Saga

Domain bridge component. Events from one domain in, commands or fact events to another domain out. Stateless translation between bounded contexts.

Abbreviation: sag

Sagas should contain minimal logic — just enough to map fields and construct target messages. If you need conditionals or business rules, that logic belongs in the source aggregate.

Sagas must be stateless — each event is processed independently with no memory of previous events.

Output Types

Sagas can emit two types of messages:

Output	Use When	Example
Command	Target aggregate should decide	ReserveFunds — aggregate validates balance
Fact Event	Reporting external reality	PaymentRecorded — Stripe already processed it

When the source event represents an external fact (payment processed, delivery confirmed), the saga emits a fact event rather than a command. The target aggregate records the fact without decision logic.

See Commands vs Facts for the full pattern.