Notification Handlers (Domain Events)

Core Principles

• Domain events represent something that happened in the domain
• Multiple handlers can react to a single event (one-to-many)
• Dispatch events after successful persistence (not before SaveChanges)
• Handlers should be idempotent for at-least-once delivery
• Events are named in past tense: OrderCreated, OrderShipped

Patterns

Domain Event Interface

public interface IDomainEvent : INotification
{
    DateTimeOffset OccurredAt { get; }
}

public abstract record DomainEvent : IDomainEvent
{
    public DateTimeOffset OccurredAt { get; } = DateTimeOffset.UtcNow;
}

Concrete Domain Events

public sealed record OrderCreatedEvent(
    Guid OrderId,
    string CustomerName) : DomainEvent;

public sealed record OrderSubmittedEvent(
    Guid OrderId,
    decimal Total) : DomainEvent;

public sealed record OrderCancelledEvent(
    Guid OrderId,
    string Reason) : DomainEvent;

public sealed record OrderItemAddedEvent(
    Guid OrderId,
    Guid ProductId,
    int Quantity) : DomainEvent;

Raising Events from Aggregates

public abstract class AggregateRoot
{
    private readonly List<IDomainEvent> _domainEvents = [];
    public IReadOnlyList<IDomainEvent> DomainEvents =>
        _domainEvents.AsReadOnly();

    protected void RaiseDomainEvent(IDomainEvent domainEvent)
        => _domainEvents.Add(domainEvent);

    public void ClearDomainEvents() => _domainEvents.Clear();
}

public sealed class Order : AggregateRoot
{
    public static Order Create(string customerName)
    {
        var order = new Order { CustomerName = customerName };
        order.RaiseDomainEvent(
            new OrderCreatedEvent(order.Id, customerName));
        return order;
    }

    public void Submit()
    {
        if (Status != OrderStatus.Draft)
            throw new DomainException("Only draft orders can submit");

        Status = OrderStatus.Submitted;
        RaiseDomainEvent(new OrderSubmittedEvent(Id, Total));
    }
}

Notification Handlers

// Handler 1: Send confirmation email
internal sealed class SendOrderConfirmationOnCreated(
    IEmailService emailService,
    ILogger<SendOrderConfirmationOnCreated> logger)
    : INotificationHandler<OrderCreatedEvent>
{
    public async Task Handle(
        OrderCreatedEvent notification, CancellationToken ct)
    {
        logger.LogInformation(
            "Sending confirmation for order {OrderId}",
            notification.OrderId);

        await emailService.SendOrderConfirmationAsync(
            notification.OrderId, notification.CustomerName, ct);
    }
}

// Handler 2: Update dashboard stats
internal sealed class UpdateDashboardOnOrderCreated(AppDbContext db)
    : INotificationHandler<OrderCreatedEvent>
{
    public async Task Handle(
        OrderCreatedEvent notification, CancellationToken ct)
    {
        var stats = await db.DashboardStats.SingleAsync(ct);
        stats.IncrementOrderCount();
        await db.SaveChangesAsync(ct);
    }
}

// Handler 3: Reserve inventory
internal sealed class ReserveInventoryOnOrderSubmitted(
    IInventoryService inventoryService)
    : INotificationHandler<OrderSubmittedEvent>
{
    public async Task Handle(
        OrderSubmittedEvent notification, CancellationToken ct)
    {
        await inventoryService.ReserveForOrderAsync(
            notification.OrderId, ct);
    }
}

Dispatch After SaveChanges (Interceptor)

public sealed class DomainEventDispatcher(IPublisher publisher)
    : SaveChangesInterceptor
{
    public override async ValueTask<int> SavedChangesAsync(
        SaveChangesCompletedEventData eventData,
        int result,
        CancellationToken ct = default)
    {
        var context = eventData.Context!;

        // Collect events from all aggregates
        var aggregates = context.ChangeTracker
            .Entries<AggregateRoot>()
            .Select(e => e.Entity)
            .Where(e => e.DomainEvents.Count > 0)
            .ToList();

        var events = aggregates
            .SelectMany(a => a.DomainEvents)
            .ToList();

        // Clear events before dispatch to prevent re-dispatch
        aggregates.ForEach(a => a.ClearDomainEvents());

        // Dispatch each event
        foreach (var domainEvent in events)
            await publisher.Publish(domainEvent, ct);

        return result;
    }
}

// Registration
builder.Services.AddSingleton<DomainEventDispatcher>();
builder.Services.AddDbContext<AppDbContext>((sp, options) =>
{
    options.AddInterceptors(
        sp.GetRequiredService<DomainEventDispatcher>());
});

Idempotent Handler

internal sealed class ProcessOrderPaymentOnSubmitted(
    AppDbContext db,
    IPaymentService paymentService)
    : INotificationHandler<OrderSubmittedEvent>
{
    public async Task Handle(
        OrderSubmittedEvent notification, CancellationToken ct)
    {
        // Idempotency check — skip if already processed
        var alreadyProcessed = await db.PaymentRecords
            .AnyAsync(p => p.OrderId == notification.OrderId, ct);

        if (alreadyProcessed)
            return;

        await paymentService.ChargeAsync(
            notification.OrderId, notification.Total, ct);

        db.PaymentRecords.Add(new PaymentRecord
        {
            OrderId = notification.OrderId,
            Amount = notification.Total,
            ProcessedAt = DateTimeOffset.UtcNow
        });

        await db.SaveChangesAsync(ct);
    }
}

Manual Dispatch (Without Interceptor)

// Alternative: dispatch events in the handler directly
internal sealed class SubmitOrderHandler(
    IOrderRepository repository,
    IUnitOfWork unitOfWork,
    IPublisher publisher)
    : IRequestHandler<SubmitOrderCommand, Result>
{
    public async Task<Result> Handle(
        SubmitOrderCommand request, CancellationToken ct)
    {
        var order = await repository.FindAsync(request.OrderId, ct);
        if (order is null)
            return Result.Failure(
                Error.NotFound("Order.NotFound", "Order not found"));

        order.Submit();
        await unitOfWork.SaveChangesAsync(ct);

        // Dispatch events after successful save
        foreach (var domainEvent in order.DomainEvents)
            await publisher.Publish(domainEvent, ct);
        order.ClearDomainEvents();

        return Result.Success();
    }
}

Anti-Patterns

• Dispatching events before SaveChanges (data may not be persisted)
• Business logic in notification handlers (keep for side effects only)
• Non-idempotent handlers with at-least-once delivery
• Throwing exceptions in handlers that block other handlers
• Synchronous cross-service calls in notification handlers

Detect Existing Patterns

1. Search for INotification and INotificationHandler< implementations
2. Look for IDomainEvent or DomainEvent base types
3. Check for RaiseDomainEvent or AddDomainEvent in entities
4. Look for SaveChangesInterceptor that dispatches events
5. Search for IPublisher.Publish calls

Adding to Existing Project

1. Define IDomainEvent interface extending INotification
2. Add event collection to aggregate root base class
3. Create concrete events for key domain state changes
4. Add dispatch interceptor to DbContext configuration
5. Create notification handlers for side effects (email, analytics, etc.)
6. Ensure idempotency in all handlers

References

• https://learn.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/domain-events-design-implementation
• https://github.com/jbogard/MediatR/wiki#notifications