Reactors

A projection answers “what does this look like now?” A reactor answers “what should happen because of this?” When an author is registered, something has to send the welcome email, tell the search index, or kick off the next step — and none of that belongs in the command, which should only record that the fact happened.

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.