What are the key principles that make a Noumena event-driven system work well?

Loosely coupled, exactly-once processing, service independence, and decentralized/parallel services.

These four principles keep an event-driven system robust:

1. Loosely coupled — the Engine integrates with external systems via AMQP messages (Advanced Message Queuing Protocol, a standard for passing messages through a broker) whose meaning is specified in the NPL business logic, not baked into service-to-service calls.
2. Process exactly once — a critical property for a financial system: an event must not be silently lost, and (often more importantly) must be processed only once.
3. Service independence — each service listens for the events relevant to it and reacts on its own, without relying on synchronous calls to other services.
4. Decentralized / parallel services — integration services (talking to blockchains, banking systems, etc.) operate independently and in parallel, so teams can develop, deploy, and scale them separately.