add-reactor

Add a Chronicle reactor that triggers automation or translation logic in response to events.

Always read the reactors.md rule first. It is the source of truth for reactor conventions, rules, and patterns.

Step 1 — Identify the event(s)

Determine which event type(s) the reactor needs to observe. These events must already exist or be created as part of the slice.

Step 2 — Create the reactor class

Place the reactor in the appropriate feature folder, co-located with the slice it belongs to.

// Copyright (c) Cratis. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

namespace MyApp.Projects.Notifications;

/// <summary>
/// Sends a notification when a project is registered.
/// </summary>
/// <param name="notifications">The notification service.</param>
public class ProjectRegisteredNotifier(INotificationService notifications) : IReactor
{
    /// <summary>
    /// Reacts to <see cref="Registration.ProjectRegistered"/> events.
    /// </summary>
    /// <param name="event">The event.</param>
    /// <param name="context">The event context.</param>
    public async Task ProjectRegistered(Registration.ProjectRegistered @event, EventContext context) =>
        await notifications.Notify($"Project '{@event.Name}' was registered.");
}

Step 3 — Follow the critical rules

• IReactor is a marker interface — no methods to implement. Method dispatch is by first-parameter event type.
• Method name — can be anything descriptive. The name is for readability, not dispatch.
• EventContext — optional second parameter. Omit if event metadata is not needed.
• Idempotent — reactors may be called more than once for the same event. Design accordingly.
• Use event data directly — never query the read model back inside the reactor.
• Return events instead of injecting IEventLog — to produce new events, return them directly as Task<TEvent>, Task<EventForEventSourceId>, or a collection thereof. For commands in other slices, inject ICommandPipeline. Avoid injecting IEventLog directly.