Live Polling App
Audience voting for live TV — a vote burst and a hot result read, at the same second.
Keep votes landing and results loading while a TV host says 'vote now' to millions at once.
Two very different traffic shapes share this system: a vote is a write burst timed to the second the host says go, and a result is a read that gets hammered by everyone watching the tally move. Neither can block the other — the vote firehose needs a queue, the results need a cache.
Components in play
- Edge LB — Spreads the audience across the Poll API fleet.
- Poll API — Accepts votes and serves the live tally.
- Vote/result split — Splits accepted traffic between the vote-write queue and the tally-read cache.
- Result cache — Serves the current tally without hitting the store.
- Poll results store — Durable vote counts and poll state.
- Vote queue — Absorbs the vote burst before it reaches the store.
- Vote workers — Drains the queue and applies votes to the store.
Graded on these SLOs
- ≥ 97.00%Votes + reads succeed
- ≤ 140 msResult p99
- ≥ 5.0k rpsSustain the vote-now spike
- ≤ 85%Headroom
- ≤ 10Lean footprint
The brief
Design a live-audience polling service like the ones used on singing-competition shows. Viewers vote for their favorite while the results update in real time on screen. The two traffic shapes are opposites: a vote is a write, timed to the instant the host says 'vote now'; a result is a read, hammered continuously by everyone watching the tally move. Neither can be allowed to block the other.
Functional
- Cast a vote for an option in an active poll
- Show a live (near-real-time) tally of results
- Close a poll and lock in the final result
Non-functional
- Vote bursts are extreme and synchronized to a single on-air moment
- Result reads must stay fast even while votes are still landing
- No vote should be silently dropped — undercounting is a trust problem
Warm-up: design decisions
Optional theory to prime the calls a senior engineer would make before you build.
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