wiki: design the business layer (session, tariff, permit, vision, shift, ops)
Pivot from the hardware/integrity layer to the parking operation. All wiki-only; no code yet. Core principle throughout: business entities are projections over the signed append-only event log, never mutable tables. New concepts: parking-session, tariff (composable/versioned, FX-ready), shift (manned-only Z-report), capacity-occupancy, validation-discounts, reporting-analytics, clock-integrity, ticket-encoding, anti-passback. New entities: permit, opencv-anpr-service, blocklist. Decisions: session-model, vision-service (host-side ANPR + vehicle verification; scoped AGPL exception for the isolated service). Updates: append-only-event-chain (new event types + vision witness), local-jwt-auth (drop 8h expiry -> until logout; code change pending), lpr-camera (host-side recognition supersedes edge-AI), standing-decisions (AGPL exception), open-questions (+FX, +pay-station money corners, backup). Deferred + flagged: intercom/help-call, receipts/refunds/change, FX engine, lane topology (#1).
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---
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type: concept
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tags: [parking, domain, business, pricing]
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sources: []
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updated: 2026-06-15
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status: open
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---
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# Tariff (Fee Model)
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How a [[parking-session]]'s fee is computed from its duration. A tariff is **admin-composed data,
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not code** — the park owner builds and constantly edits the rate card at runtime (like a
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[[permit]]), in a selectable currency, with **no numbers hard-coded anywhere** and no code change to
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reprice. The computation is **pure and offline** ([[offline-first]]: no network, no clock authority
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beyond the host).
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> Decisions (2026-06-15): (1) tariffs are **effective-dated, immutable versions** — editing
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> publishes a new version, never mutates an old one; (2) **one active tariff per site** (versioned
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> over time), modelled with an id/scope so multiple rate cards can be added later without migration;
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> (3) **currency is selectable** (ISO 4217) and the money model is **FX-ready but FX is deferred**.
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## Design principles
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- **Pure function of (entry time, charge time, tariff).** `fee = f(enteredAt, asOf, tariff)`. No
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side effects, deterministic, unit-testable. The pay station calls it with `asOf = now`; the exit
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lane re-checks against the recorded payment.
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- **Data-driven.** The tariff lives as a config record (its own table or seeded config), versioned,
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so a historical session always reprices against the tariff in force when it was incurred. Never
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hard-code rates (this is an [[open-questions|open-question]]-adjacent procurement input — sites
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differ).
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- **Integer minor units.** Money is integer cents (or the site currency's minor unit) — never
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floats. Avoids rounding drift across a revenue ledger.
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- **The fee, once paid, is a signed `payment` event** ([[parking-session]]) — the computation is
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reproducible, but the *charged* amount is fixed in the chain.
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## The composable structure — stepped blocks + daily cap
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The admin composes a **rate card** the fee function interprets. The general model is an **ordered
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list of duration blocks** (flat rate is just one block) plus a daily cap — chosen because it
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expresses every common operator shape (first-hour pricing, tapering, caps) with no special cases in
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code. All amounts are **integer minor units** in the tariff's currency.
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```jsonc
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{
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"currency": "EUR", // ISO 4217; selectable per tariff version
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"gracePeriodEntryMin": 15, // free if exited within this (drop-off/turnaround)
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"incrementMin": 60, // billing granularity; partial increments round UP
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"blocks": [ // consumed in order as duration accrues
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{ "uptoMin": 60, "priceMinorPerIncrement": 200 }, // first hour
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{ "uptoMin": 180, "priceMinorPerIncrement": 150 }, // 60→180 min
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{ "uptoMin": null, "priceMinorPerIncrement": 100 } // null = open-ended, thereafter
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],
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"dailyCapMinor": 1200, // cap per rolling 24h (null = no cap)
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"lostTicketMinor": 2000, // flat charge when there's no entry id
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"gracePeriodExitMin": 15, // pay-on-foot walk-back window
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"overstay": "reprice" // top-up = recompute(entry→now) − alreadyPaid (decided)
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}
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```
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> **The numbers above are illustrative, not defaults to ship.** "No one knows the pricing and it
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> changes constantly" — so the admin authors all of it; the system ships with **no rate card** and
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> the owner must compose + publish one before the lot can charge (until then: free, or gated —
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> operator policy, see Open).
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**Lost ticket** is not just the flat `lostTicketMinor`: the admin may **override with an arbitrary
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amount** at the moment (operator judgement — establish entry time from [[opencv-anpr-service|plate]]
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capture/CCTV and charge real duration, or apply a set penalty). The configured flat fee is the
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default; the chosen amount is recorded in the signed `payment` event ([[parking-session]]).
