---
title: Reactors
---

Reactors are observers that react to events and execute side effects. They are ideal for integrating with external systems, sending notifications, or emitting follow-up actions when events occur.

```mermaid
flowchart LR
    E[Event appended] --> R[Reactor]
    R --> S["Side effect<br/>(notify · call an API)"]
    R -.->|via command pipeline| C[New command]
```

## Key Concepts

- **Event-driven side effects** - Reactors run when events are appended and can call external services or enqueue work
- **Convention-based methods** - Public methods with supported signatures are discovered automatically
- **Event context access** - Optional `EventContext` provides metadata like timestamps and identifiers
- **Event-source isolation** - Events are processed per event source, in order, to keep behavior consistent

## When to Use Reactors

Reactors are a good fit when you need to:

- Trigger notifications or workflows when specific events occur
- Synchronize with external systems
- Run validations or side effects that do not belong in projections or reducers
- Emit follow-up events based on patterns in the event stream

## Basic Example

```csharp
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Reactors;

[EventType]
public record EmailConfirmed(string Email);

public class EmailNotificationsReactor : IReactor
{
    public Task Confirmed(EmailConfirmed @event, EventContext context)
    {
        return SendConfirmationAsync(@event.Email, context.Occurred);
    }

    Task SendConfirmationAsync(string email, DateTimeOffset occurred) => Task.CompletedTask;
}
```

## Topics

- [Getting Started](/chronicle/reactors/getting-started/)
- [Subscribe to External Event Stores](/chronicle/reactors/external-event-store-subscriptions/)
- [Event Processing](/chronicle/reactors/event-processing/)
- [Returning Side Effects](/chronicle/reactors/side-effects/)
- [Event Sequence](/chronicle/reactors/event-sequence/)
- [Once-Only Processing](/chronicle/reactors/once-only/)
- [Filtering by appended event metadata](/chronicle/events/filtering/)
- [Tagging Reactors](/chronicle/concepts/tagging-reactors/)