HLD-108·Caching & Edge
Easy · 500 ptsEvent QR Badge Lookup & Gate Check-In API
Design a stadium and conference gate check-in service validating scanned QR attendee badges at turnstiles with sub-30ms tail latency when venue doors open.
Max p99
≤ 30 ms
Availability
≥ 99.5%
Budget
≤ $2,150
Functional Requirements
- FR-1POST /badges — Issues signed QR badge metadata for a registered attendee.
- FR-2GET /badges/{badge_id} — Verifies ticket entitlement and gate access zone at turnstile scanners.
- FR-3PATCH & DELETE /badges/{badge_id} — Updates, revokes, or deletes badge_id entries with bounded TTL or explicit cache/index invalidation.
- FR-4Turnstile scans must remain fast and available if the primary badge database crashes.
Workload Profile
Early-Bird Scans16,000 Read RPS / 2,500 Check-In Write RPS
Doors-Open Peak48,000 Read RPS / 8,000 Write RPS
Active Working Set6 GB active event badge records
Traffic DistributionBurst skew on active gate badge_id prefixes
Consistency Model
Bounded eventual replication (< 2s) across scanner POPs acceptable; newly issued badges must persist durably.
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