parking_solution/wiki/concepts/parking-session.md

type, tags, sources, updated, status

type	tags	sources	updated	status
concept	parking domain business anti-fraud		2026-06-15	open

Parking Session

The core business-domain entity: one vehicle's stay, from entry to exit, plus the money owed and paid for it. Everything on the business side — tariff, payment, reconciliation, revenue reporting — hangs off the session. This page defines what a session is and, just as importantly, what it is not.

Scope decision (2026-06-15): build the transient (casual, pay-for-duration) session first; layer permit holders on top as a second identity source that short-circuits payment. Mixed site, transient-first — see entry-exit-readers ("two populations, one shared relay") and session-model.

A session is a PROJECTION over the signed event log — not a mutable table

This is the single most important rule, and it falls straight out of the threat-model (the adversary is the insider who can edit the database) and the append-only-event-chain:

• The events table is the ledger and the only source of truth. vehicle_entry, vehicle_exit, payment, void are all appended + signed, never updated or deleted.
• A session is a read-model folded from those events — open when an entry has no matching exit, paid when a payment event references it, closed when an exit lands. It MAY be cached in a table for query speed (dashboards, "cars currently in"), but that cache is always rebuildable from the chain and never authoritative. Same pattern as the LaneMap (append-only-event-chain), scaled to the business domain.
• Why this matters: a mutable sessions row that stored "amount owed / paid" would reopen exactly the fraud hole the whole system exists to close (operator marks a session paid, pockets the cash). With sessions as a projection, "paid" is a signed payment event an operator can't forge or silently delete — a deletion breaks the chain visibly. See session-model for the rejected mutable-table alternative.

Identity — how an entry is tied to its exit

A session needs a key that survives from entry to exit. Two populations, two keys (entry-exit-readers):

• Transient: a ticket id (printed, ideally on pre-numbered stock — see reconciliation) or a plate read by lpr-camera. This id is carried in the event's identity field.
• Permit holder: a credential (card / plate / QR) matched to a permit record. A valid permit means the session owes nothing — the PAY step is skipped (see below).

Lifecycle (pay-on-foot / pay station model)

Payment is decoupled from exit (decision 2026-06-15, matching the [[autonomous-direction| unmanned]] roadmap): the customer pays at a central station before walking back to the car; the exit lane only validates that the session is settled.

ENTRY (lane)      vehicle_entry event              → session OPEN
                  (ticket printed / plate read; barrier opens)
PAY  (pay station) payment event {sessionRef, fee, paidAt}
                                                    → session PAID  (grace window starts)
EXIT (lane)       validate: PAID && now ≤ paidAt + graceMinutes ?
                    yes → vehicle_exit event        → session CLOSED → pulseOpen
                    no  → reject → re-pay overstay top-up at station, then exit

States, as derived from events:

State	Condition (over the event chain)
OPEN	a vehicle_entry with no later matching vehicle_exit
PAID	OPEN + a payment event covering the fee due, within its grace window
CLOSED	a matching vehicle_exit event exists
VOIDED	a void event references the session (lost ticket written off, error correction)

Permit sessions skip PAID: a valid permit at exit is itself the authorization to close.

Edge cases the model must name (not yet designed in full)

• Overstay after payment — exited the grace window; needs a top-up payment. The one genuinely stateful rule; handled as a second payment event, fee = f(time since paid).
• Lost ticket — no entry id to match. A default flat "lost ticket" fee (see tariff), or an amount the admin sets at the moment (operator judgement — e.g. they can establish entry time from opencv-anpr-service capture or CCTV and charge accordingly, or apply a fixed penalty). Recorded as a payment (with the chosen amount + a reason) + a void/annotation so it reconciles; the admin-set amount is captured in the signed event, attributed.
• Manual override — an operator/admin opens the barrier for a stuck or disputed car, or writes off a session, as a deliberate act. Each is a signed, reason-coded event (barrier_open_command / a void with reason) — so an override is authorized and logged, while an open with no such signed event remains the fraud signal (append-only-event-chain). The override is the legitimate counterpart to the out-of-band-open anomaly.
• Forced / fail-open exit — barrier failed open (fail-state-safety): the vehicle leaves with no vehicle_exit. This is an open session that never closes — a reconciliation anomaly by design (append-only-event-chain's "physical open with no signed command"), not something to paper over. (A manual override above is the signed, non-anomalous version.)
• Re-entry / never-exited — stale open sessions (drove out tailgating, sensor missed). Surface as anomalies; never auto-close silently.

What this unblocks (build order)

The device layer left the entry flow dangling — the session domain is that next step. Schema + code follow this page and tariff; the decision is recorded in session-model.

As-built (2026-06-15)

• Entry flow (apps/server/src/entry-flow.ts): access-device input edge → print ticket (failover) → signed vehicle_entry → pulseOpen. Holds (anomaly, no open, no entry) if printing fails. See device-input-flow.
• Exit flow (apps/server/src/exit-flow.ts): a credential read (new read bus channel) → fold the signed ledger for that identity → validate open + PAID + within gracePeriodExitMin → signed vehicle_exit → pulseOpen. Unpaid / expired / unknown → signed anomaly, barrier stays closed. Validation folds the ledger (authoritative), then updates the sessions cache.
  • Not a fail-state: an unpaid reject keeps the barrier closed deliberately (driver returns to the pay station); "exit fails open" (fail-state-safety) is about the system being unable to decide (host/power loss), not an unpaid car.
  • Currently every transient exit rejects — no payment events exist until the pay station is built; the validation is the correct end-state, just not passable yet.

Design gap (flagged): lane_devices has no entry/exit direction model. Entry is button-driven and exit is read-driven, so they don't currently collide — but a lane with both an entry reader and an exit reader can't yet be distinguished. A lane-direction/role model is needed before multi-reader lanes (relates to open-questions #1 topology).