marketplaces/docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md

Phase 1 Backend Contract — Money, FX, Price Snapshot, Payment State Machine

Companion to PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md Phase 1 (Sprints 1.1–1.4) and PRODUCT-PLAN-v3.1-GAP-ANALYSIS.md §3.3/§3.5/§3.6.

Status: unblocked (2026-08-17). BACKEND-API-REFERENCE.md §7 previously marked the cart/payment call chain frozen. Per the delivery plan's Sprint 0.1 decision, the freeze is lifted — this contract can move to implementation once backend ownership (also Sprint 0.1, still open) is confirmed.

This doc is the frontend's ask, in the same style as BACKEND-API-REFERENCE.md. It does not prescribe backend implementation (DB schema, service boundaries) — only the wire contract and the invariants the frontend needs to hold.

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1. Why this exists

Current behaviour (services/currency-rates.service.ts, pages/cart/cart.component.ts):

• Currency conversion rates are typed by an admin into Admin Settings and persisted to browser localStorage. They never update and drift from market.
• The amount charged is computed client-side and sent as CartPaymentRequest.amount to POST /cart. The backend currently trusts this number.
• No record exists anywhere of which FX rate produced a given displayed price, or when it was captured.

Result: bank/NSPK settlement totals don't reconcile against order counts, because nothing on the backend can reconstruct why a given amount was charged. This document's contract exists to close that gap — it is the same complaint as Product Plan v3.1 §3.3/§3.8, and our own §12.7 raised it first.

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2. Money representation

All money fields in every new endpoint below use minor units, never float.

interface Money {
  amountMinor: number; // integer, no float. 4990 = 49.90 for a 2-decimal currency.
  currency: string;    // ISO 4217, e.g. "RUB" | "USD" | "EUR" | "AMD"
}

Currency	Minor unit	Decimals
RUB	kopeck	2
USD	cent	2
EUR	cent	2
AMD	luma	2

Rounding rule for any conversion: round half up to the currency's minor-unit precision, applied once, at the point of conversion — never re-rounded on redisplay.

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3. FX Quote

3.1 Endpoint

GET /api/v2/pricing/fx-quote?base=RUB&quote=USD

{
  "quoteId": "fxq_8a3f1c2a",
  "base": "RUB",
  "quote": "USD",
  "rate": 0.0108,
  "source": "rapira",
  "observedAt": "2026-08-20T09:14:00Z",
  "expiresAt": "2026-08-20T09:19:00Z"
}

Field	Notes
quoteId	Opaque, referenced by every PriceSnapshot that used this quote.
rate	1 base = rate * quote. Float is acceptable here — it's a market rate, not a money amount.
source	Adapter name. Frontend never hardcodes a provider; treat as an opaque label for display in the backoffice reconciliation panel.
expiresAt	TTL, provider-configurable. Frontend must not use an expired quote to display or charge.

3.2 Stale-quote policy

• If the frontend holds a quote past expiresAt, it must re-fetch before checkout can proceed.
• If the rate source is unavailable, the backend decides: block (503 SERVICE_UNAVAILABLE with error.code: "FX_SOURCE_UNAVAILABLE") or serve a configured fallback quote explicitly marked "source": "fallback". Which policy applies is a tenant setting, not a frontend choice — see delivery-plan Sprint 0.1 decision on FX source.
• Outlier detection (e.g. a quote >X% off the previous one) is a backend concern; the frontend has no opinion on the threshold, only on obeying expiresAt.

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4. PriceSnapshot

Created once, at checkout, immutable afterward. This is what makes a total explainable months later.

interface PriceSnapshot {
  id: string;
  offerId: string;
  amount: Money;          // price in the offer's base currency
  displayAmount: Money;    // price in the currency the customer checked out in
  fxQuoteId: string | null; // null when displayAmount.currency === amount.currency
  capturedAt: string;      // ISO 8601
}

Rule: once a PriceSnapshot exists on an order line, it is never recalculated — not on rate update, not on currency-setting change, not on replay. An old order shows the price it was actually charged at.

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5. Server-authoritative checkout amount

This is the contract change with the highest priority in Phase 1 — it removes the client-trusted amount field entirely.

5.1 Current (to be replaced)

POST /cart
{ "amount": 4990, "currency": "RUB", "items": [{ "itemID": 101, "price": 4990, ... }], ... }

The backend trusts amount and each line's price as sent by the browser.

