# IoT — Derived Rules

> Derives from `domains/edge/first-principles.md`. Applies P3
> (Resources are Constrained and Declared) primarily, with P5
> (command idempotency), P7 (partial degradation when devices drop),
> P9 (device identity and provisioning), and P10 (telemetry from
> devices). Cross-links `domains/security/secrets` for device
> credentials and `domains/messaging/queues` for the MQTT QoS
> parallels to delivery semantics.

## What IoT at the Edge Is (P3 Resources are Constrained and Declared)

- IoT at the edge is the engineering discipline of operating
  constrained devices — sensors, actuators, gateways, microcontrollers
  — as first-class participants in a distributed system. The
  distinguishing constraint is per-device resource bounds (P3): a
  battery-powered sensor has kilobytes of RAM, a constrained
  protocol, and a multi-year sleep budget. These constraints are
  declared per device class, never assumed infinite.
- The boundary is per D-061: edge owns the constrained-device
  reality; performance owns the generic unbounded-growth-is-a-bug
  principle (`performance/P4 Resource Bounds`); concurrency owns
  in-process primitives. IoT is an edge concern because its defining
  traits are constrained resources (P3), geographic distribution as
  a fleet (P8), partition-prone operation (P2), and device-scoped
  identity (P9) — concerns that only arise at the network edge.
- See `domains/edge/offline-first.md` for the partition-survival
  discipline that constrained devices depend on, and
  `domains/edge/sync.md` for the reconciliation of device state
  across partitions.

## Device Resource Classes (P3, C8 Economy)

- A device resource class declares the bounds for a class of
  devices: CPU (MHz, cores), memory (KB/MB), power (battery mAh,
  duty-cycle budget), bandwidth (bytes/sec, latency budget), and
  storage (KB/MB). Every device in the fleet is assigned to a class;
  every operation is budgeted against its class.
- An undeclared budget is a defect (P3 violation): a sensor that
  sends telemetry every second without a duty-cycle budget exhausts
  its battery in days, not years. The budget is the correctness
  bound (C1) and the economy bound (C8).
- A device class implies a protocol choice: a class-0 device
  (constrained sensor, KB RAM) speaks CoAP; a class-1 device
  (gateway, MB RAM) speaks MQTT; a class-2 device (edge compute
  node, GB RAM) speaks HTTP. The protocol follows the constraint,
  not the reverse.

| Class | RAM | Power | Protocol | Typical role |
|-------|-----|-------|----------|--------------|
| 0 (constrained sensor) | < 10 KB | Battery, multi-year | CoAP, LoRaWAN | Telemetry only, no inbound commands |
| 1 (actuator, gateway) | 10 KB – 1 MB | Battery or wired, weeks-months | MQTT, CoAP | Telemetry + commands, queue-and-forward |
| 2 (edge compute) | > 1 MB | Wired, continuous | HTTP, MQTT | Local aggregation, gateway, edge inference |

