Chronicle reactors for follow-up work

A Cratis Chronicle reactor runs follow-up work when an event has been committed. Use it in Cratis Arc to send a welcome email after registration or update a search index after a book enters the catalog. The original command decides whether the change is allowed; the reactor acts on the recorded fact. A projection, by contrast, builds state for queries.

For immediate work chosen inside a model-bound command, use returned command operations. Return those operations from the command’s Handle(), not from a reactor. They do not provide durable delivery; keep event-driven follow-up in a reactor or a durable workflow/outbox.

Set up the reaction

Reactors need Arc’s Chronicle integration and registered event types. Follow Arc and Chronicle setup, including the separate Chronicle server, before adding a reactor. If you already supply authentication, use that page’s split-registration setup.

Implement IReactor and give each handler the event type it observes as its first parameter. React to an event walks through calling a service or executing a command. To let Arc execute a follow-up command through its validation and authorization pipeline, return the command as a side effect.

Choose replay behavior

A replay processes recorded events again, for example after an observer rewind. Put [OnceOnly] on a handler when replay must not repeat its effect, such as sending a welcome email. At class level, [OnceOnly] excludes the whole reactor from replay.

Replay exclusion does not prevent retries. Recovering a failed event-source partition can deliver the same event again, including to a [OnceOnly] handler. Make effects idempotent, meaning that repeating them does not repeat the business change. Chronicle’s delivery identity gives you a stable key to recognize a delivery, but your application or the receiving service must use it to prevent duplicate external effects.

Topics

Topic	Description
Returning commands as side effects	Return one or several commands, choose their system actor, and handle refusal or partial failure.