feat(monitor): adaptive poll rate, unreachable backoff, device-offline alert

Three changes to cut wasted device/cellular load and surface outages:

- Adaptive interval: full-rate (~1.25s) while a browser is subscribed for a
  smooth chart; relaxed cadence (MONITOR_IDLE_POLL_INTERVAL, default 10s) when
  the feed is keepalive-only (alerting). ~8x fewer polls with no viewer ->
  ~8x less cellular traffic on a metered SIM. Note: idle interval also sets the
  alert sampling resolution when nobody is watching.
- Exponential backoff when the device is unreachable (1->2->...->60s cap),
  reset on the first good poll, so a dead/asleep device stops churning
  reconnects (log spam + wasted SYN traffic). Capped at 5s while a browser is
  watching so a recovery still surfaces quickly.
- Device-offline alert: the reachable->unreachable transition raises a
  connectivity AlertEvent (sentinel rule_id=0, metric="connectivity") through
  the existing evaluator/dispatch seam; recovery clears it. Deduped in memory
  and via the DB (so a restart mid-outage doesn't duplicate the event).

MonitorManager.status() now reports reachable + current mode (watched/idle/
backoff) for observability.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/alerts.py

@@ -112,6 +112,7 @@ class AlertEvaluator:
     def __init__(self):
         self._states: Dict[Tuple[str, int], RuleState] = {}
         self._rule_cache: Dict[str, Tuple[float, list]] = {}  # unit_id -> (fetched_at, rules)
+        self._offline_events: Dict[str, int] = {}  # unit_id -> open connectivity AlertEvent id
         logger.info("[ALERT] rule-based evaluator ready")
 
     async def evaluate(self, unit_id: str, snap) -> None:
@@ -235,9 +236,71 @@ class AlertEvaluator:
             f"recovered to {value:.1f} dB (peak {peak:.1f} dB)",
         )
 
+    # -- connectivity (device offline/online) -------------------------------
+    #
+    # Raised by the live monitor when it loses / regains contact with a device.
+    # Persisted as an AlertEvent (sentinel rule_id=0, metric="connectivity") so it
+    # lands in the same events/inbox/ack pipeline as threshold alerts. The in-memory
+    # map dedupes; the DB query also dedupes across a process restart.
+
+    async def device_offline(self, unit_id: str) -> None:
+        if unit_id in self._offline_events:
+            return  # already flagged offline
+        from app.database import SessionLocal
+        from app.models import AlertEvent
+        db = SessionLocal()
+        try:
+            existing = db.query(AlertEvent).filter_by(
+                unit_id=unit_id, metric="connectivity", status="active").first()
+            if existing:  # already open in the DB (e.g. carried across a restart)
+                self._offline_events[unit_id] = existing.id
+                return
+            evt = AlertEvent(
+                rule_id=0, unit_id=unit_id, rule_name="Device unreachable",
+                metric="connectivity", threshold_db=0.0, status="active",
+            )
+            db.add(evt)
+            db.commit()
+            db.refresh(evt)
+            self._offline_events[unit_id] = evt.id
+        except Exception as e:
+            logger.warning(f"[ALERT] failed to record offline for {unit_id}: {e}")
+        finally:
+            db.close()
+        await self._dispatch_raw("OFFLINE", unit_id, "Device unreachable",
+                                 "live monitor lost contact with the device")
+
+    async def device_online(self, unit_id: str) -> None:
+        self._offline_events.pop(unit_id, None)
+        from app.database import SessionLocal
+        from app.models import AlertEvent
+        db = SessionLocal()
+        cleared = 0
+        try:
+            opened = db.query(AlertEvent).filter_by(
+                unit_id=unit_id, metric="connectivity", status="active").all()
+            for evt in opened:
+                evt.clear_at = datetime.utcnow()
+                evt.status = "cleared"
+                cleared += 1
+            if cleared:
+                db.commit()
+        except Exception as e:
+            logger.warning(f"[ALERT] failed to record online for {unit_id}: {e}")
+        finally:
+            db.close()
+        if cleared:  # only announce recovery if it was actually flagged offline
+            await self._dispatch_raw("ONLINE", unit_id, "Device recovered",
+                                     "live monitor regained contact with the device")
+
+    # -- event persistence + dispatch ---------------------------------------
+
     async def _dispatch(self, kind: str, unit_id: str, rule, detail: str) -> None:
+        await self._dispatch_raw(kind, unit_id, rule.name, detail)
+
+    async def _dispatch_raw(self, kind: str, unit_id: str, name: str, detail: str) -> None:
         """POC dispatch: server log. Swap in a Terra-View webhook (email/SMS) here."""
-        logger.warning(f"[ALERT:{kind}] {unit_id} '{rule.name}': {detail}")
+        logger.warning(f"[ALERT:{kind}] {unit_id} '{name}': {detail}")
 
 
 # Module-level singleton (the monitor calls alert_evaluator.evaluate per snapshot)

app/monitor.py

@@ -27,10 +27,27 @@ from app.alerts import alert_evaluator
 
 logger = logging.getLogger(__name__)
 
