Event

An event is a time-bearing occurrence with a value. It marks, reports, or induces change depending on how it is interpreted by an observer relative to a boundary.

Structurally, an event is a value with a notion of occurrence. Semantically, an event's role is observer and boundary relative.

Common Uses of Event

The word event is used for several related but distinct concepts:

Domain events are events relevant inside a domain boundary, such as OrderPlaced, PaymentCaptured, or ShipmentDispatched. They may be used as event-sourcing events, persisted in a transactional outbox, or published best-effort after a state change. In Cohesive terms, a domain event is endogenous relative to the domain boundary in which it occurs or is accepted.
Event-sourcing events are domain events interpreted as state actions and committed as the authoritative history for an entity. They carry both the event-action aspect and the committed modality: they advance version and can be folded or replayed into state.
Telemetry or metric events are system events about application behavior, runtime behavior, measurements, traces, or logs. They often have softer persistence guarantees and narrower retention windows. They may be exogenous to a domain model while endogenous to the observability system that records or aggregates them.
External events are observations of a process outside the observer's boundary. They arrive as exogenous events and may be interpreted as commands, queries, or ignored inputs. A command interpretation may then lead to an accepted endogenous event, state change, or committed event record depending on the realization.
Machine events are occurrences in a runtime, host, operating system, orchestrator, or infrastructure boundary, such as restart, out-of-memory termination, timeout, placement change, or network partition. They are usually exogenous to an application or domain observer, while endogenous to the machine or infrastructure boundary that emits them.

These names are not mutually exclusive, and they are not intrinsic types of event. Endogenous, exogenous, input, and output are roles assigned relative to an observer and boundary.

Boundary Roles

Relative to an observer's boundary:

An exogenous event arrives from outside the observer boundary.
An input event is an exogenous event in the role of entering a system or observer.
A command is an input event interpreted as an attempted transition for a target subject.
A query is an input event interpreted as a request to observe, compute, or return information.
An endogenous event occurs or is accepted within the observer's own semantic history.
An output event is an endogenous event emitted across a boundary.
A nil endogenous event is the modeled absence of an accepted domain transition after the observer interpreted the input.

The same occurrence may therefore be an output event for one system, an exogenous input event for another, and a command or query only after interpretation by an observer relative to that system's boundary, current state, policies, authority, and target subject.

State Actions and Commitment

An event becomes a state action only when an observer interprets it through a transition or apply relation for a subject. In event-sourced semantics, the relevant events are committed endogenous events:

\begin{align} \mathrm{decide}&\colon\mathrm{state}_v\times\mathrm{Input}\times\mathrm{Context}\to\mathrm{Endogenous event}_v\lor\mathrm{nil}\lor\mathrm{Rejection} \\ \mathrm{apply}&\colon\mathrm{state}_v\times\mathrm{Endogenous event}_v \to\mathrm{state}_{v+1} \end{align}

Only committed endogenous events advance entity version and become part of the event-sourced state history. Attempted inputs, rejected commands, retries, telemetry, and nil outcomes may be recorded elsewhere, but they are not state actions for the target entity unless committed inside that entity boundary.

Commitment is a modality, not 'event-ness' itself. An event may be ephemeral, delivered through a broker, recorded in a log, retained as telemetry, persisted in an outbox, or committed as authoritative entity history. Each claim is scoped to a boundary and persistence mechanism.

Events participate in event-state duality:

Events can be folded or integrated into current state.
State histories can be compared to derive deltas that an observer may interpret as events.
Event streams can form behavior, and behavior can be sampled or detected as events.