Event-Driven Architecture
eventsarchitecturepatterns+1

Event-Driven Architecture Patterns in C#: A Complete Developer Guide

Master Event-Driven Architecture patterns with this comprehensive guide. Learn how to build scalable, resilient systems using C#, MassTransit, RabbitMQ, and Azure Service Bus. Explore real-world implementation patterns including Event Sourcing, CQRS, Saga patterns, and modern microservices communication strategies used by industry leaders.

JG
Junrill GalvezAuthor
15 min read
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Event-Driven Architecture (EDA) has become the backbone of modern distributed systems. After spending years building everything from e-commerce platforms to financial trading systems, I've learned that understanding EDA patterns isn't just academic—it's essential for creating systems that can handle real-world complexity and scale.

Whether you're working on microservices, building reactive systems, or designing cloud-native applications, event-driven patterns will fundamentally change how you think about system design.

Why Event-Driven Architecture Matters

In traditional synchronous systems, services are tightly coupled through direct API calls. Event-driven architecture changes this by using asynchronous events to coordinate different components of a system, creating several key advantages:

  • Loose Coupling - Services don't need to know about each other directly
  • Scalability - Components can scale independently based on event load
  • Resilience - System continues functioning even if individual services fail
  • Real-time Responsiveness - Events enable immediate reactions to state changes
  • Business Alignment - Events naturally model business processes

An event is simply a change in state, or an update, like an item being placed in a shopping cart on an e-commerce website. But the power lies in how we architect systems around these events.

Core Event-Driven Patterns

1. Publish-Subscribe (Pub/Sub) Pattern

The foundation of event-driven systems where publishers emit events without knowing who will consume them.

1// Event definition
2public record OrderCreatedEvent(
3    Guid OrderId, 
4    Guid CustomerId, 
5    decimal TotalAmount, 
6    DateTime CreatedAt);
7
8// Publisher service
9public class OrderService
10{
11    private readonly IMessagePublisher _publisher;
12    
13    public OrderService(IMessagePublisher publisher)
14    {
15        _publisher = publisher;
16    }
17    
18    public async Task<Order> CreateOrderAsync(CreateOrderRequest request)
19    {
20        var order = new Order
21        {
22            Id = Guid.NewGuid(),
23            CustomerId = request.CustomerId,
24            TotalAmount = request.TotalAmount,
25            CreatedAt = DateTime.UtcNow
26        };
27        
28        // Save to database
29        await SaveOrderAsync(order);
30        
31        // Publish event - decoupled from consumers
32        await _publisher.PublishAsync(new OrderCreatedEvent(
33            order.Id, 
34            order.CustomerId, 
35            order.TotalAmount, 
36            order.CreatedAt));
37        
38        return order;
39    }
40}
41
42// Consumer services
43public class InventoryService : IConsumer<OrderCreatedEvent>
44{
45    public async Task ConsumeAsync(OrderCreatedEvent @event)
46    {
47        // Reserve inventory items
48        await ReserveInventoryAsync(@event.OrderId);
49        Console.WriteLine($"Inventory reserved for order {@event.OrderId}");
50    }
51}
52
53public class EmailService : IConsumer<OrderCreatedEvent>
54{
55    public async Task ConsumeAsync(OrderCreatedEvent @event)
56    {
57        // Send confirmation email
58        await SendOrderConfirmationAsync(@event.CustomerId, @event.OrderId);
59        Console.WriteLine($"Order confirmation sent for {@event.OrderId}");
60    }
61}

2. Event Sourcing Pattern

Instead of storing current state, store the sequence of events that led to that state.

