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api-docs/adr/ADR-297-multi-node-semantic-correctness.md

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+# ADR-297: Multi-node semantic correctness — per-node inference, node-keyed rate limiting, stale state
+
+- **Status**: Accepted — initial implementation (this PR)
+- **Date**: 2026-08-11
+- **Deciders**: ruv
+- **Tags**: multi-node, mqtt, home-assistant, correctness, sensing-server
+
+## Context
+
+The external review confirmed three defects on the multi-node path — the core
+mechanism RuView uses to reduce blind spots and room dependence:
+
+1. The active `NodeInfo` payload carries RSSI/position/subcarrier/sync but **no
+   per-node classification**; the MQTT mapper reads `node.classification` and
+   falls back to the room aggregate when absent, so every node can publish the
+   same aggregate presence value (issues 1540, 1554).
+2. The MQTT `RateLimiter` is keyed by `EntityKind` only
+   (`mqtt/state.rs:65`), so one node consumes the numeric publish slot and the
+   others are suppressed until the interval expires, while availability still
+   says online (issue 1541).
+3. In the UDP vital path, top-level classification is taken from the
+   latest-arriving node while other features are fused, so with disagreeing
+   nodes room presence can flip at packet frequency (issue 1555).
+
+## Decision
+
+- **Separate the types.** Introduce `NodeInference` (per-node classification +
+  confidence + freshness) distinct from `RoomInference` (the fused room
+  aggregate). `NodeInfo` carries a `NodeInference`; the room aggregate is
+  computed explicitly and never overwrites node state. No silent fallback from
+  node to room.
+- **Key the rate limiter by (node, entity).** `RateLimiter` becomes keyed on
+  `(NodeId, EntityKind)` so nodes no longer starve each other; per-entity
+  behavior per node is preserved.
+- **Deterministic fusion.** Room classification is a pure function of the set
+  of current per-node inferences (e.g. freshness-weighted vote), not
+  last-writer-wins; identical inputs yield identical room state.
+- **Stale entities cannot stay online.** An entity whose backing node has not
+  reported within N expected publish intervals transitions to unavailable/
+  stale rather than holding a frozen value while availability says online.
+
+## Consequences
+
+- Multi-node HA/MQTT output becomes semantically correct; distinct nodes
+  report distinct state and no longer suppress one another.
+- Schema change to `NodeInfo`/the MQTT contract; existing single-node
+  deployments keep working (one node = one inference). Consumers reading the
+  old aggregate-only shape need the migration accessor.
+- Aligns with ADR-295 (freshness) and the review's call for one canonical
+  `NodeInference`/`RoomInference` contract.
+
+## Validation
+
+- Unit/integration tests: per-node classification round-trips through the MQTT
+  mapper with no room fallback; two nodes with different rates both publish
+  (no starvation); disagreeing nodes produce deterministic, non-flapping room
+  state; a silent node's entities go stale, not frozen-online.
+- `cargo test -p wifi-densepose-sensing-server`.