Complete designs worked end to end: URL shortener, news feed, chat, ride-hailing, and more.
14 questions in System Design.
Evaluate feature flags in an SDK from a cached ruleset, assign users by hashing a per-experiment salt with the user id so assignment is sticky, log exposure where the arms first diverge, gate readouts on an automated SRM check, and make the kill switch a top-precedence rule that works with the control plane down.
A notification system at 50 million a day is sized by bursts and provider limits, not its average of about 580 a second. Accept with an idempotency key, apply preferences once, fan out to per-channel queues with transactional and bulk lanes, rate-limit each provider, and never fail over blindly on a timeout.
A distributed job scheduler cannot promise exactly-once execution when workers die mid-job, so it promises at-least-once with idempotent jobs. Materialise each firing as a run row, claim it with a lease and heartbeats, judge time by one clock, and key side effects on job id plus scheduled time.
In file sync, the file itself does not travel. Changed content-hashed chunks go to a block store, then a small metadata commit names the version it replaced. Other laptops pull the new manifest and the chunks they lack. A concurrent edit fails that commit and becomes a conflicted copy, never a silent overwrite.
Video streaming startup time is won before playback: transcode into an adaptive ladder, cut aligned segments, package a manifest, and cache the first low-bitrate segment near the viewer. At play time the system can only afford manifest fetch, licence check, first segment and decoder start. It also connects adaptive bitrate to the point an interviewer is testing.
Size it first: 200M daily actives opening the feed six times a day is 42k reads/s at peak, while 100M posts fanned out to 200 followers each is 231k feed writes/s, so writes dominate. Push to ordinary accounts, pull for high-follower ones, and store ids so deletes and blocks filter on read.
Clarify scale first, then size it: 100M new links a day at a 100:1 read ratio gives roughly 1.2k writes/s and 116k redirects/s, needing 7-character base62 keys. Generate keys from leased counter blocks scrambled by a bijection, serve reads from cache and CDN, and redirect with 302 rather than 301.
Ride matching system design should rank drivers by estimated pickup time and market impact, not straight-line distance. Offer the ride under a short soft lock, use a small batching window when it improves assignment quality, and keep the rider wait within a few seconds. Use this dispatch answer to show the decision, trade-off, and evidence rather than a memorised definition.
An authoritative breakdown of horizontal matchmaking services, constraint relaxation algorithms, and distributed locking for ticket-based lobbies. Use this matchmaking answer to show the decision, trade-off, and evidence rather than a memorised definition. It also connects SBMM to the point an interviewer is testing.
CDN caching for global reads works when cacheable bytes are separated from personalised fragments. Use content-addressed assets, tag-based purges for pages that must change, and plan for purge storms because invalidation sends cold traffic back to the one origin region. It also connects http caching to the point an interviewer is testing.
Paid video streaming DRM raises the cost of copying rather than making copying impossible. Start with signed short-lived URLs, add encrypted segments, and use DRM when keys must stay outside application code, while budgeting for licence latency and unsupported devices.
Real-time pricing system design should publish numbered price versions instead of calculating a different answer per request. Compute prices by region and time window, serve the current version consistently, and freeze quotes briefly so users are not surprised during checkout.
Selling a specific seat is a uniqueness problem, not a throughput one, so the seat row is the serialisation point and the design lives or dies on the reservation hold: a short expiring claim written atomically, plus a queue in front so the contention never reaches the database at full force.
Preventing double booking seats means claiming the seat before payment, not racing two payments and reconciling later. A short atomic hold serialises buyers, payment only starts for the winner, and uncertain payment outcomes are settled by reconciliation rather than silent expiry.