Observers

Observers describe how the semantic observer role is arranged in the system graph.

At the structure level, observers define placement, responsibility, logical execution context, and boundaries for observation, interpretation, routing, hosting, projection, and event participation. This page describes structural use of the observer role, not the primitive definition of observer and not a specific runtime implementation.

Observers may correspond to actors, services, request handlers, workflow activities, projections, processes, entities, brokers, clients, schedulers, fibers, tasks, or infrastructure participants. Concrete mechanisms such as actor systems, HTTP pipelines, workflow engines, brokers, green-thread schedulers, or OS threads belong to substrate concerns such as actor systems, application hosts, workflow engines, brokers, and runtimes.

The same structural observer may be realized by different substrate contexts over time when the runtime preserves the logical execution context. Conversely, a single substrate mechanism, such as a worker thread, may realize many short-lived observers across different operations.

Observers provide structure for:

Boundaries of interpretation.
Exogenous and endogenous event classification.
Command interpretation.
Routing and interaction.
Projection and hosting responsibilities.
Alignment between transition interpretation and the realization context that commits effects.