julian
1efa77bf56
A parking lot is one pool of spaces with a flexible set of entry/exit
points — no "lane". Direction is a property of each RELAY inside an access
controller; readers/cameras bind to a controller relay and inherit it.
Schema:
- drop `lane` from ledger_events, device_events, sessions
- rename lane_devices -> devices (no lane/direction columns)
- access config.relays=[{relay,direction,button?}]; reader/camera
config.controllerId+relay binding
- fresh 0000_baseline migration (history reset; dev data was throwaway)
Signed ledger:
- remove `lane` from canonicalize(); bump signer keyId sw-hmac-v1 -> v2
(v1 events won't verify under v2 — intentional, gated per-event by keyId)
Server:
- new device-resolve.ts (replaces lane-map.ts): relayForButton,
relayForDevice, firstRelayByDirection, devicesByDirection
- entry-flow: button terminal -> its relay; exit/permit: reader's bound
relay; dispatcher resolves the bound relay + inherited direction
- camera snapshots fire by direction site-wide, async, never block open
- DeviceConfig widened to nested JSON for relays[]
Web:
- wizard: no lane selector; add controllers (relay map + entry-button
terminal) first, then bind readers/cameras/printers to a controller relay
Wiki: new entry-exit-points.md (replaces lane-direction); reworked
entry-exit-readers, parking-session, first-run-setup, device-registry,
append-only-event-chain, device-events; removed stale lane/LaneMap mentions.
54 lines
2.4 KiB
Markdown
54 lines
2.4 KiB
Markdown
---
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type: concept
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tags: [parking, printer, device, reliability]
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sources: []
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updated: 2026-06-14
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---
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# Printer roles & failover
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A lane runs **more than one printer**, and the system knows each one's job so it can fail over
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automatically. This is a reliability decision, not a threat-model one: an entry ticket must
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still print when the outside dispenser jams or drops off the network.
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## Roles
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Each printer instance (a `devices` row, category `printer`) declares a **role** in its
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config:
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- **`entry-dispenser`** — outside, at the lane. Prints the entry ticket the driver takes.
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- **`booth-receipt`** — inside the booth. Prints receipts at exit/payment, AND serves as the
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**backup** for entry tickets.
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It also declares a **`failoverRank`** (higher = preferred within a role) to order multiple
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printers of the same role deterministically (ties broken by id).
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## Failover rule (asymmetric, on purpose)
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For an **entry ticket** (`wantRole = entry-dispenser`): try the entry dispensers (best rank
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first), then fall back to the **booth printer**. So a driver still gets a ticket when the
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outside unit is offline — the operator hands it over from the booth.
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The reverse is **deliberately not** done: a **receipt** never prints on the outside dispenser.
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Receipts are a booth-only job; an entry dispenser falling back to print receipts makes no
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physical sense.
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## Where the logic lives
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- The driver (`rongta`) is **role-agnostic** — role/rank are just config; the transport doesn't
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care. Keeps [[device-adapter-pattern|adapters]] swappable.
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- Selection is pure logic in `packages/devices/printer-routing.ts`: `orderForRole()` ranks
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candidates; `printWithFailover()` attempts the print down the list and throws
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`NoPrinterAvailableError` only when every candidate fails.
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- It **attempts the print directly** rather than racing a `healthCheck` first — the print is
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the real reachability test, and a health probe that passes can still be followed by a failed
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print.
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## Open: the all-printers-down policy
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When `printWithFailover` exhausts every candidate, what should entry do — raise the barrier
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with no paper ticket (the plate/[[lpr-camera]] is the independent record), or hold? That policy
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belongs to the **entry flow** ([[device-input-flow]], [[fail-state-safety]]), not the printer
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layer, and is **not yet decided**. The signed event ([[append-only-event-chain]]) is created
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regardless of whether paper prints.
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