Event

An Event is a fact that something happened. It is named in the past tense – RoundStarted, BookBorrowed, PaymentCaptured – and it carries the data that makes the fact concrete: who, what, when.

Events are immutable. Once published, an Event never changes. It records the state of the world at the moment it was emitted, and any later change is itself an Event. That is the foundation of Event Sourcing: state is not the truth; the sequence of Events is.

Publisher and Consumers

In ESDM, every Event is published by exactly one consistency unit. Most often that publisher is an Aggregate, but it can also be a Process Manager, a Dynamic Consistency Boundary, or another publishing kind. The single-publisher rule is what keeps responsibility traceable: when you ask who emits this Event?, the model gives one answer.

An Event may have any number of consumers – Read Models that project it, Process Managers that react to it, Policies that derive Commands from it. The linter expects every Event to have at least one consumer; an Event nobody listens to is almost always a modeling mistake.

Data and Identity

An Event carries a payload – the structured data that describes the fact. It also carries an identifier – the natural key of the consistency unit it belongs to – so that downstream consumers know which entity the fact is about.

Events do not carry intent, decisions, or future tense. OrderShouldShip is not an Event; it is a Command that has not happened yet. OrderShipped is the Event that records the shipment as a fact. Keeping the tense rigorous is one of the simplest disciplines you can apply, and it pays off every time you read the model later.