5.2 Target

POST /api/v2/storefront/checkout
{
  "offers": [{ "offerId": "off_9a1", "qty": 2 }],
  "currency": "USD",
  "deliveryOptionId": "del_standard"
}

{
  "checkoutSessionId": "chk_7f2e",
  "lines": [
    {
      "offerId": "off_9a1",
      "qty": 2,
      "unitPrice": { "amountMinor": 5390, "currency": "USD" },
      "lineTotal": { "amountMinor": 10780, "currency": "USD" },
      "priceSnapshotId": "snap_3b1c"
    }
  ],
  "subtotal": { "amountMinor": 10780, "currency": "USD" },
  "discount": { "amountMinor": 0, "currency": "USD" },
  "delivery": { "amountMinor": 500, "currency": "USD" },
  "total": { "amountMinor": 11280, "currency": "USD" },
  "fxQuoteId": "fxq_8a3f1c2a",
  "expiresAt": "2026-08-20T09:19:00Z"
}

The frontend sends offer IDs and quantities. The backend computes every price, using the offer's live price and the current FX quote. No amount or price field is ever accepted from the client for anything that affects the charge.

POST /api/v2/storefront/payments/intents then references checkoutSessionId only — the amount charged is read server-side from the checkout session, never re-sent by the client.

5.3 Total formula (must be reconstructable, per line)

order.total = sum(line.unitPrice * line.qty)
              - discounts
              + delivery
              + taxes/fees (if applicable)

Backoffice must be able to render this formula, with the FX quote used, for any order — this is what Product Plan §7.2 asks for and what a bank reconciliation needs.

---

6. Payment state machine

6.1 States

PaymentIntent: created -> pending -> authorized/paid -> failed/cancelled
Payment:       received -> confirmed -> captured/settled -> refunded/partially_refunded
Order:         pending_payment -> paid -> processing -> fulfilled/completed

6.2 Required fields per transition

interface PaymentEvent {
  id: string;
  paymentIntentId: string;
  fromState: string;
  toState: string;
  providerEventId: string;   // idempotency key from the provider
  providerTimestamp: string; // when the provider says it happened
  receivedAt: string;        // when our webhook received it
  processedAt: string;       // when our system finished processing it
}

No fixed delays anywhere in this chain. The frontend already complies with this (polls real provider status via /qr/dynamic/{partnerId}/{qrId} and /card/{partnerId}/{orderId} on an interval bounded by QR TTL) — this section documents the backend side of the same principle.

6.3 Webhook contract

POST /api/providers/v1/payments/{provider}/webhook

• Signature verification is mandatory; reject unsigned/invalid-signature payloads with 401, do not silently accept.
• Idempotency key = provider + providerEventId. A repeated delivery of the same event must be a no-op — same PaymentEvent row, no second order, no second notification.
• On success, emit payment.confirmed / payment.failed onto the platform event bus (Phase 2) so Order creation is driven by the event, not by the webhook handler doing double duty.

6.4 Idempotent order creation

POST /api/admin/v2/orders  (internal, from the payment-confirmation handler)
Idempotency-Key: <checkoutSessionId>

A retried call with the same checkoutSessionId must return the existing order, not create a second one. This is the mechanism that makes "double-click doesn't create two orders" true regardless of frontend debouncing.

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7. What the frontend will stop doing once this ships

• Delete CurrencyRatesService's localStorage-persisted admin-typed rates and hardcoded DEFAULT_RATES fallback (USD: 0.011, AMD: 4.3).
• Delete the Admin Settings currency-rate editor UI.
• Stop sending amount / price in any checkout-related request.
• Replace client-side float conversion (CurrencyRatesService.convert()) with server-supplied Money values everywhere a price is displayed.

8. What the frontend will start doing

• Fetch GET /api/v2/pricing/fx-quote on currency switch; block checkout if the held quote has expired.
• Render the backoffice "total formula" panel (lines × qty − discounts + delivery + fees, FX quote used) once §5.2 and the admin Orders API exist (Phase 2).
• Surface FX_SOURCE_UNAVAILABLE and error.code-driven stale-quote UI per the error envelope in BACKEND-API-REFERENCE.md §5.

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9. Resolved / open questions (Sprint 0.1, 2026-08-17)

1. Payment chain freeze — lifted. §5 can proceed.
2. FX rate source/provider — ours, in-house, as the default (not just a fallback). No external provider committed. Backend computes and serves the quote itself; the source field in §3.1 can legitimately read "internal" as the normal case. Revisit if an external provider is chosen later — the contract shape doesn't need to change, only the value of source.
3. Backend-converted prices vs. frontend-requested display currency — still open, needs confirmation before implementation. This doc's §5.2 models the frontend sending a target currency and the backend returning the converted total. Confirm this is the intended flow before backend implementation starts.
4. Backend ownership — still open. This contract is ready regardless of who builds against it, but implementation can't be scheduled until this is answered.