الإشعارات والأحداث

الطلبات مقابل الإشعارات

// Request:      one handler, returns a result
// Notification: many handlers, fire-and-forget

INotification

public record OrderPlacedNotification(int OrderId, int CustomerId)
    : INotification;

INotificationHandler

public class SendConfirmationEmailHandler
    : INotificationHandler<OrderPlacedNotification>
{
    public Task Handle(
        OrderPlacedNotification n, CancellationToken ct)
    {
        // send the email...
        return Task.CompletedTask;
    }
}

مشتركون متعددون

public class UpdateInventoryHandler
    : INotificationHandler<OrderPlacedNotification>
{
    public Task Handle(OrderPlacedNotification n, CancellationToken ct)
    {
        // decrement stock...
        return Task.CompletedTask;
    }
}

النشر

public class CreateOrderHandler(AppDbContext db, IPublisher publisher)
    : IRequestHandler<CreateOrderCommand, int>
{
    public async Task<int> Handle(CreateOrderCommand c, CancellationToken ct)
    {
        var order = new Order { CustomerId = c.CustomerId };
        db.Orders.Add(order);
        await db.SaveChangesAsync(ct);
        await publisher.Publish(
            new OrderPlacedNotification(order.Id, c.CustomerId), ct);
        return order.Id;
    }
}

فك اقتران الآثار الجانبية

// CreateOrderHandler does NOT know about email or inventory

استراتيجية النشر

builder.Services.AddMediatR(cfg =>
{
    cfg.RegisterServicesFromAssembly(typeof(Program).Assembly);
    cfg.NotificationPublisher = new TaskWhenAllPublisher();
});

معالجة حالات الفشل

public async Task Handle(OrderPlacedNotification n, CancellationToken ct)
{
    try { await _email.SendAsync(n.CustomerId, ct); }
    catch (Exception ex) { _logger.LogError(ex, "email failed"); }
}

نمط أحداث المجال

await db.SaveChangesAsync(ct);
foreach (var domainEvent in order.Events)
    await publisher.Publish(domainEvent, ct);
order.Events.Clear();

الإشعارات في مقابل قوائم انتظار الرسائل

// In-process reactions  -> MediatR notifications
// Cross-service / durable -> message broker

اختبار الإشعارات

var publisher = new Mock<IPublisher>();
var handler = new CreateOrderHandler(db, publisher.Object);
await handler.Handle(new CreateOrderCommand(1, []), default);
publisher.Verify(p => p.Publish(
    It.IsAny<OrderPlacedNotification>(), default), Times.Once);

تحقق سريع

مراجعة