Social-feed cases: free-recall review
Free-recall prompts across the social-feed cases. Reconstruct each design from memory — notification pipeline, feed fan-out hybrid, chat push/ordering, autocomplete precompute — before revealing the model answer.
Retrieval beats re-reading. For each prompt, reconstruct the full design from memory — the pipeline, the tradeoff, the failure mode — before opening the model answer. The effort of rebuilding the architecture is what makes these four cases stick.
Rebuild each design without looking back: the notification fan-out pipeline and what prevents the retry storm; the feed’s fan-out tradeoff and the celebrity hybrid; chat’s push transport, ordering, and delivery ticks; and why autocomplete precomputes everything offline.
- 01Reconstruct the notification pipeline and the three pieces that prevent the retry storm.
- 02State the feed's fan-out tradeoff, the read:write ratio that decides it, and the celebrity hybrid.
- 03Explain chat's transport, the persist-before-ack rule, ordering, and the three ticks.
- 04Why must autocomplete precompute, and how does the trie with top-k per node serve in O(1)?
If you can rebuild each design from memory, you hold the unit’s spine. The notification system is an async fan-out pipeline whose retry storm is prevented by backoff+jitter, a bounded-retry DLQ, and an idempotency key. The news feed defaults to push (read-heavy, ~100:1) but pulls celebrities in a hybrid because the power-law tail breaks a single strategy. The chat system is server-push over WebSockets routed via a connection registry, persists before it acks, and orders messages with a per-conversation sequence number. Autocomplete precomputes top-k per trie node offline and serves an O(1) read behind an edge cache, never ranking live per keystroke. The through-line: at this depth, every read-heavy social system moves the expensive work to the write or build side so the user-facing path is a cheap, safe lookup.
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