-# Extra idle between DOD polls. The 1s device rate-limit already paces consecutive
+# Extra idle between DOD polls WHEN A BROWSER IS WATCHING. The 1s device rate-limit
-# DOD? commands, so this just needs to be small — the rate-limit is the real floor.
+# already paces consecutive DOD? commands, so this just needs to be small — the
+# rate-limit is the real floor (~1.25s/poll effective).
 MONITOR_POLL_INTERVAL = float(os.getenv("MONITOR_POLL_INTERVAL", "0.25"))
 
+# Idle cadence when NO browser is subscribed and the feed is only kept alive for
+# alerting. Same data, ~8x fewer polls -> ~8x less cellular traffic on a metered
+# SIM (~1 GB/device/month at full rate -> ~125 MB). NOTE: this also sets the alert
+# sampling resolution when nobody is watching, so keep it <= the smallest alert
+# duration_s you rely on (default 10s comfortably catches a "sustained 30/60s" rule).
+MONITOR_IDLE_POLL_INTERVAL = float(os.getenv("MONITOR_IDLE_POLL_INTERVAL", "10"))
+
+# Exponential backoff once the device is unreachable, so a powered-off / asleep /
+# out-of-signal device stops churning reconnects every cycle (log spam + a trickle
+# of wasted cellular data on failed SYNs). delay = min(BASE * 2**(fails-1), MAX),
+# reset to full-rate on the first good poll. While a browser is actively watching we
+# cap the backoff lower (WATCHED_MAX) so a recovery surfaces quickly for the viewer.
+MONITOR_BACKOFF_BASE_S = float(os.getenv("MONITOR_BACKOFF_BASE_S", "1"))
+MONITOR_BACKOFF_MAX_S = float(os.getenv("MONITOR_BACKOFF_MAX_S", "60"))
+MONITOR_BACKOFF_WATCHED_MAX_S = float(os.getenv("MONITOR_BACKOFF_WATCHED_MAX_S", "5"))
+
 # How often to refresh the run state (Measure?). It changes rarely, so we cache it
 # and skip that second rate-limited command on most polls — roughly halving the
 # per-update latency (~2.5s -> ~1.3s).
@@ -131,6 +148,12 @@ class DeviceMonitor:
             while self._has_demand():
                 snap, mst = await self._poll_once()
                 if snap is not None:
+                    if not self._reachable:
+                        # Recovered from an outage — clear the connectivity alert.
+                        try:
+                            await alert_evaluator.device_online(self.unit_id)
+                        except Exception as e:
+                            logger.warning(f"[MONITOR] {self.unit_id}: online alert failed: {e}")
                     self._consec_fail = 0
                     self._reachable = True
                     payload = _snapshot_payload(snap, self.unit_id, mst)
@@ -143,7 +166,8 @@ class DeviceMonitor:
                         logger.warning(f"[MONITOR] {self.unit_id}: alert eval failed: {e}")
                 else:
                     # Tell clients the device went offline — once, on transition, after a
-                    # few failures so a momentary blip doesn't flap the UI.
+                    # few failures so a momentary blip doesn't flap the UI. Same edge
+                    # raises the device-offline alert.
                     self._consec_fail += 1
                     if self._reachable and self._consec_fail >= 3:
                         self._reachable = False
@@ -153,6 +177,10 @@ class DeviceMonitor:
                             "feed_status": "unreachable",
                         })
                         last_send = loop.time()
+                        try:
+                            await alert_evaluator.device_offline(self.unit_id)
+                        except Exception as e:
+                            logger.warning(f"[MONITOR] {self.unit_id}: offline alert failed: {e}")
 
                 # Heartbeat: during quiet/offline stretches, send a keepalive so an
                 # idle WS isn't dropped by a reverse proxy. Not cached (new subscribers
@@ -166,10 +194,23 @@ class DeviceMonitor:
                     }, cache=False)
                     last_send = loop.time()
 
-                await asyncio.sleep(MONITOR_POLL_INTERVAL)
+                await asyncio.sleep(self._next_delay())
         finally:
             logger.info(f"[MONITOR] {self.unit_id}: feed stopped")
 
+    def _next_delay(self) -> float:
+        """Inter-poll delay: exponential backoff while unreachable, full-rate while a
+        browser is watching, relaxed cadence when the feed is keepalive-only."""
+        if self._consec_fail > 0:
+            shift = min(self._consec_fail - 1, 6)  # cap growth at 2**6 = 64x base
+            delay = min(MONITOR_BACKOFF_BASE_S * (2 ** shift), MONITOR_BACKOFF_MAX_S)
+            if self._subscribers:
+                delay = min(delay, MONITOR_BACKOFF_WATCHED_MAX_S)
+            return delay
+        if self._subscribers:
+            return MONITOR_POLL_INTERVAL       # a browser is watching — smooth chart
+        return MONITOR_IDLE_POLL_INTERVAL      # keepalive-only (alerting) — save data
+
     async def _poll_once(self):
         """One DOD poll: read, persist, return (snapshot, measurement_start_iso)."""
         db = SessionLocal()
@@ -267,6 +308,11 @@ class MonitorManager:
                 "running": m.running,
                 "subscribers": m.subscriber_count(),
                 "keepalive": m._keepalive,
+                "reachable": m._reachable,
+                # what cadence the loop is currently using, for observability
+                "mode": ("backoff" if m._consec_fail > 0
+                         else "watched" if m._subscribers
+                         else "idle"),
             }
             for uid, m in self._monitors.items()
         }