feat(booth): active sessions panel + audited barrier re-open
Active Sessions panel lists sessions that are open OR exited-but-within-grace (barrier state is unconfirmed, so a paid car is presumed possibly-present until grace expires). Row click → pay/exit modal; 'Open barrier' (paid sessions only — no payment, no button) fires a human-intervention re-pulse signed as an attributed anomaly, never a second vehicle_exit. Wiki: booth-exit-flow.md. Note: the backend (PayStation.activeSessions, ExitFlow.reopenBarrier, routes, api.ts) landed with the prior commit's shared files.
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---
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type: concept
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tags: [parking, domain, booth, exit, payment, threat-model]
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sources: []
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updated: 2026-06-17
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status: open
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---
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# Booth Exit Flow — pay-at-booth, voucher vs. immediate exit
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How the **manned booth** takes payment for a transient ticket and lets the car out. Complements the
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unattended reader path in [[parking-session]] / the exit flow: same signed events, a booth-driven
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trigger. Decided 2026-06-17.
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## Operator flow
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1. **Ticket input** on the booth screen. The operator scans (HID scanner types the id + Enter) or
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keys the ticket number.
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2. On submit, the booth **looks up the session** and opens a **modal**: entry time, exit time (now),
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**duration**, **total owed** (the [[tariff]] quote), tender (cash/card), and a checkbox
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**"Printo biletë dalje"** (print exit ticket).
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3. The operator takes payment → a signed `payment` event ([[parking-session]]). What happens next
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depends on the checkbox:
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- **Checked → print an exit voucher.** The customer carries it to a (distant) exit and
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**self-exits by scanning it** there; that scan runs the normal reader exit flow. The booth does
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NOT open the barrier.
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- **Unchecked → immediate exit.** When the modal closes after a successful payment, the booth
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**signs `vehicle_exit`, pulses the exit relay, and fires the exit snapshot** right away (booth is
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at/near the exit).
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## Settled decisions (2026-06-17)
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- **Voucher carries the SAME ticket id** (reprinted as the Code128 barcode). At the exit reader it
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runs the existing exit validation — which now finds the session **paid + within walk-back grace**,
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so it opens. No new identity or code type; the "biletë dalje" is a *paid reprint* of the entry
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ticket id. Reuses [[tariff|walk-back grace]] exactly.
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- **The checkbox default lives in `site_config`** (`exit_voucher_default`, a site-wide boolean edited
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in Site settings) — because it's booth geography, not per-ticket. The operator may override per
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transaction. (Per-exit-point config deferred until a site has both a near and a far exit.)
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- **Payment is never rolled back.** If the checkbox is OFF and `pulseOpen` fails (offline
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controller), the signed `payment` + `vehicle_exit` already stand (money was taken, the car is
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owed an exit). The booth surfaces a clear error and an **audited `anomaly`** so the operator opens
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manually — we never silently drop the payment, and never leave a paid car without an exit event.
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## Threat-model notes ([[threat-model|operator as adversary]])
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- The booth exit reuses the **same validation as the reader path** (paid + within grace, or free
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entry-grace) — there is no booth-only bypass that admits an unpaid car. An unpaid ticket sends the
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operator to take payment first.
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- Every booth action is a **signed ledger event attributed to the operator's session**: the payment,
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the exit, and any `anomaly` (failed open / override). A colluding operator can't wave a car out
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without leaving a signed, attributed trail visible to [[reconciliation]].
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- The voucher path keeps the **camera snapshot at the physical exit** (the self-scan fires it), so
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the evidence is captured where the car actually leaves, not where it paid.
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## Active sessions & human-intervention barrier open
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**The barrier state is ASSUMED, never confirmed.** We send "open" intent and never truly know the car
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cleared ([[barrier-not-a-door]], no wired loop/sensor feedback). So a signed `vehicle_exit` does NOT
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mean the car is gone — it may be stuck (damaged ticket / dead scanner, or the barrier re-closed on a
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phantom obstacle: an animal, a person, a cardboard box or bag in the wind). These edge cases need a
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**human in the booth** to open the barrier, leaving a signed trace.
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**A session is "active" (shown in the booth Active Sessions list) while it is EITHER:**
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- **open** — entered, no `vehicle_exit` yet (still inside), OR
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- **exited but `now ≤ graceExpiresAt`** — paid and/or the voucher scanned, but still within the
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walk-back grace window. Because the barrier is unconfirmed, the car is presumed *possibly still
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present* until grace expires. **Payment and a successful voucher scan do NOT remove it from the
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list** — only grace expiry does.
