Design a Camera Health Dashboard

# Design a Camera Health Dashboard Design a system that displays how many of one million cameras are healthy. Each camera sends a signal every 20 seconds and counts as healthy when at least one signal has arrived during the most recent 60 seconds. ### Constraints & Assumptions - Signals may be duplicated, delayed, or arrive out of order. - The displayed count should have a stated freshness and consistency contract. - A camera changes state only when its latest accepted signal crosses the 60-second boundary. ### Clarifying Questions to Ask - Is health based on server receive time or device event time? - How fresh must the dashboard count be? - Must users also drill down to individual unhealthy cameras? ```hint Model state transitions Counting every signal is less important than detecting when a camera moves between healthy and unhealthy. ``` ### What a Strong Answer Covers - Ingestion, identity, deduplication, and latest-signal state. - Efficient expiration scheduling rather than full scans every second. - Atomic or reconcilable updates to the aggregate count. - Dashboard delivery, clock handling, failure recovery, and monitoring. ### Follow-up Questions 1. How would regional outages be distinguished from camera failures? 2. How would you support different heartbeat intervals per camera model?