Design an Expense Management System

1. Problem Statement

2. Live Coding Format

3. Key Focus Areas

• 1
  Expense entity modeling and state machine (Draft → Submitted → Approved → Reimbursed; Rejected is final)
• 2
  ApprovalPolicy abstraction for routing rules
• 3
  Idempotency at the service boundary
• 4
  Immutable audit log derived from transitions
• 5
  Separation of entity, policy, and service responsibilities

4. What Strong Candidates Should Demonstrate

• Model an Expense entity with explicit state machine invariants and guarded transitions.
• Separate approval routing policy from the Expense entity itself.
• Design idempotent service operations at the object/service boundary.
• Produce an immutable append-only audit history from state transitions.
• Write tests, run or traced by hand, that cover state transitions and policy routing.