parking_solution/wiki/concepts/booth-exit-flow.md

---
type: concept
tags: [parking, domain, booth, exit, payment, threat-model]
sources: []
updated: 2026-06-18
status: open
---

# Booth Exit Flow — pay-at-booth, voucher vs. immediate exit

How the **manned booth** takes payment for a transient ticket and lets the car out. Complements the
unattended reader path in [[parking-session]] / the exit flow: same signed events, a booth-driven
trigger. Decided 2026-06-17.

## Operator flow

1. **Ticket input** on the booth screen. The operator scans (HID scanner types the id + Enter) or
   keys the ticket number.
2. On submit, the booth **looks up the session** and opens a **modal**: entry time, exit time (now),
   **duration**, **total owed** (the [[tariff]] quote), tender (cash/card), and a checkbox
   **"Printo biletë dalje"** (print exit ticket).
3. The operator takes payment → a signed `payment` event ([[parking-session]]). What happens next
   depends on the checkbox:
   - **Checked → print an exit voucher.** The customer carries it to a (distant) exit and
     **self-exits by scanning it** there; that scan runs the normal reader exit flow. The booth does
     NOT open the barrier.
   - **Unchecked → immediate exit.** When the modal closes after a successful payment, the booth
     **signs `vehicle_exit`, pulses the exit relay, and fires the exit snapshot** right away (booth is
     at/near the exit).

## Settled decisions (2026-06-17)

- **Voucher carries the SAME ticket id** (reprinted as the Code128 barcode). At the exit reader it
  runs the existing exit validation — which now finds the session **paid + within walk-back grace**,
  so it opens. No new identity or code type; the "biletë dalje" is a *paid reprint* of the entry
  ticket id. Reuses [[tariff|walk-back grace]] exactly.
- **The checkbox default lives in `site_config`** (`exit_voucher_default`, a site-wide boolean edited
  in Site settings) — because it's booth geography, not per-ticket. The operator may override per
  transaction. (Per-exit-point config deferred until a site has both a near and a far exit.)
- **Payment is never rolled back.** If the checkbox is OFF and `pulseOpen` fails (offline
  controller), the signed `payment` + `vehicle_exit` already stand (money was taken, the car is
  owed an exit). The booth surfaces a clear error and an **audited `anomaly`** so the operator opens
  manually — we never silently drop the payment, and never leave a paid car without an exit event.

## Threat-model notes ([[threat-model|operator as adversary]])

- The booth exit reuses the **same validation as the reader path** (paid + within grace, or free
  entry-grace) — there is no booth-only bypass that admits an unpaid car. An unpaid ticket sends the
  operator to take payment first.
- Every booth action is a **signed ledger event attributed to the operator's session**: the payment,
  the exit, and any `anomaly` (failed open / override). A colluding operator can't wave a car out
  without leaving a signed, attributed trail visible to [[reconciliation]].
- The voucher path keeps the **camera snapshot at the physical exit** (the self-scan fires it), so
  the evidence is captured where the car actually leaves, not where it paid.

## Active sessions & human-intervention barrier open

**The barrier state is ASSUMED, never confirmed.** We send "open" intent and never truly know the car
cleared ([[barrier-not-a-door]], no wired loop/sensor feedback). So a signed `vehicle_exit` does NOT
mean the car is gone — it may be stuck (damaged ticket / dead scanner, or the barrier re-closed on a
phantom obstacle: an animal, a person, a cardboard box or bag in the wind). These edge cases need a
**human in the booth** to open the barrier, leaving a signed trace.

**A session is "active" (shown in the booth Active Sessions list) while it is EITHER:**
- **open + unpaid** — entered, no `vehicle_exit`, owing money. **Always shown** — a car that owes
  money never ages out; it's genuinely still inside until it pays, however long that takes. OR
- **open + paid, still within grace** — paid but no exit recorded yet, `now ≤ graceExpiresAt`. OR
- **exited but `now ≤ graceExpiresAt`** — paid and/or the voucher scanned, but still within the
  walk-back grace window. Because the barrier is unconfirmed, the car is presumed *possibly still
  present* until grace expires. **Payment and a successful voucher scan do NOT remove it from the
  list** — only grace expiry does.

A session drops off the list once it is **past grace** and EITHER exited OR **paid** (presumed truly
gone). The **paid age-out** is important: a paid session whose walk-back grace lapsed has left, so it
is omitted **even if no `vehicle_exit` was ever signed**. Without this, a paid car that left via a
manual barrier re-open (which historically signed no exit — see below) would linger **forever**
(ticket T-397815c0, 2026-06-18). The signed log is untouched — this is purely the list's display
filter (`PayStation.activeSessions()`).

