Health Check / Heartbeat Service — Simple

Problem statement (interviewer prompt)

Design a health-check / heartbeat service used by thousands of microservices for liveness / readiness probes + dashboards. Aggregate at 1Hz, detect failures within seconds, distinguish flap vs hard-down, and feed into the LB / orchestrator's removal decisions.

flowchart LR
  N[Nodes]
  HB[Heartbeat collector]
  STATE[(Health state)]
  ALERT[Alerts]
  N -. heartbeat .-> HB --> STATE --> ALERT

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class N,HB service;
    class STATE datastore;
    class ALERT obs;