APIs, Ledgers, and Orchestration
Design idempotent APIs, model money movements with double-entry principles, and log every state transition with tamper-evident metadata. Use circuit breakers, retries with backoff, and dead-letter queues for resilience. Separate command, query, and reconciliation paths to simplify audits. Mirror minimal user data, encryption-at-rest everywhere, and role-based access for operators. Treat webhooks as untrusted input and verify signatures. Finally, build observability early: trace IDs across services, real-time dashboards for anomalies, and playbooks that reduce uncertainty when seconds truly matter.