## Constrained Protocols — MQTT and CoAP (P3, P5)

- **MQTT** is the canonical pub/sub protocol for constrained devices.
  It is lightweight (2-byte header), broker-backed, and provides QoS
  levels (0, 1, 2) that map to delivery semantics. MQTT is the
  cross-process analog of message-queue delivery — see
  `domains/messaging/queues` for the general queue/delivery-semantics
  discipline; the cross-link is one-directional outward (edge →
  messaging) per D-062 and D-026 extended.
- **CoAP** is the REST analog for constrained devices: UDP-based,
  low-overhead, with confirmable (CON) and non-confirmable (NON)
  message types. CoAP fits class-0 devices where TCP is too heavy.
- A blocking synchronous call on a constrained device with no
  timeout is the `blocking-call-on-constrained-device` chaos
  anti-pattern: it blocks the node, has no timeout (= hang), and
  retries are unsafe without idempotency (P3 + P5 breach). Every
  device operation must be async with a timeout, and every retried
  operation must be idempotent.

```json
// MQTT publish/subscribe payload with QoS levels (P5 idempotency,
// P3 constrained protocol).
// QoS 0 — at-most-once: fire-and-forget, no ack. For telemetry
// where a dropped sample is acceptable (P3 economy of the
// constrained link).
{
  "topic": "devices/sensor-7/temperature",
  "qos": 0,
  "payload": {
    "device": "sensor-7",
    "ts": 1700000000,
    "value": 21.4,
    "unit": "C"
  }
}

// QoS 1 — at-least-once: acked, may duplicate. The consumer must
// be idempotent (P5) — dedup by (device, ts) or an idempotency key.
{
  "topic": "devices/actuator-3/command",
  "qos": 1,
  "payload": {
    "device": "actuator-3",
    "idempotencyKey": "cmd-1700000000-1",
    "command": "set-point",
    "value": 22.0
  }
}

// QoS 2 — exactly-once: four-step handshake, no duplication. The
// heaviest QoS; use only where duplicates are intolerable AND the
// device has the budget for the handshake (class-1+ only, P3).
{
  "topic": "devices/actuator-3/irreversible-command",
  "qos": 2,
  "payload": {
    "device": "actuator-3",
    "idempotencyKey": "cmd-1700000000-2",
    "command": "calibrate"
  }
}
```

- The QoS choice is a P5 (idempotency) and P3 (resource) decision:
  QoS 0 is cheapest (no ack) but lossy; QoS 1 requires consumer
  idempotency (P5); QoS 2 is exactly-once but costs a four-step
  handshake on a constrained link. The default for telemetry is QoS
  0 or 1; the default for commands is QoS 1 with an idempotency key
  (P5); QoS 2 is reserved for irreversible commands where the
  device budget permits.

## Device Identity and Provisioning (P9 Identity is Constrained at the Edge)

- Every device holds a per-device identity: a unique device ID and a
  scoped credential (X.509 certificate, API token, or rotating
  key). No shared fleet credential — one compromise must not equal
  a fleet compromise (P9). The credential scope is minimal: a
  device can publish to `devices/<its-id>/+` and subscribe to
  `devices/<its-id>/commands`, nothing else.
- Provisioning is the act of assigning a device identity at
  enrollment time. The provisioning manifest declares the device,
  its class, its allowed topics, and its credential. The manifest is
  the P9 contract — a device operating outside its manifest scope is
  a violation.
- A device that is provisioned with a shared fleet key (the
  `shared-edge-device-credential` anti-pattern) is a P9 violation:
  blast radius is unbounded. See `domains/security/secrets` for the
  general secret-hygiene discipline (per-identity credentials,
  rotation, minimal scope) that device provisioning builds on.

```yaml
# Device provisioning manifest (P9 per-device identity + scoped
# credentials). The manifest is the contract; the device operates
# only within its declared scope.
device:
  id: sensor-7
  class: 0                    # P3 resource class
  model: temp-sensor-v2
  firmware: 1.4.2
identity:
  cert: "sha256-of-device-cert"
  credentialScope:
    publish:
      - "devices/sensor-7/temperature"
      - "devices/sensor-7/status"
    subscribe:
      - "devices/sensor-7/commands"
    # No wildcard, no fleet-wide topics (P9).
provisioning:
  enrolledAt: 2024-01-15T00:00:00Z
  rotatesEvery: 90d
  # Per-device credential; never shared (P9, domains/security/secrets).
```

## Telemetry from Devices (P10 Edge Observability Survives Partition)

- Device telemetry is local-first (P10): the device buffers telemetry
  on-node and forwards on reconnect. A fire-and-forget telemetry
  pipeline loses data when the link drops; a buffered pipeline
  survives. The buffer is bounded by the device class (P3): a
  class-0 sensor buffers minutes of telemetry, not hours.
- Telemetry is observable in aggregate: the operator sees the fleet's
  behavior, not just per-device. A device that has not reported in
  its expected interval is itself a signal (a dead device, a
  partitioned device, a drained battery). See
  `domains/observability/metrics` for the generic structured-metrics
  discipline; edge owns the partition-survivable, local-first angle.
- Telemetry must not be a secrets channel (P9 analog, see
  `domains/observability/P6 No Secrets in Observability`): device
  credentials, PII, and personally-identifying location must not
  enter telemetry payloads.

