Checkout validates the cart, calculates totals, reserves inventory, initiates payment, and confirms or cancels the order. Persist each state transition and use idempotency keys for client retries. Never rely on a single long HTTP request to complete every external action.

Use a saga or durable workflow for cross-service steps. If payment fails, release the reservation; if payment succeeds but the response is lost, query or reconcile provider state before charging again. Webhooks must be authenticated, deduplicated, and safe to process out of order.

Publish order events through a transactional outbox so downstream systems—email, analytics, fulfillment—can react without losing the relationship between a committed order and its event. Consumers should be idempotent and expose backlog metrics.