julian
f87e4c0d6b
Generalise printer-only monitoring to every configured device. New DeviceMonitor polls all enabled devices each tick (default 8s): printers via rich readStatus(), relays/readers/cameras via the generic healthCheck() reachability probe, flattened to one traffic-light (ready/degraded/offline) + detail, deduped (emit on change only), fail-toward-offline. - device-status bus event + GET /api/devices/status snapshot. - Pushed over the existing /api/ws (hello carries the initial set; device-status frame per change). - Web: live-store devices map, WS handler, DeviceFooter chip-per-device (role label not vendor; click a degraded/offline chip for an issues panel). Verified roleKind resolution + change-only emit on a fresh DB. Note: the footer's UI surface (api type, router mount, i18n devices) rides in the subsequent subscription commit due to shared-file overlap. Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V
80 lines
4.0 KiB
Markdown
80 lines
4.0 KiB
Markdown
---
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type: concept
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tags: [parking, printer, device, monitoring, reliability]
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sources: []
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updated: 2026-06-14
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---
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# Printer status monitoring
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> **Generalised 2026-06-18:** the booth's all-device status **footer** is a separate, unified
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> monitor across every category (relays/readers/cameras/printers) — see
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> [[device-status-monitoring]]. This page remains the authority for the *printer-specific* rich
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> status (paper/cover/cutter) + its SSE stream; both monitors run.
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The booth must know a printer is in trouble **before** a driver presses the entry button and no
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ticket comes out. So the system polls each printer's live status (paper out, cover open, cutter
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jam, off-line) and pushes changes to the operator UI. A reliability control, like
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[[printer-roles-failover]] — not a threat-model one.
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## Where the status comes from (the safe-decode decision)
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The raw print socket (TCP 9100) is write-only for us — it returns no paper/cover feedback. ESC/POS
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printers expose status via real-time queries (`DLE EOT n`). On the [[rongta-printer]] clone we
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probed, **`DLE EOT` replies do NOT follow the canonical ESC/POS bit layout** (the spec's fixed
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validation bits were wrong, verified on hardware 2026-06-14). Decoding those bits ourselves risked
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a **false-healthy** — reporting "paper OK" when it's empty — which is the dangerous direction for
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an entry lane.
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Instead we scrape the device's **own status web page** (`http://<host>/prn_stat.htm`). The board
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decodes the bits itself into labelled Yes/No rows (Cover Is Open, Cutter Error, Paper End, Paper
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Near End, Printer Off-Line). We trust the device's decode over hand-decoding an undocumented clone.
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This is captured as a device capability: `MonitorableDevice.readStatus(): PrinterStatus` in
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`packages/devices`. The Rongta driver implements it; the monitor is device-agnostic via
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`isMonitorable()`. A future printer with a different status mechanism just implements the same
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interface.
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## Status mapping (fail safe)
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`readStatus()` maps to `ready | degraded | offline`:
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- status page unreachable / times out → **offline** (same signal as a dead printer; never throws),
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- page reachable but a recognised field is missing → **degraded** ("unexpected status page") —
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we do NOT claim "ready" off a page we didn't fully parse,
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- any fault flag true (paper end, cover open, cutter error, off-line) → **degraded** + a detail
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string ("paper out", …),
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- all five clear → **ready**.
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## The monitor (server)
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`PrinterMonitor` (`apps/server/src/printer-monitor.ts`):
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- reloads the monitored set from `devices` each tick (so a newly-assigned printer is picked
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up without a restart), keeping only enabled, monitorable printers;
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- polls every `PRINTER_POLL_MS` (default 5000ms), never overlapping ticks;
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- caches the latest status per device id;
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- emits a `printer-status` event on the device bus **only when status changes** (deduped).
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## API / live UI
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- `GET /api/printers/status` — cached snapshot of all printers (no device round-trip).
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- `GET /api/printers/status/stream` — **Server-Sent Events**: full snapshot on connect, then one
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event per change. The booth SPA subscribes for real-time paper-out / offline indicators.
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- Any authenticated role may read (operational, not a setup action).
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## Verified on hardware (2026-06-14)
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`readStatus()` against 10.0.10.6 → `ready` (all flags false); against an unreachable host →
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`offline` with "status page timeout" (no throw); bus emits on change and suppresses unchanged
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reads. Full repo typechecks.
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## Open / not yet done
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- **Fault-state capture**: we've only observed the all-clear page. The exact label text for an
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active fault (e.g. does "Paper End" flip to "Yes"?) should be confirmed by physically removing
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paper / opening the cover, to be 100% sure the scrape catches it. The parser is built to match
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Yes/No and degrade on anything unexpected, so this is a confidence check, not a blocker.
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- Tying a `degraded`/`offline` entry-dispenser into [[printer-roles-failover]] failover and the
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(not-yet-built) entry flow's all-printers-down policy ([[device-input-flow]]).
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