### The one operator action — "Open barrier" (audited re-pulse)

For an active session, the operator can open the barrier as a **human intervention**. This:
- **re-pulses an exit relay** (resolved site-wide, as the booth exit does), and
- signs an **`anomaly`** (`source: booth`, attributed to the operator, reason "manual barrier open"), and
- **closes the session IF it is still open** — i.e. if no `vehicle_exit` exists yet, the re-open *is*
  this car leaving, so it also signs a **`vehicle_exit`** (`source: manual`, reason "human-intervention
  exit"). If the session is **already exited** (the phantom re-close case — a second exit would
  double-count occupancy), it signs **no** second exit: anomaly only.

> **Refined 2026-06-18 (was "NEVER a `vehicle_exit`").** The original rule never signed an exit on a
> re-open, on the assumption a normal `vehicle_exit` had already happened. But when the re-open was the
> *only* way a car left (its walk-back grace had expired, so a normal exit was refused), the session
> kept **no exit event** and lingered as "open" forever (ticket T-397815c0). Fix: sign the exit only
> when the session is **still open**, preserving the no-double-count guarantee for the already-exited
> case. The [[#a-session-is-active|paid age-out]] above is the belt-and-braces safety net for any
> paid session that still slips through.

**Guard — no payment, no button.** The "Open barrier" action is shown/active **only for sessions that
have a payment** (paid, or paid-and-exited-in-grace). An **unpaid** open session has **no barrier-open
affordance at all** — the row routes to the [[#operator-flow|pay/exit modal]] instead. The
no-unpaid-bypass rule is enforced structurally: the button simply does not exist for an unpaid car.
(A future reason-required *force exit* for genuine disputes would be a separately-audited path — see
Open.)

This single mechanism covers both edge cases: a **damaged ticket / dead scanner** (find the still-open
session in the list → pay/exit modal, or if already paid → Open barrier, no scan needed), and a
**phantom-obstacle re-close** (the just-exited car is still in the list within grace → Open barrier).

## ⚠ Open question — walk-back grace renews on every payment (voucher overstay)

**Found 2026-06-17. Not yet fixed.** Scenario: customer pays at the booth, takes an exit voucher,
then dawdles past the walk-back grace before reaching the exit.

What the code does today (`exit-flow.ts`, `pay-station.ts`):
- The exit reader's grace check is `now − paidAt ≤ graceExitMin`, reading **the latest payment's**
  `graceExitMin`. Over the window → exit **refuses** ("top-up required"). ✓ *Correct — no free exit.*
- The re-quote (`computeFee(enteredAt, now, …)`) always prices from **entry**, never from the last
  payment. So a top-up charges the **full** entry→now fee (minus what's paid is implicit via the
  ledger). ✓ *Correct — the timer does NOT restart; the customer pays the true total.*
- BUT every `payment` writes its own `graceExitMin`, and the exit flow reads the **latest** one — so
  **each top-up grants a fresh, full grace window.** ✗ *This is the bug.*

**The leak is time, not money.** It is not a free-exit hole (the fee always catches up from entry).
But the grace window — meant as a one-time walk-from-pay-to-gate allowance — is re-granted in full on
every payment, so a customer could pay → wait → pay a tiny delta → get another full window → repeat,
riding the gap between "paid" and "next increment accrues." With coarse [[tariff]] increments the
abuse is bounded but real.

**Candidate fixes (business call — fairness vs. anti-abuse):**
1. **Grace on top-up only when the top-up charged new money** (recommended). Kills the "tiny delta
   forever" loop while staying fair to a genuine overstay; re-price stays from entry.
2. **Single non-renewing window** anchored to the FIRST payment — cleanest anti-abuse, but can unfairly
   trap someone who legitimately paid, walked, then hit a slow elevator after a top-up.
3. **Cap total grace** granted per session regardless of payment count.

Decided halves: **refuse-on-expiry** and **reprice-from-entry** are deliberate and correct. The
**grace-renews-fully-per-payment** consequence was an unintended side effect of reading `graceExitMin`
off the latest payment. See [[tariff]] (walk-back grace) for the pricing side of the same question.

## As-built / open
- Backend: `GET /api/session/:identity` (lookup + quote), `POST /api/exit { identity }` (validated
  booth exit), `site_config.exit_voucher_default`. Exit validation shared between the booth and the
  reader path (one code path, two triggers).
- **Open: walk-back grace renews on every payment** — see the flagged section above (voucher overstay
  re-grants a full grace window; pick a fix before production).
- Voucher print = reprint the ticket id barcode on the booth printer ([[ticket-encoding]]).
- Open: a force-open **override** (lost ticket / equipment fault) — deferred; would be a separately
  audited signed event, not folded into the validated path.