Getting Started with Reactors

Reactors provide a straightforward way to execute side effects when events are appended. This guide walks through creating your first reactor and registering it for discovery.

Prerequisites

Before you begin, ensure you have:

• A Chronicle-enabled application
• A basic understanding of events and event sourcing
• At least one event type marked with [EventType]

Creating a Reactor

1. Define an Event

[EventType]
public record OrderPlaced(Guid OrderId, string CustomerEmail, decimal TotalAmount);

2. Implement the Reactor

Create a class that implements IReactor and add one or more handler methods that match supported signatures:

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Reactors;

public class OrderNotificationsReactor : IReactor
{
    public Task Placed(OrderPlaced @event, EventContext context)
    {
        return NotifyAsync(@event.CustomerEmail, @event.TotalAmount, context.Occurred);
    }

    Task NotifyAsync(string email, decimal amount, DateTimeOffset occurred) => Task.CompletedTask;
}

3. Register the Reactor (Optional)

If your application uses dependency injection, ensure the reactor can be created by the configured service provider. For example, register it as a transient service in your DI container.

4. Customize with the Reactor Attribute (Optional)

You can override the default identifier or event sequence using the [Reactor] attribute:

using Cratis.Chronicle.Reactors;

[Reactor(id: "order-notifications", eventSequence: "orders")]
public class OrderNotificationsReactor : IReactor
{
    public Task Placed(OrderPlaced @event) => Task.CompletedTask;
}

Method Signatures

Reactor methods are discovered by convention. For a full list of supported signatures and guidance, see Event Processing.

Next Steps

• Explore Event Processing for method patterns and error handling
• Learn about Tagging Reactors to organize reactors