Event-driven architecture

Event-driven architecture organises a system around facts that have happened — order placed, payment received, user deleted — which are published once and consumed by whichever components care. The publisher does not know who is listening.

This makes a system extensible in one specific and valuable way: adding a new reaction to an existing event requires no change to the code that emits it. Analytics, notifications and audit logging are natural consumers, added without touching the transaction that produced the event.

The price is that no single place in the code describes what happens when an order is placed. Understanding a flow means following it across several components, which is why event-driven systems need good tracing and a documented catalogue of events far more urgently than a monolith does.