ADR-0007: Standalone battery sensors for offline flood detection

Status

Accepted — 2026-03-31

Context

The OT sensor pipeline (ESP32 → MQTT → Telegraf → InfluxDB → Grafana → SentinelBot Telegram) provides flood alerts when fully operational. However, the highest-risk scenario is a power outage — the exact moment the full stack may be offline, and the sump pump is most likely to fail.

A Pi-based alerting solution was initially proposed. The flaw: no internet during a power outage means no Telegram alerts, which is exactly when flood detection matters most.

Decision

Standalone battery-powered water sensors (e.g., Basement Watchdog) are the primary offline flood detection mechanism. Independent of internet, power grid, and homelab stack.

The ESP32 OT pipeline remains as the telemetry and monitoring layer — historical data, dashboards, online alerts. It does not replace purpose-built offline sensors.

Battery backup sump pump is the recommended complement — addresses root cause, not just alerting.

Alternatives Considered

Pi-based alerting only — Fails during power outages. Internet-dependent alerts are unreliable for the primary risk scenario.

ESP32 + local buzzer — Viable for local audible alert but adds firmware complexity and doesn't solve remote notification.

Consequences

• Two detection layers: standalone hardware (offline) + OT pipeline (online)
• ESP32 pipeline scope remains: telemetry, monitoring, dashboards, online alerts only

References

• ADR-0006 (power outage strategy)