---
type: concept
tags: [parking, domain, business, devices, entry-flow]
sources: []
updated: 2026-06-15
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.
- **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: QR (preferred) — printed by the booth dispenser

- The [[rongta-printer]] prints the ticket id as a **2D barcode (QR)** plus human-readable text and
  entry time. QR over 1D barcode: denser, tolerant of crumpling/partial reads, easy for a cheap
  camera/imager to read.
- **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 **[[gee-qr-er80]]** QR access reader (`-Q-W`: QR scanner, Wiegand/RS-232/RS-485,
    Linux-supported) — the concrete scanner for this path. A serial `ReaderDevice` adapter feeds the
    `read` bus; pending the reader's RS-232 frame/baud (see [[gee-qr-er80]] open questions).

## 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

- QR symbology/error-correction level + what else prints (site name, tariff summary, help number).
- Scanner hardware (imager model; same unit at pay station and exit?).
- Lost/damaged ticket → the lost-ticket path ([[parking-session]], [[tariff]] admin-arbitrary
  amount).