---
type: concept
tags: [parking, domain, business, devices, entry-flow]
sources: []
updated: 2026-06-19
status: open
---

# Ticket Encoding & Scanning

How a transient [[parking-session]]'s **ticket id** is printed, carried by the customer, and read
back at the pay station and exit. This is the **physical backbone of the transient flow** — the
thing that links entry → pay → exit when there's no plate.

## The ticket id is the session key

At entry the system mints a `vehicle_entry` event with a **ticket id** (`identity`) and prints a
ticket the customer keeps. That same id is read back later to find the session. Properties the id
must have:

- **Opaque + unguessable** — a random id (not a sequential count an attacker could iterate to claim
  someone else's cheaper session). Sequential **physical** stock numbering is a separate
  reconciliation aid ([[reconciliation]] pre-numbered stock), not the scan key.
- **All-numeric** (as-built 2026-06-17) — so ANY legacy 1D barcode scanner reads it and an operator
  can hand-key it. Random (not sequential), so "all-numeric" does not weaken the unguessable
  property. Format (**shortened 13→11 on 2026-06-19**): **11 digits = 10 cryptographically-random
  digits + 1 Luhn check digit**. **Length is driven by guess-resistance, not volume** — with 10^10
  valid ids and the Luhn digit rejecting 9/10 malformed guesses, a blind attempt at a currently-OPEN
  ticket lands at ~1-in-10^7 even with thousands parked (the [[threat-model|booth-operator]] threat),
  which is the property that matters; raw count (a billion) was never the constraint. Collisions stay
  negligible at lot scale. `newTicketId()` in `apps/server/src/entry-flow.ts`; validate with
  `validateTicketCode()` — now **length-agnostic** (`\d{10,14}` + Luhn) so legacy 13-digit tickets in
  circulation keep validating. Gate MANUAL entry only — a scanned/looked-up id already in the ledger
  is authoritative regardless of format.
  > **Why 11, not the requested 9 (2026-06-19):** 9 digits (10^8 space) against ~1000 live tickets
  > gives ~1-in-10^5 per blind guess — an operator scripting guesses could find a valid open ticket
  > in minutes. 11 keeps a strong anti-forgery margin AND was the actual fix for a printer overflow
  > (below). The shorter id is *why* the width-3 barcode now fits the 72mm Cashino line.
- **Format is a property of minting, not the schema** — `identity` / `sessions.id` are free-form
  `text`, so changing the id format is a code change with **no migration**. Legacy `T-<uuid>` ids
  (pre-2026-06-17) remain valid keys and coexist with numeric ones.
- **Single logical session** — scanning it at the pay station finds the open session; after payment
  it's the proof-of-paid the exit checks.

## Encoding: Code128 numeric barcode — printed by the booth dispenser

- The [[rongta-printer]] prints the ticket id as a **1D Code128 barcode** (the all-numeric code),
  with the **same code in large human-readable digits below it**, then the entry time. Code128 over
  QR for the primary symbology because the booth's reader hardware is unknown and a legacy 1D laser
  scanner is the lowest common denominator — and the printed digits mean total reader failure still
  leaves a hand-keyable code. A **QR for phone/imager scanning may be added later behind an admin
  toggle** (deferred — see Open).

> **As-built (2026-06-17).** `renderTicket()` in [[rongta-printer]]
> (`packages/devices/src/drivers/printer-rongta.ts`) emits the Code128 via ESC/POS `GS k` (code set
> B) — **rendered by the printer firmware**, so there is no image-rendering step and no new
> dependency (keeps the MIT/Apache/BSD constraint). Resilience rationale: the booth's reader is
> uncertain, so the id is carried in two independently-readable forms (1D barcode / printed digits).
> The "operator scans with a phone" path reuses the
> existing dispatch flow ([[entry-exit-readers]]) and is tracked separately (not yet built).

> **Barcode GEOMETRY must fit the paper width — root cause of a real garbage-print incident
> (2026-06-19).** The Cashino [[rongta-printer|KP-300H]] entry dispenser printed tickets as **raster
> garbage** (solid black bars / banding) while the [[rongta-printer|Rongta]] printed the *identical*
> byte stream fine. Not data corruption: the **Code128 symbol overflowed the print line.** Math: a
> Code128-B symbol is `(11·chars + 35) · moduleWidth` dots. At 203 dpi the KP-300H prints **72mm =
> 512 usable dots** (the Rongta runs 80mm = 576). The old **13-digit** id at `GS w 3` = ~534 dots
> **overran 72mm**, and the firmware rendered the overflow as pixels; the Rongta's 80mm had just
> enough room — which is why only the Cashino failed. The **11-digit** id at width 3 = **~468 dots**,
> fits both widths, and scanned the full value at the exit reader (verified on hardware). Lessons:
> (1) keep `(11·len+35)·moduleWidth` under the **narrowest** deployed printer's usable dots; (2) a
> too-NARROW module (`GS w 2`) is also bad — it scanned but returned **truncated** values (partial
> reads logged as `exit.refused.noSession` anomalies). Width 3 + 11 digits is the verified sweet
> spot. `code128()` in `packages/devices/src/drivers/printer-escpos.ts`.
- **Scan points** (both host-side reads — [[entry-exit-readers]]):
  - **Pay station** — customer scans the ticket → host finds the session → shows fee → takes
    payment ([[tariff]], pay-on-foot) → appends `payment`.
  - **Exit lane** — customer scans the (now paid) ticket → host validates paid + within
    `gracePeriodExit` → `vehicle_exit` → `pulseOpen`.
- The **scanner is a device behind an adapter** ([[device-adapter-pattern]]): a new `ReaderDevice`
  kind (QR/barcode imager) — likely the same `IdentitySource = "ticket"` / `"qr"` path. Keeps the
  app device-agnostic; hardware model is procurement ([[bom]], [[open-questions]]).
  - **On hand:** the **[[dingtian-dt008-reader|Dingtian DT-008]]** QR/RFID reader (Wiegand/TCP-IP/USB/RS485,
    Linux-supported) — the concrete scanner for this path. A serial `ReaderDevice` adapter feeds the
    `read` bus; via HTTP-GET push (see [[dingtian-dt008-reader]]).

## Ticketless alternative (plate as the ticket)

Where the [[opencv-anpr-service|vision service]]/LPR captures the plate, the **plate can be the
session key** instead of a printed ticket — drive in, plate read, drive to pay station and enter
plate (or it's looked up), pay, exit by plate. No paper. The two can coexist per lane
([[entry-exit-readers]] "both share a relay"); a printed QR ticket is the fallback when a plate
isn't captured or is low-confidence (recognition is advisory — [[opencv-anpr-service]]).

## Open

- Primary symbology **decided**: Code128 set B over the all-numeric id (as-built above). Still open:
  what *else* prints (site name, tariff summary, help number).
- **Optional QR (deferred)** — an admin toggle to ALSO print a QR for phone/imager users. The
  `code128()`/`qrCode()` ESC/POS helpers were prototyped 2026-06-16; QR was dropped 2026-06-17 in
  favor of "1D barcode + hand-keyable numeric code" because the booth's reader hardware is unknown.
  Revisit when mobile scanning is wanted.
- **Phone-scan fallback** (operator scans a ticket with a phone when a reader is down) — designed
  but not built: an authenticated route feeding the same dispatcher + a minimal mobile scan UI.
- Scanner hardware (imager model; same unit at pay station and exit?).
- Lost/damaged ticket → the lost-ticket path ([[parking-session]], [[tariff]] admin-arbitrary
  amount).