Events and Collaboration

After a domain event enters EventStore, it is authoritative history. The downstream collaboration in this section runs after the append. It consumes committed facts, but its effects are not part of the source aggregate transaction and cannot roll back an appended event.

Choose the collaboration mechanism by goal before opening its implementation and failure boundaries.

Choose a Processing Mechanism

Need	Choose	Responsibility boundary
Ordinary side effects such as notifications, audit, cache invalidation, or external integration	Processor	Invoke matching functions; the side effect owns idempotency and recovery
An event must generate cross-aggregate follow-up commands	Saga	Map the event to 0..N commands and cross the command-send boundary
A failed processing function needs durable recording, scheduling, and replay	Event Compensation	Persist failure-recovery state; it does not express a business reverse action
Maintain a query read model	Projection	Own only the read model, not ordinary integrations or cross-aggregate orchestration

Choose from the required business result, not from annotation names. One flow may use several branches, but their completion signals remain independent.

Typical Collaboration Path

command -> aggregate -> append domain event
                         |-> Processor -> ordinary side effect
                         |-> Saga -> 0..N follow-up commands
                         `-> processing failure -> Compensation

• The source event is already committed; a downstream failure is not an EventStore rollback.
• EVENT_HANDLED means only that the matching Processor function completed.
• SAGA_HANDLED means only that the matching Saga function completed and its command sends completed; it does not mean those commands were processed.
• For caller-visible waiting at these stages, see Completion Semantics.

Reading Order

1. Start with Event Processor for ordinary application or integration side effects.
2. Read Saga when an event must drive other aggregates.
3. When a processing failure needs durable recovery, continue to Event Compensation.