Add Event Handler

Add Event Handler

When the user asks to add an event handler, scaffold a WolverineFx handler for a domain or integration event.

Arguments

• {EventName} -- Name of the event to handle (e.g., OrderCreatedEvent)
• {Service} -- Target microservice (ask if ambiguous)

Configuration

Read cloudstack.json from the project root at the start of execution. Extract:

• NAMESPACE = project.namespace (default: detect from *.sln name or first *.csproj root namespace)
• DOMAIN_PATH = backend.sharedDomainPath (default: find directory matching **/Domain/ containing Entities/)
• SERVICES = backend.services[] (default: discover from src/*/ directories containing .Application/ subfolders)

If cloudstack.json does not exist, auto-detect by scanning the project structure.

1. Find the Event

Search for the event class:

grep -rn "class {EventName}" src/

If the event doesn't exist, suggest running /add-entity first or create it inline.

2. Event Handler (Application/Events/{EventName}Handler.cs)

namespace {Namespace}.{Service}.Application.Events;

public class {EventName}Handler
{
    // Constructor-inject repositories, services, or ILogger<T>

    public async Task Handle({EventName} @event, CancellationToken cancellationToken)
    {
        // React to the event:
        // - Update read models
        // - Send notifications
        // - Publish integration events
        // - Trigger side effects
    }
}

For handlers that publish to a message broker:

public class {EventName}Handler
{
    private readonly IMessagePublisher _messagePublisher;
    private readonly ILogger<{EventName}Handler> _logger;

    public {EventName}Handler(IMessagePublisher messagePublisher, ILogger<{EventName}Handler> logger)
    {
        _messagePublisher = messagePublisher;
        _logger = logger;
    }

    public async Task Handle({EventName} @event, CancellationToken cancellationToken)
    {
        _logger.LogInformation("Handling {Event} for {Id}", nameof({EventName}), @event.Id);

        await _messagePublisher.PublishAsync(
            "{domain}.events.{event-type}",
            @event,
            cancellationToken);
    }
}

3. Integration Event (if cross-service)

If the handler needs to publish an event for other services:

namespace {Namespace}.{Service}.Application.Events;

public record {Name}IntegrationEvent(
    Guid Id,
    // relevant fields
    DateTimeOffset OccurredAt);

Checklist

• Handler class in Application/Events/ folder
• Event parameter named @event (reserved keyword escaping)
• CancellationToken propagated
• Logging at appropriate level
• No return value (event handlers are fire-and-forget)
• Idempotent handling (events may be delivered more than once)
• No exceptions thrown for business logic (log and continue)

Guidelines

• WolverineFx discovers handlers by convention -- no explicit registration needed
• One handler per event per concern (don't mix read model updates with notifications)
• Use ILogger for observability, not Console.WriteLine
• For message broker subjects/topics, follow the project's naming conventions

Output

After scaffolding, report:

## Scaffolded: {EventName}Handler

Files created:
- `src/{Service}/{Service}.Application/Events/{EventName}Handler.cs`
{if integration event:}
- `src/{Service}/{Service}.Application/Events/{Name}IntegrationEvent.cs`

Next: Run `/run-tests` to verify handler registration.

Error Handling

• Event class not found: Suggest running /add-entity first to create the entity and its domain events.
• Message publisher interface missing: Check if IMessagePublisher is registered in the service's DI. If not, it may need to be added to the Infrastructure layer.

Related Skills

• /add-entity to create the domain entity and events first
• /add-command if the handler needs to trigger a command