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## The fee algorithm (pure, integer, offline)
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```
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fee(enteredAt, asOf, tariff):
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minutes = roundUp(asOf − enteredAt, incrementMin)
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if minutes ≤ gracePeriodEntryMin: return 0
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total = 0
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for each rolling 24h segment of the stay:
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segMinutes = minutes within this segment
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segFee = walk `blocks` in order, charging priceMinorPerIncrement for each
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incrementMin that falls in each block's [prevUpto, uptoMin) range
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if dailyCapMinor: segFee = min(segFee, dailyCapMinor)
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total += segFee
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return total
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```
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Deterministic, side-effect-free, unit-testable; the daily cap is applied **per rolling 24h** (so an
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overnight stay doesn't hit the cap twice). Rounding and segment edges are part of the settled spec
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because the chain + reconciliation depend on the result being reproducible.
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## The pay-on-foot consequence
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Because payment is decoupled from exit ([[parking-session]] lifecycle), the tariff has **two
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time references**, not one:
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1. At the **pay station**: `fee = f(enteredAt, now, tariff)` — charge for time parked so far.
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2. At the **exit lane**: the session is valid to leave iff `now ≤ paidAt + gracePeriodExit`.
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Past that, an **overstay top-up** = `f(paidAt, now, tariff.overstayRate)` is due before exit.
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`gracePeriodExit` is therefore a real revenue/UX parameter, not a nicety: too short traps people
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who paid; too long gives free parking between pay and exit.
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## Permit holders
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A valid [[permit]] bypasses tariff computation entirely for the covered period (subscription
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already paid out-of-band). A permit that has lapsed mid-stay falls back to the transient tariff for
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the uncovered time — an edge case to design with [[permit]].
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## Versioning — edits publish immutable, effective-dated versions
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Prices change constantly, **and** a historical [[parking-session]] must reprice against the rate
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that was in force when it was incurred — never today's. So a tariff is **never edited in place**:
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- Each save **publishes a new version** with an `effectiveFrom` timestamp; prior versions are
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**immutable**. Picking the version for a session = "the latest version with `effectiveFrom ≤
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session entry time`".
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- The session's **`payment` event records the `tariffVersionId`** it was priced under
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([[parking-session]], [[append-only-event-chain]]). The charged amount is then both reproducible
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*and* fixed in the signed chain — an admin can't retroactively rewrite prices to alter what a past
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session "should have" paid without it being visible.
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- An **in-progress** session that crosses a version boundary uses the version in force at **entry**
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(consistent, predictable) — confirm vs. pro-rating if an operator ever wants the latter.
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## Data model (first cut — with [[session-model]])
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| Table / field | Notes |
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| --- | --- |
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| `tariffs` | a logical rate card: `id`, `scope` (site/lane/zone — only "site" used now), `name`. |
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| `tariff_versions` | `id`, `tariffId`, `effectiveFrom`, `currency`, `structure` (the JSON above), `createdBy`, `createdAt`. **Immutable.** |
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| (active) | "one active tariff per site" = one `tariffs` row; multiple `tariff_versions` over time. The `scope`/`id` exist so multiple rate cards can be added later **without migration**. |
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Unlike the event log, tariff data is **mutable master data** in the sense that new versions are
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*added*; but each version row, once published, is never changed — close to append-only, and the
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*use* of it is fixed in the signed `payment` event.
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## Currency & FX — selectable now, FX deferred
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- Each `tariff_version` names its **`currency`** (ISO 4217), admin-selectable. Amounts everywhere
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are `{ minorUnits, currency }` — never a bare number, never a float.
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- A `payment` event stores its **`currency`** and a reserved **`fxRate` (null for now)** + optional
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`baseCurrency`. So when an exchange-rate system is added later, historical payments stay
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reproducible (you know the currency charged and, once FX exists, the rate applied) — **no
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migration** of stored amounts.
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- **FX engine is NOT built now.** When it is, it needs an *offline* rate source (rates can't depend
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on the network — [[offline-first]]), a base currency, and a rounding policy. Deferred to
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[[open-questions]].
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## Open
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- The **actual rate cards** are owner-authored at runtime — nothing to confirm at build time; the
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composer UI + validation (sane blocks, non-negative, ordered `uptoMin`) is the work.
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- **Time-of-day / weekday tiers** — not in the block model yet; add as a tier wrapper if a site
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needs day/night/weekend cards (deferred until asked).
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- **Blank-tariff policy** — free vs. gated until a rate card is published (operator policy).
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- **In-progress version-boundary** — entry-version (decided) vs. pro-rate (revisit if needed).
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- **FX** — exchange-rate system, offline rate source, base currency ([[open-questions]]).
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