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.

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

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;
}

Reactors

Key Concepts

When to Use Reactors

Basic Example

Topics