Everything you need to ace the system design interview

Core Topics

Deep dives into every building block: load balancing, caching, sharding, consistency, queues, and more.

Case Studies

Classic interview questions solved end to end - requirements, estimation, APIs, deep dives, and a rapid MVP build with real code.

Interview Framework

A battle-tested 4-step framework with a minute-by-minute plan for a 45-minute interview.

Back-of-Envelope Calculator

Turn DAU and usage assumptions into QPS, storage, bandwidth, and cache estimates instantly.

Flashcards

Rapid-fire Q&A across all categories to pressure-test your recall before the interview.

Glossary & Latency Numbers

Every term you might get asked, plus the latency numbers every engineer should know.

Design a service that converts long URLs into short, unique aliases and redirects users who visit the alias to the original URL. The core challenges are generating collision-free short codes at scale, serving redirects with very low latency, and handling a read-heavy workload that can be two orders of magnitude larger than writes.

Design a service that limits how many requests a client can make in a time window (e.g., 100 requests per minute per user) across a fleet of many API servers. The core challenges are choosing an algorithm with the right burst and accuracy tradeoffs, sharing counter state across servers with low latency, and deciding how the limiter should fail.

Design the system that lets users post content and see a ranked, near-real-time feed of posts from people they follow. The defining challenge is the fan-out problem: how a single post reaches millions of followers' feeds efficiently, and how to blend fan-out on write with fan-out on read to handle celebrity accounts without melting the infrastructure.

Design a messaging system supporting one-on-one and group chats with real-time delivery, message ordering, online presence, and read receipts. The core challenges are maintaining millions of long-lived WebSocket connections, routing messages between users connected to different servers, guaranteeing per-conversation ordering, and syncing state across a user's multiple devices.

Design a platform where creators upload videos and viewers stream them worldwide. The system splits into an asynchronous upload-and-transcode pipeline and a read-dominated streaming path built on adaptive bitrate protocols (HLS/DASH) and a CDN. The core challenges are parallelizing transcoding, serving petabytes of video with low startup latency, and keeping origin traffic tiny relative to what viewers consume.

Design a crawler that downloads billions of pages for a search index while being a polite citizen of the web. The core components are a URL frontier that balances priority against per-host politeness, a dedup layer (Bloom filters and content fingerprints), robots.txt compliance, trap avoidance, and a recrawl strategy that keeps the index fresh.