## Command Idempotency (P5 Edge Operations are Idempotent)

- Device commands are retried by nature (the network is
  partition-prone). Every command carries an idempotency key so a
  retried command does not double-apply (P5). A `set-point` command
  retried with the same idempotency key sets the point once, not
  twice; an `open-valve` command retried is safe because the valve
  is already open.
- Irreversible commands (a calibration burn-in, a firmware flash)
  require stronger idempotency: the device tracks applied
  idempotency keys and refuses re-application. A retried irreversible
  command without idempotency tracking double-applies the effect
  (P5 violation, possibly a physical-side-effect bug).
- The idempotency key is per-command, not per-device. A device that
  dedups by device ID alone will drop distinct commands issued in
  the same window. Use `(device, command-id, ts-window)` or a
  UUID per command.

## Partial Degradation When Devices Drop (P7 Partial Degradation is Engineered)

- A fleet degrades when devices drop (battery exhaustion, partition,
  hardware failure). The system must continue to operate with the
  remaining devices; a whole-system crash on one device's failure is
  a P7 violation. The degraded mode is documented: which
  aggregations are valid with N-1 devices, which alerts fire, which
  fallbacks engage.
- A device that drops is not an incident by itself — fleets expect
  churn. The operator-facing signal is the *aggregate* health (X%
  of devices reporting, Y% partitioned for >Z minutes), not the
  per-device drop. Per-device drop alerts are noise; aggregate
  degradation alerts are signal (see `domains/observability/metrics`).
- A command to a dropped device must time out (P5 — idempotent
  retry) and degrade (P7 — the fleet continues without that
  device). A command that blocks forever waiting for a dropped
  device is the `blocking-call-on-constrained-device` chaos
  anti-pattern (P3 + P5 breach).

## Cross-Link to Messaging (P5, cross-link messaging/queues)

- MQTT QoS 0/1/2 maps to at-most-once / at-least-once / exactly-once
  delivery semantics — the same three-way tradeoff documented in
  `domains/messaging/queues`. The cross-link is one-directional
  outward (edge → messaging) per D-026 extended: edge owns the
  constrained-device protocol angle; messaging owns the generic
  cross-process delivery-semantics angle.
- This link dangles until P2 (the messaging domain is authored in
  P2); it is verified bidirectional in P5 (ATELIER-114 per
  IDEATE-40). Acceptable per D-053 (vertical-slice integrity — P1
  ships the edge domain self-consistent; the messaging cross-link
  resolves by the P6 ship).
- The parallel: a constrained device's QoS 1 publish is the
  device-flavored instance of an at-least-once queue delivery — the
  consumer (the broker or the downstream service) must be
  idempotent (P5 here, `messaging/P3 Consumers are Idempotent`
  there). The idempotency discipline is the same; the protocol and
  failure model differ (constrained-device link vs broker-backed
  network).

## What Violates IoT-at-the-Edge Discipline

| Violation | Principle |
|-----------|-----------|
| Undeclared device resource budget (assumes infinite battery/RAM) | P3 Resources are Constrained and Declared |
| Shared fleet credential (one key for all devices) | P9 Identity is Constrained at the Edge |
| Non-idempotent device command (retried command doubles the effect) | P5 Edge Operations are Idempotent |
| Blocking synchronous call on a constrained device with no timeout | P3, P5 (blocks the node; retry unsafe) |
| Fire-and-forget telemetry with no on-device buffer (lost on partition) | P10 Edge Observability Survives Partition |
| Whole-system crash on one device's failure (no degradation contract) | P7 Partial Degradation is Engineered |
| Device credential scope that includes fleet-wide topics (over-scoped) | P9, `domains/security/secrets` |
| QoS 2 used on a class-0 device (no budget for the handshake) | P3 Resources are Constrained and Declared |
| Per-device-drop alert (noise; aggregate degradation is the signal) | P7, `domains/observability/metrics` |
| Telemetry payload that includes device credentials or PII | P9, `domains/observability/P6 No Secrets in Observability` |