System design interview questions

The questions asked in real onsites, with what to clarify, what interviewers probe, common mistakes and the bar at each level. Practice any of them out loud with an AI interviewer.

• Design a URL shortenerRead-heavy key-value service: unique short-code generation, hot-key caching, redirect semantics.Medium
• Design a distributed rate limiterAlgorithm trade-offs (fixed window, sliding log, sliding counter, token bucket), placement, distributed atomic counting, failure modes.Medium
• Design a notification systemAsync processing with durable queues, per-channel isolation, retries/DLQ, at-least-once + idempotency, outbox, priorities.Medium
• Design a news feedFan-out on write vs read, celebrity hybrid, feed caches, outbox, ranking as an extension.Hard
• Design a chat app like WhatsAppStateful connections, routing between chat servers, offline delivery, receipts, ordering, group fan-out, media via object storage.Hard
• Design a ticket booking system like TicketmasterContention on scarce inventory: seat holds with TTL, locking/optimistic concurrency, payments, flash-sale traffic and virtual queues.Hard
• Design a video streaming service like YouTubeUpload pipeline, transcoding DAG, adaptive bitrate streaming, CDN economics, metadata and view counts.Hard
• Design a ride hailing app like UberHigh-frequency location updates, geospatial indexing, matching with contention, trip state machine.Hard
• Design a web crawlerURL frontier, politeness, deduplication at scale, distributed workers, failure and trap handling.Medium

Coming next: AI system design questions, including LLM serving, RAG search and agent platforms.