---
title: Reactors
description: Topics for building reactors that react to Chronicle events and trigger side effects.
---


[React to an event](/arc/backend/chronicle/react-to-an-event/) covers the fundamentals of reactors — when to reach for one and how they observe events. The topics here go deeper on specific reactor patterns.

## Topics

| Topic | Description |
| ------- | ----------- |
| [Returning commands as side effects](/arc/backend/chronicle/reactors/command-side-effects/) | Let a reactor trigger follow-up commands by returning them — Arc executes them through the command pipeline automatically. |