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Typing Indicators
High write rate, ephemeral, do not persist.SavedSaved on this device — Saved on this device
01Clarifications
What would you ask before drawing a single box?
Ambiguity you would resolve with the interviewer: scope, scale, who uses it, what counts as done.
AI staff engineer
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About Typing Indicators
High write rate, ephemeral, do not persist.
- Difficulty
- beginner
- Time
- about 50 minutes
- Stages
- 10
- Topic
- Real-Time: Chat, Presence & Live Updates
How this problem is worked
Ten stages, from the questions you would ask an interviewer to the trade-offs you would defend. Each asks one question, and the simulator runs the architecture you draw against the requirements you wrote.
- 01ClarificationsWhat would you ask before drawing a single box?
- 02Functional reqsWhat must this system actually do?
- 03Non-functionalWhat must it promise about speed, uptime and correctness?
- 04Capacity estimationHow much load and data does this have to hold?
- 05API designWhat does the outside world call, and what comes back?
- 06Data modelWhat gets stored, and what is it looked up by?
- 07Use-case breakdownHow does each requirement actually get served?
- 08High-level designWhich components handle a request, and in what order?
- 09Deep divesWhich part breaks first, and what do you do about it?
- 10Trade-offsWhat did this design cost, and what breaks at 10×?
More in Real-Time: Chat, Presence & Live Updates
Long-lived connections and the things you push down them — messages, cursors, green dots, scores, prices and bids.
- WhatsApp / MessengerHundreds of millions of long-lived sockets, sub-second 1:1 + group delivery, E2E-encrypted, multi-device, multi-region active-active.
- Slack / DiscordChannels and history. Push or pull — and how a hot-channel fanout doesn't melt the gateway.
- Live Comments / Score UpdatesPub/sub at scale. WebSocket vs SSE vs long polling. Approximate by design — mega-rooms drop comments on purpose.
- Collaborative Editor (Google Docs)OT vs CRDT. Causal ordering. Real conflict-freedom. Server-authoritative single-writer doc-actor with WAL-before-ack — per-doc serialization, persisted before broadcast, pinned to a home region.
- Online IndicatorGreen dot for contacts. Mind the N² watch problem. Approximate by design — never quote presence more precisely than reality.
- Concurrent Hotel Viewers"X users viewing this right now." Hot keys, HLL.
Browse the full problem catalog, or see what the simulator does and does not model.