parking_solution/wiki/concepts/device-status-monitoring.md

---
type: concept
tags: [parking, device, monitoring, reliability, ui]
sources: []
updated: 2026-06-18
status: open
---

# Device status monitoring (the booth footer)

The booth shows a **fixed footer** with the live status of every configured device — relays,
readers, cameras, printers — so an operator sees at a glance that the barrier relay is reachable,
the exit scanner is up, and the ticket printer has paper. This generalises the printer-only
[[printer-status-monitoring]] to **all four [[device-adapter-pattern|device categories]]**. A
reliability control, not a threat-model one. (Built 2026-06-18.)

## What gets polled, and how

Every **enabled** row in `devices` is polled on an interval, regardless of category — the monitor
talks only to the adapter interfaces ([[device-adapter-pattern]]), never a driver SDK:

- **Printers** → their rich `MonitorableDevice.readStatus()` (paper end / near-end, cover open,
  cutter error, off-line) — the same capability the existing [[printer-status-monitoring|PrinterMonitor]]
  uses. The footer surfaces the fault detail.
- **Relays / readers / cameras** → the generic `Device.healthCheck()` **reachability** probe every
  adapter implements (`ready | degraded | offline`). This is presence/up-ness, not a deep fault
  model — a relay either answers or it doesn't.

Both collapse to one **traffic-light**: `ready | degraded | offline`, plus a `detail` string. Fail
**toward "there's a problem"**, never false-healthy: a probe that throws or times out reads
`offline` (consistent with [[printer-status-monitoring]]'s fail-safe mapping); a driver that's no
longer registered reads `offline` ("driver not registered") rather than vanishing.

## The monitor (server)

`DeviceMonitor` (`apps/server/src/device-monitor.ts`), modelled on the PrinterMonitor:

- re-reads the device set each tick (a newly-assigned/removed device appears/disappears without a
  restart); drops cached status for devices that are gone or disabled;
- polls every `DEVICE_POLL_MS` (default **8000ms**), never overlapping ticks;
- caches the latest unified status per device id;
- emits a `device-status` bus event **only when a device's state or detail changes** (deduped).

> **Relationship to the PrinterMonitor.** Both run. The PrinterMonitor stays the authority for the
> printer-specific live detail + its SSE stream (`/api/printers/status*`) that the entry flow may
> later depend on for [[printer-roles-failover]]. The DeviceMonitor is the **unified footer feed**
> across all categories. They poll independently (printers get probed by both — cheap HTTP reads);
> the small duplication is deliberate, to avoid coupling the footer to printer internals. Could be
> consolidated later if the overlap ever matters.

## API / live UI

- `GET /api/devices/status` — cached snapshot of all devices (no device round-trip). Any
  authenticated role (operational, not a setup action).
- Live updates ride the **one booth [[booth-console|WebSocket]]** (`/api/ws`): the `hello` frame
  carries the initial device-status set; a `device-status` frame is pushed per change. The web
  [[booth-console|live store]] holds the set keyed by device id; the REST snapshot seeds it / fills
  in if the socket is briefly down.
- **`DeviceFooter`** (`apps/web/src/ui/DeviceFooter.tsx`) renders one **compact** chip per device —
  a coloured dot + a **role label, never the vendor** — ordered access → reader → camera → printer,
  with a right-aligned roll-up ("N with issues" / "all ready"). Mounted in the app shell so it's
  visible on every screen.

### Label = role, not vendor (refinement 2026-06-18)

The chip shows **what the device does, not who made it**: the localised category + a role/direction
suffix → `Lexuesi hyrje`, `Printer kabina`, `Kamera dalje`. The server sends a structured
**`roleKind`** token (not a composed string), the client localises it:
- **reader / camera** → the direction inherited from its bound relay (`directionOf()` in
  [[entry-exit-points|device-resolve]]): `entry | exit | both`.
- **access controller** → `entry | exit | both` from its `relays[]`, or **`mixed`** when it spans
  more than one direction; `null` if it declares none yet.
- **printer** → `lane` (entry-dispenser) | `booth` (booth-receipt) — the [[printer-roles-failover]] role.
- `null` → the chip shows the category alone.

### Detail does NOT pollute the footer (refinement 2026-06-18)

Chips stay short — **no inline fault text**. A device that is `degraded`/`offline` is clickable (so
is the roll-up); clicking opens a small **issues panel** anchored above the footer that lists only
the problem devices with their role label, state, the `detail` string, and the last-checked time.
`ready` chips are non-interactive. The panel closes on outside-click / Escape (a lightweight
popover — no extra dependency; only Radix Dialog is installed).

## Verified (2026-06-18)

On a fresh DB seeded with a stub relay, a TCP reader, and two printers (one reachable, one not):
relay + reader → `ready` via `healthCheck`; the unreachable printer → `offline` (with a detail
string, never threw); the bus emitted once per device on first observation, and a second unchanged
tick was silent (change-only emit). Server + web build clean.

## Open / not yet done

- **Reachability ≠ correctness.** `healthCheck()` says a relay/reader answers, not that it's wired
  to the right barrier or reading cards — that's a setup/precondition concern ([[first-run-setup]],
  the Dingtian [[access-controller-button-flow|precondition checks]]).
- **No per-device history / alerting.** The footer is point-in-time; a flapping device isn't
  tracked over time. Reconciliation-style alerting is out of scope here.
- **Cameras** only expose `healthCheck` reachability today; a "last snapshot age" health signal
  could be richer ([[lpr-camera]], [[opencv-anpr-service]]).
- Possible later **consolidation** of PrinterMonitor + DeviceMonitor (see the note above).