When a user opens a hotel detail page, show "127 people are viewing this right now." The display refreshes as users arrive and leave. Sounds trivial — until you realize that on a viral hotel during a holiday weekend, you have one entity (one hotel ID) that 100K users are pinging every few seconds. Every detail of this design — TTL choice, push vs poll, exactness, where you keep state — flips when you account for the hot-key reality.

This canonical is sized for 5M concurrent active hotel pages globally (production-tier 3) with viral hotels reaching 100K concurrent viewers on a single ID. The design tilts heavily on three load-bearing tricks: edge fan-in (per-PoP coalescing collapses 280 PoPs of writes into one batched origin call per heartbeat-interval), hashtag-pinned sharded HyperLogLog (16–64 parallel HLLs per viral hotel co-located in one Redis slot for atomic PFMERGE), and a 2-bucket sliding window on top of the HLL to kill the 30-second roll-over cliff. Push fan-out follows Pusher's published coalesce-and-broadcast pattern (≤100 subscribers broadcast every change; >100 broadcast at most once per 5 s) layered over Discord-style sticky-hash gateway routing.