The delivery layer for EventEngine: the durability level ladder (0-4), transactional outbox, retry, dead-letter handling, and pluggable transports. Depends on event_engine for event definitions and schema.

The durable record layer for EventEngine: an immutable, append-only event table the host owns, a handler that records every dispatched event, and event-sourcing replay. Depends on event_engine for event definitions.

The subscriber layer for EventEngine: runs your in-app subscribers for events routed to it. Registers itself with the event_engine runtime as the :inline and :background processors, running subscribers synchronously or in a background job depending on which one an event's rule names.