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

type, tags, sources, updated

type	tags	sources	updated
concept	parking printer device monitoring reliability		2026-06-14

Printer status monitoring

The booth must know a printer is in trouble before a driver presses the entry button and no ticket comes out. So the system polls each printer's live status (paper out, cover open, cutter jam, off-line) and pushes changes to the operator UI. A reliability control, like printer-roles-failover — not a threat-model one.

Where the status comes from (the safe-decode decision)

The raw print socket (TCP 9100) is write-only for us — it returns no paper/cover feedback. ESC/POS printers expose status via real-time queries (DLE EOT n). On the rongta-printer clone we probed, DLE EOT replies do NOT follow the canonical ESC/POS bit layout (the spec's fixed validation bits were wrong, verified on hardware 2026-06-14). Decoding those bits ourselves risked a false-healthy — reporting "paper OK" when it's empty — which is the dangerous direction for an entry lane.

Instead we scrape the device's own status web page (http://<host>/prn_stat.htm). The board decodes the bits itself into labelled Yes/No rows (Cover Is Open, Cutter Error, Paper End, Paper Near End, Printer Off-Line). We trust the device's decode over hand-decoding an undocumented clone.

This is captured as a device capability: MonitorableDevice.readStatus(): PrinterStatus in packages/devices. The Rongta driver implements it; the monitor is device-agnostic via isMonitorable(). A future printer with a different status mechanism just implements the same interface.

Status mapping (fail safe)

readStatus() maps to ready | degraded | offline:

• status page unreachable / times out → offline (same signal as a dead printer; never throws),
• page reachable but a recognised field is missing → degraded ("unexpected status page") — we do NOT claim "ready" off a page we didn't fully parse,
• any fault flag true (paper end, cover open, cutter error, off-line) → degraded + a detail string ("paper out", …),
• all five clear → ready.

The monitor (server)

PrinterMonitor (apps/server/src/printer-monitor.ts):

• reloads the monitored set from lane_devices each tick (so a newly-assigned printer is picked up without a restart), keeping only enabled, monitorable printers;
• polls every PRINTER_POLL_MS (default 5000ms), never overlapping ticks;
• caches the latest status per device id;
• emits a printer-status event on the device bus only when status changes (deduped).

API / live UI

• GET /api/printers/status — cached snapshot of all printers (no device round-trip).
• GET /api/printers/status/stream — Server-Sent Events: full snapshot on connect, then one event per change. The booth SPA subscribes for real-time paper-out / offline indicators.
• Any authenticated role may read (operational, not a setup action).

Verified on hardware (2026-06-14)

readStatus() against 10.0.10.6 → ready (all flags false); against an unreachable host → offline with "status page timeout" (no throw); bus emits on change and suppresses unchanged reads. Full repo typechecks.

Open / not yet done

• Fault-state capture: we've only observed the all-clear page. The exact label text for an active fault (e.g. does "Paper End" flip to "Yes"?) should be confirmed by physically removing paper / opening the cover, to be 100% sure the scrape catches it. The parser is built to match Yes/No and degrade on anything unexpected, so this is a confidence check, not a blocker.
• Tying a degraded/offline entry-dispenser into printer-roles-failover failover and the (not-yet-built) entry flow's all-printers-down policy (device-input-flow).