Add Integration Event

Cross-module communication goes through integration events + the Outbox (transactional, crash-safe) — never a direct in-process call into another module's runtime, and never IEventBus.PublishAsync from a handler. Full model: .agents/rules/eventing.md.

Step 1 — Define the event (source module's Contracts)

Modules.{Source}.Contracts/Events/{Event}IntegrationEvent.cs — implement IIntegrationEvent:

public sealed record {Event}IntegrationEvent(
    Guid Id,
    DateTime OccurredOnUtc,
    string? TenantId,
    string CorrelationId,
    string Source,
    Guid {Entity}Id,
    string SomePayload) : IIntegrationEvent;

⚠️ Don't rename/move this type later — the outbox stores its assembly-qualified name; a rename makes Type.GetType() return null and the message dead-letters. Keep the type name + namespace stable.

Step 2 — Publish via the Outbox (source handler)

The source module must have eventing wired (add-module Step 1): AddEventingCore + AddEventingForDbContext<{Source}DbContext>. Inject IOutboxStore and add the event in the same unit of work:

public sealed class Do{Thing}CommandHandler({Source}DbContext db, IOutboxStore outbox)
    : ICommandHandler<Do{Thing}Command, Unit>
{
    public async ValueTask<Unit> Handle(Do{Thing}Command command, CancellationToken cancellationToken)
    {
        // … mutate entities, db.SaveChangesAsync …
        var evt = new {Event}IntegrationEvent(
            Id: Guid.CreateVersion7(),
            OccurredOnUtc: DateTime.UtcNow,
            TenantId: /* current tenant */,
            CorrelationId: Guid.NewGuid().ToString(),
            Source: "{Source}",
            {Entity}Id: entity.Id,
            SomePayload: "…");
        await outbox.AddAsync(evt, cancellationToken).ConfigureAwait(false);
        return Unit.Value;
    }
}

The OutboxDispatcherHostedService later publishes it via IEventBus.

Step 3 — Handle it (consumer module)

Modules.{Consumer}/IntegrationEventHandlers/{Event}IntegrationEventHandler.cs — sealed, implement IIntegrationEventHandler<T>:

public sealed class {Event}IntegrationEventHandler({Consumer}DbContext db /*, IHubContext<AppHub> hub */)
    : IIntegrationEventHandler<{Event}IntegrationEvent>
{
    public async Task HandleAsync({Event}IntegrationEvent @event, CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(@event);
        // … write to the consumer's tables / push a notification …
        await db.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }
}

Register the consumer's handlers in its ConfigureServices:

builder.Services.AddIntegrationEventHandlers(typeof({Consumer}Module).Assembly);

Gotchas

• Idempotency is free with the in-memory bus (the Inbox dedups by {eventId, handlerName}) — don't hand-roll it.
• The in-memory bus runs handlers synchronously in the publisher's scope — keep the handler lean; a throw surfaces to the originating request.
• If the handler reads a tenant-filtered DbContext from a background path (open-generic handler, Hangfire job), restore Finbuckle context first via IMultiTenantContextSetter (see WebhookFanoutHandler).
• Module load order: the consumer must load before the publisher if it must react (Order in [assembly: FshModule]) — e.g. Notifications (750) before Chat (800).

Checklist

• Event in source Contracts, implements IIntegrationEvent, stable type name
• Source module has AddEventingCore + AddEventingForDbContext<T>; published via IOutboxStore.AddAsync (not the bus)
• Consumer handler sealed : IIntegrationEventHandler<T>; AddIntegrationEventHandlers(assembly) registered
• Background readers restore tenant context; module Order lets the consumer load first
• Build + tests green