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, etc.), see reading topics under system design.
Facebook Feed System Design (Ranking & Scale)
From the user’s side, the home feed is simple: open the app, scroll, see posts from people and pages you follow—ideally ranked or at least fresh enough that the product feels alive. From your side as the engineer, the ha
Instagram Feed System Design (Feed Ranking & Scale)
From the user’s side, Instagram is scroll, tap, watch: photos, carousels, Reels, and Stories that disappear tomorrow. From your side, the product is bytes: huge video objects on cellular networks, thumbnails that must be
Rate Limiter System Design (Token Bucket Explained)
An API gateway must enforce per-client limits (by user, IP, API key, or route) to protect backends and keep usage fair. The interesting part is doing it at millions of checks per second with sub-millisecond overhead, cor
Distributed Cache System Design (Sharding, Eviction & Scale)
You’re building an in-memory cluster that feels like Redis or Memcached at scale: sub-millisecond reads, horizontal add/remove of nodes, and explicit policies for eviction, replication, and what “consistent” means when f
Job Scheduler System Design (Queues, Retries & Reliability)
Teams need cron-like scheduling, one-off delayed jobs, and dependency graphs at huge scale: nothing should run twice as observed by the business without you saying why, and nothing should silently never run when a worker
WhatsApp System Design (Messaging Reliability at Scale)
Billions of users expect private chat that feels instant: delivery and read receipts, ordering you can reason about, media that does not toast mobile networks, and end-to-end encryption so the server is not reading plain
Dropbox System Design (File Sync & Conflict Resolution)
Users expect folders to stay in sync across laptops and phones: edits should converge, dedup should save money, and sharing should not turn into a permissions nightmare. The interview stress is metadata scale, conflict h
Nearby Friends System Design (Geo Indexes, Privacy & Scale)
Users who opt in want to see which friends are roughly nearby without broadcasting exact addresses. The system must combine location streams (noisy, bursty) with the social graph, respect privacy and freeze modes, and an
Airbnb Reservation System Design (Mock Interview + No Double Booking)
Guests search listings and book date ranges on one calendar per listing. The job is to never double-book overlapping nights while still feeling fast enough to complete checkout—holds expire, payments fail, and two people
Vaccine Booking System Design (Mock Interview + Slots & Fairness)
A public program must verify eligibility under changing regional rules, expose appointment inventory honestly, and survive thundering herds when new cohorts qualify—without double-booking slots or leaking sensitive recor
Booking Waitlist System Design (Fair Queuing & Notifications)
When inventory is gone, users join a waitlist; when a slot frees, the next eligible person gets a time-limited offer to pay or confirm. The system must never give the same released slot to two people, must notify reliabl
Notification System Design (Fan-Out & Delivery at Scale)
Product teams enqueue notifications; your platform fans out to push, email, and SMS with preferences, templates, retries, and provider rate limits—without becoming a spam cannon or melting APNs/FCM quotas. The interview
Facebook Post Search System Design (Indexing & Query at Scale)
Users search posts they can see—by keywords, author, time—with latency and freshness expectations that don’t forgive naive “grep the database.” The hard part is inverted index scale plus authorization: two posts might ma
Auth System Design (Sessions & Tokens Guide)
You’re building username/password auth with sessions for web and mobile: sign-up, login, logout, optional remember device, and password reset—while attackers run credential stuffing, brute force, and session theft agains
Logging System Design (Ingestion, Storage & Query at Scale)
Services emit structured logs at massive scale; operators need search, dashboards, and retention with compliance for audit trails without paying petabyte prices for every debug printf. The interview is ingestion pipeline
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