System design interviews
Interview guides & deep dives
Free, readable guides for classic system design questions—how interviewers phrase them, what strong answers include, tradeoffs, and FAQs. Each guide links to the matching interactive practice problem with AI feedback.
Guides are curated for clarity and interview realism. For fundamentals (CAP, sharding, caching, etc.), see system design topics.
Facebook Feed System Design
Your friend posted ten minutes ago but your feed still looks like yesterday— fan-out lag or a slow ranker is the product failure, not a missing database box. Hybrid fan-out, a celebrity policy, and honest degradation whe
Instagram Feed System Design
You post a Reel on spotty LTE. The app says "posted" in two seconds—but playback stutters because only a poster frame exists while the full bitrate ladder is still encoding. Your friend's feed still shows yesterday's ord
Rate Limiter System Design
An API gateway must enforce per-client limits (by user, IP, API key, or route) to protect backends and keep usage fair. The hard part is three things at once: the check has to stay very fast even when traffic is huge, ev
Distributed Cache System Design
A flash sale starts and one product id owns a single Redis shard—CPU looks fine while the database melts. The interview is consistent hashing, eviction, replication lag, and cache-aside invalidation, not memorizing RDB f
Parking Lot System Design
Saturday mall rush: entry gates disagree with payment kiosks on whether level 3 is full because spot counts updated eventually. Two cars can both be told to park in the same space unless assignment is atomic. The intervi
Job Scheduler System Design
Midnight cron fires 50K jobs at once and your cluster oversubscribes CPUs while a payment reconciliation job waits behind a low-priority email batch. Scheduling, priority, retries, and exactly-once side effects define th
WhatsApp System Design
Imagine billions of people treating chat like a utility—one tap to send, one glance at delivery ticks, and trust that no one at the company can read the thread. Within seconds of a viral moment, a family group shares a v
Dropbox System Design
A user edits a 500 MB deck on a train while another laptop saves a different copy from the office. Hotel Wi-Fi drops halfway through a large upload. A shared folder sends thousands of change notifications at once. A Drop
Nearby Friends System Design
A user opens a map at a concert and sees "three friends nearby." One friend paused sharing. Another moved across town ten minutes ago. A bad actor keeps querying the same person every few seconds. The design must feel fr
Airbnb Reservation System Design
A guest tries to book a beach house for a holiday weekend while another guest is checking out and a host calendar import is running in the background. The system must let many people browse quickly, but only one reservat
Vaccine Booking System Design
Imagine millions of people trying to book a vaccine appointment the moment registration opens. Within seconds, the system receives an enormous spike in traffic, available slots disappear instantly, and thousands of users
Booking Waitlist System Design
A sold-out concert has a huge waitlist. When one ticket is cancelled, the system must offer it fairly, give one user a time-limited window, handle notification delay, and prevent two waitlisted users from accepting the s
Notification System Design
A product sends password reset codes, receipts, marketing campaigns, and urgent alerts through email, SMS, and push. A marketing blast must not delay password resets, providers rate-limit traffic, and retries must not sp
Facebook Post Search System Design
A user searches Facebook posts for "beach wedding 2019" and expects results from public posts, friends, groups, and their own history. The hard part is not only finding matching words. The system must never show a privat
Auth System Design
A product needs email and password authentication for web and mobile users. People must be able to sign up, log in, stay signed in, reset a forgotten password, optionally use MFA, and sign out from a stolen device. At th
Logging System Design
It is 2am during an incident. Services produce hundreds of terabytes of logs per day, one bad deployment turns on noisy debug lines, and on-call needs to tail fresh logs and search the last few days without a rogue query
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