Reactors

A projection answers “what does this look like now?” A reactor answers “what should happen because of this?” After an author registration commits, a reactor can send a welcome email, update the search index, or start the next step. Keep follow-up that depends on committed facts separate from the original command’s decision.

Choose when the work must happen

Keep reactors or a durable workflow/outbox for reliable follow-up to committed facts. For immediate work chosen within a model-bound command, prefer returned command operations. They do not provide durable delivery and are returned from the command, not directly from a reactor.

React to an event is the place to start: when to reach for a reactor, how method dispatch works by event type, and why idempotency matters. The topics here go deeper.

Topics

Topic	Description
Returning commands as side effects	Let a reactor trigger follow-up commands by returning them — Arc executes them through the command pipeline automatically.