This pattern deploys a Lambda durable function that implements an expense approval workflow. After an expense is submitted, the function creates a callback token and suspends execution — incurring zero compute cost while waiting for a manager's decision.

A manager can approve or reject the expense via a separate API endpoint that sends the callback. If no response arrives within 72 hours, a durable timer automatically rejects the expense.

This demonstrates the human-in-the-loop pattern with Lambda durable functions: WaitForCallback for external events, durable timers for deadlines, and DynamoDB for persisting expense state.