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A session drops off the list once it is exited **and** past grace (presumed truly gone).
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### The one operator action — "Open barrier" (audited re-pulse)
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For an active session, the operator can open the barrier as a **human intervention**. This:
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- **re-pulses an exit relay** (resolved site-wide, as the booth exit does), and
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- signs an **`anomaly`** (`source: booth`, attributed to the operator, reason "manual barrier open")
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— **NEVER a second `vehicle_exit`** (a second exit would double-count occupancy and corrupt the
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ledger's meaning). It is an audited *re-open*, not a new exit.
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**Guard — no payment, no button.** The "Open barrier" action is shown/active **only for sessions that
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have a payment** (paid, or paid-and-exited-in-grace). An **unpaid** open session has **no barrier-open
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affordance at all** — the row routes to the [[#operator-flow|pay/exit modal]] instead. The
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no-unpaid-bypass rule is enforced structurally: the button simply does not exist for an unpaid car.
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(A future reason-required *force exit* for genuine disputes would be a separately-audited path — see
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Open.)
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This single mechanism covers both edge cases: a **damaged ticket / dead scanner** (find the still-open
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session in the list → pay/exit modal, or if already paid → Open barrier, no scan needed), and a
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**phantom-obstacle re-close** (the just-exited car is still in the list within grace → Open barrier).
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## ⚠ Open question — walk-back grace renews on every payment (voucher overstay)
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**Found 2026-06-17. Not yet fixed.** Scenario: customer pays at the booth, takes an exit voucher,
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then dawdles past the walk-back grace before reaching the exit.
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What the code does today (`exit-flow.ts`, `pay-station.ts`):
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- The exit reader's grace check is `now − paidAt ≤ graceExitMin`, reading **the latest payment's**
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`graceExitMin`. Over the window → exit **refuses** ("top-up required"). ✓ *Correct — no free exit.*
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- The re-quote (`computeFee(enteredAt, now, …)`) always prices from **entry**, never from the last
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payment. So a top-up charges the **full** entry→now fee (minus what's paid is implicit via the
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ledger). ✓ *Correct — the timer does NOT restart; the customer pays the true total.*
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- BUT every `payment` writes its own `graceExitMin`, and the exit flow reads the **latest** one — so
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**each top-up grants a fresh, full grace window.** ✗ *This is the bug.*
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**The leak is time, not money.** It is not a free-exit hole (the fee always catches up from entry).
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But the grace window — meant as a one-time walk-from-pay-to-gate allowance — is re-granted in full on
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every payment, so a customer could pay → wait → pay a tiny delta → get another full window → repeat,
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riding the gap between "paid" and "next increment accrues." With coarse [[tariff]] increments the
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abuse is bounded but real.
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**Candidate fixes (business call — fairness vs. anti-abuse):**
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1. **Grace on top-up only when the top-up charged new money** (recommended). Kills the "tiny delta
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forever" loop while staying fair to a genuine overstay; re-price stays from entry.
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2. **Single non-renewing window** anchored to the FIRST payment — cleanest anti-abuse, but can unfairly
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trap someone who legitimately paid, walked, then hit a slow elevator after a top-up.
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3. **Cap total grace** granted per session regardless of payment count.
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Decided halves: **refuse-on-expiry** and **reprice-from-entry** are deliberate and correct. The
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**grace-renews-fully-per-payment** consequence was an unintended side effect of reading `graceExitMin`
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off the latest payment. See [[tariff]] (walk-back grace) for the pricing side of the same question.
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## As-built / open
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- Backend: `GET /api/session/:identity` (lookup + quote), `POST /api/exit { identity }` (validated
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booth exit), `site_config.exit_voucher_default`. Exit validation shared between the booth and the
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reader path (one code path, two triggers).
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- **Open: walk-back grace renews on every payment** — see the flagged section above (voucher overstay
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re-grants a full grace window; pick a fix before production).
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- Voucher print = reprint the ticket id barcode on the booth printer ([[ticket-encoding]]).
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- Open: a force-open **override** (lost ticket / equipment fault) — deferred; would be a separately
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audited signed event, not folded into the validated path.
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