ADR-0002. Event-driven posting; record finance at payment, not checkout

Field	Value
Status	Accepted
Date	2026-04-28
Deciders	finance-team, sale-team
Supersedes	—

Context

• Finance must not be on the critical path of POS checkout — a finance outage cannot block taking money.
• Revenue is only real when money actually settles. Recording at checkout (before payment) would book income for orders that are later abandoned or fail payment.
• COGS and inventory-asset value are owned by @nx/inventory, which knows cost basis only after it issues stock — finance cannot compute it.
• The chosen payment account/category is a customer-time decision carried on the sale payment payload (payment.attempt.finance), not known at checkout.

Decision

We will make finance react to Kafka events, never call it synchronously from checkout. Finance subscribes to five topics and posts vouchers as side effects:

Inbound event	Voucher
PAYMENT_SUCCESS (sale)	RECEIPT against the account on attempt.finance.source.id
PURCHASE_ORDER_RECEIVED (inventory)	PAYMENT to vendor (+ inventory asset leg)
INVENTORY_ISSUED_FOR_SALE (inventory)	ADJUSTMENT — DEBIT COGS / CREDIT INVENTORY
INVENTORY_ADJUSTED (inventory)	single-line ADJUSTMENT on INVENTORY
MERCHANT CDC (commerce)	reconcile default + control accounts

Income is booked on PAYMENT_SUCCESS, not at checkout. If the payment payload carries no chosen account (attempt.finance.source.id), finance INFO-skips rather than guessing.

Consequences

Pros	Cons
Finance outage never blocks checkout	Postings are eventually-consistent, not synchronous
Income reflects settled money, not intent	Requires idempotency for at-least-once delivery (see ADR-0003)
COGS/asset posting uses inventory's authoritative cost basis	Finance depends on upstream payload completeness (account id)
Clean separation of concerns across services	Debugging spans multiple services + the broker

Alternatives Considered

Option	Pros	Cons	Why rejected
Synchronous HTTP from sale checkout	Simple, immediate	Couples checkout latency/availability to finance	Violates "finance off the critical path"
Record at checkout (intent)	Earliest signal	Books revenue for unpaid/abandoned orders	Wrong accounting
Finance computes COGS itself	Fewer events	Finance would duplicate inventory costing logic	Cost basis is inventory's domain

References

• packages/finance/src/components/kafka/component.ts (SUBSCRIBED_TOPICS, _dispatchMessage)
• packages/finance/src/services/finance-worker.service.ts (all handle* methods)
• Sale ADR-0003 — payment via webhook
• Integration