question · advanced

Architecture Patterns

Monolith vs Microservices: Making the Call

The most over-answered question in system design. When splitting genuinely helps, why most microservice migrations fail, and how to defend keeping the monolith.

15 min · 1 question · advanced

API Gateways, Service Mesh and BFF

Where cross-cutting concerns live once you have more than one service. What belongs in the gateway, what belongs in the mesh, and when a BFF earns its keep.

13 min · 1 question · intermediate

Event-Driven Architecture, CQRS and Event Sourcing

Three related patterns that are constantly conflated. What each actually gives you, what each costs, and why event sourcing is far more expensive than it looks.

15 min · 2 questions · expert

Reliability, Security & Observability

Designing for Failure: Bulkheads, Degradation and Chaos

Everything fails eventually. The discipline is deciding in advance how it fails - which features degrade, which stay up, and how much data you are willing to lose.

15 min · 1 question · advanced

Rate Limiting Algorithms and Distributed Throttling

Four algorithms, one correct default, and the hard part nobody mentions: enforcing a global limit across fifty servers without a round trip on every request.

15 min · 1 question · advanced

Observability: Logs, Metrics and Distributed Tracing

You cannot operate what you cannot see. What each signal is for, why high-cardinality metrics will bankrupt you, and how to alert on things users actually notice.

14 min · 1 question · intermediate

Security in System Design

The security decisions an interviewer expects you to raise unprompted: how users authenticate, how services trust each other, where secrets live, and what a breach costs you.

15 min · 2 questions · advanced

Design Case Studies

Design a URL Shortener (TinyURL / bit.ly)

The classic warm-up question. Deceptively simple functionally, and a perfect vehicle for ID generation, extreme read/write skew and cache design.

16 min · 2 questions · intermediate

Design a News Feed (Twitter / Facebook Timeline)

The canonical fan-out problem. Why neither pure push nor pure pull works at scale, and how the hybrid model that every major platform actually uses is derived.

18 min · 2 questions · advanced

Design a Chat System (WhatsApp / Slack)

Real-time messaging at scale. Connection management, per-conversation ordering, guaranteed delivery across flaky mobile networks, and group fan-out.

17 min · 2 questions · advanced

Design a Notification System

Push, email and SMS at scale. Provider failover, preference enforcement, deduplication and the digest logic that stops you spamming your own users.

15 min · 2 questions · intermediate

Design a Video Streaming Platform (YouTube / Netflix)

The largest-scale case study there is. Transcoding pipelines, HLS and DASH, why 95%+ of bytes must never touch your origin, and counting views at a billion a day.

17 min · 2 questions · expert

Design a Ride-Hailing System (Uber / Lyft)

Real-time geospatial matching. How to index a moving fleet, find nearby drivers in milliseconds, and run a matching algorithm that is correct under heavy concurrency.

16 min · 2 questions · expert

Design a Web Crawler and Search Index

Crawling billions of pages politely, deduplicating near-identical content, and building an inverted index that answers a query in under 200 ms.

17 min · 2 questions · expert

The Interview Playbook

The System Design Interview Framework

A script you can run on any prompt. Six steps, a time budget for each, and the exact sentences that move you between them.

15 min · 1 question · intermediate

The Trade-off Cheat Sheet

System design has no right answers, only defended ones. Every recurring decision, what you gain, what you pay, and the sentence that makes it sound deliberate.

13 min · 1 question · advanced

How You Are Levelled and Evaluated

What actually gets written on the feedback form. The behaviours that map to each level, the rubric dimensions, and how to signal the level you are targeting.

13 min · 1 question · advanced