1// Domain events
2public abstract record DomainEvent(Guid AggregateId, DateTime OccurredAt);
3
4public record AccountOpenedEvent(Guid AccountId, string AccountNumber, decimal InitialBalance, DateTime OccurredAt) 
5    : DomainEvent(AccountId, OccurredAt);
6
7public record MoneyDepositedEvent(Guid AccountId, decimal Amount, DateTime OccurredAt) 
8    : DomainEvent(AccountId, OccurredAt);
9
10public record MoneyWithdrawnEvent(Guid AccountId, decimal Amount, DateTime OccurredAt) 
11    : DomainEvent(AccountId, OccurredAt);
12
13// Aggregate that builds state from events
14public class BankAccount
15{
16    public Guid Id { get; private set; }
17    public string AccountNumber { get; private set; } = string.Empty;
18    public decimal Balance { get; private set; }
19    public DateTime CreatedAt { get; private set; }
20    
21    private readonly List<DomainEvent> _uncommittedEvents = new();
22    
23    // Create account from events (Event Sourcing)
24    public static BankAccount FromEvents(IEnumerable<DomainEvent> events)
25    {
26        var account = new BankAccount();
27        foreach (var @event in events)
28        {
29            account.Apply(@event);
30        }
31        return account;
32    }
33    
34    // Apply events to build state
35    private void Apply(DomainEvent @event)
36    {
37        switch (@event)
38        {
39            case AccountOpenedEvent opened:
40                Id = opened.AccountId;
41                AccountNumber = opened.AccountNumber;
42                Balance = opened.InitialBalance;
43                CreatedAt = opened.OccurredAt;
44                break;
45                
46            case MoneyDepositedEvent deposited:
47                Balance += deposited.Amount;
48                break;
49                
50            case MoneyWithdrawnEvent withdrawn:
51                Balance -= withdrawn.Amount;
52                break;
53        }
54    }
55    
56    // Business operations that produce events
57    public void Deposit(decimal amount)
58    {
59        if (amount <= 0) 
60            throw new ArgumentException("Amount must be positive");
61            
62        var @event = new MoneyDepositedEvent(Id, amount, DateTime.UtcNow);
63        Apply(@event);
64        _uncommittedEvents.Add(@event);
65    }
66    
67    public void Withdraw(decimal amount)
68    {
69        if (amount <= 0) 
70            throw new ArgumentException("Amount must be positive");
71        if (Balance < amount) 
72            throw new InvalidOperationException("Insufficient funds");
73            
74        var @event = new MoneyWithdrawnEvent(Id, amount, DateTime.UtcNow);
75        Apply(@event);
76        _uncommittedEvents.Add(@event);
77    }
78    
79    public IReadOnlyList<DomainEvent> GetUncommittedEvents() => _uncommittedEvents.AsReadOnly();
80    
81    public void MarkEventsAsCommitted() => _uncommittedEvents.Clear();
82}
83
84// Event store implementation
85public interface IEventStore
86{
87    Task SaveEventsAsync(Guid aggregateId, IEnumerable<DomainEvent> events);
88    Task<IEnumerable<DomainEvent>> GetEventsAsync(Guid aggregateId);
89}
90
91public class EventStore : IEventStore
92{
93    private readonly IDbContext _context;
94    
95    public async Task SaveEventsAsync(Guid aggregateId, IEnumerable<DomainEvent> events)
96    {
97        foreach (var @event in events)
98        {
99            var eventData = new EventData
100            {
101                AggregateId = aggregateId,
102                EventType = @event.GetType().Name,
103                EventData = JsonSerializer.Serialize(@event),
104                OccurredAt = @event.OccurredAt
105            };
106            
107            _context.Events.Add(eventData);
108        }
109        
110        await _context.SaveChangesAsync();
111    }
112    
113    public async Task<IEnumerable<DomainEvent>> GetEventsAsync(Guid aggregateId)
114    {
115        var events = await _context.Events
116            .Where(e => e.AggregateId == aggregateId)
117            .OrderBy(e => e.OccurredAt)
118            .ToListAsync();
119            
120        return events.Select(DeserializeEvent);
121    }
122}

3. CQRS (Command Query Responsibility Segregation)

CQRS is often combined with event-driven patterns to handle command and query operations separately.

1// Commands (Write Operations)
2public record CreateProductCommand(string Name, decimal Price, int StockQuantity);
3public record UpdateProductPriceCommand(Guid ProductId, decimal NewPrice);
4
5// Queries (Read Operations)  
6public record GetProductQuery(Guid ProductId);
7public record SearchProductsQuery(string SearchTerm, int PageSize, int Page);
8
9// Command Handler
10public class ProductCommandHandler
11{
12    private readonly IEventStore _eventStore;
13    private readonly IMessagePublisher _publisher;
14    
15    public async Task<Guid> HandleAsync(CreateProductCommand command)
16    {
17        var productId = Guid.NewGuid();
18        var @event = new ProductCreatedEvent(
19            productId, 
20            command.Name, 
21            command.Price, 
22            command.StockQuantity,
23            DateTime.UtcNow);
24        
25        await _eventStore.SaveEventsAsync(productId, new[] { @event });
26        await _publisher.PublishAsync(@event);
27        
28        return productId;
29    }
30}
31
32// Query Handler (reads from optimized read models)
33public class ProductQueryHandler
34{
35    private readonly IProductReadRepository _readRepository;
36    
37    public async Task<ProductReadModel?> HandleAsync(GetProductQuery query)
38    {
39        return await _readRepository.GetByIdAsync(query.ProductId);
40    }
41    
42    public async Task<PagedResult<ProductReadModel>> HandleAsync(SearchProductsQuery query)
43    {
44        return await _readRepository.SearchAsync(query.SearchTerm, query.Page, query.PageSize);
45    }
46}
47
48// Read Model Projector (updates read models from events)
49public class ProductProjector : IConsumer<ProductCreatedEvent>, IConsumer<ProductPriceUpdatedEvent>
50{
51    private readonly IProductReadRepository _readRepository;
52    
53    public async Task ConsumeAsync(ProductCreatedEvent @event)
54    {
55        var readModel = new ProductReadModel
56        {
57            Id = @event.ProductId,
58            Name = @event.Name,
59            Price = @event.Price,
60            StockQuantity = @event.StockQuantity,
61            CreatedAt = @event.OccurredAt
62        };
63        
64        await _readRepository.SaveAsync(readModel);
65    }
66    
67    public async Task ConsumeAsync(ProductPriceUpdatedEvent @event)
68    {
69        var product = await _readRepository.GetByIdAsync(@event.ProductId);
70        if (product != null)
71        {
72            product.Price = @event.NewPrice;
73            product.UpdatedAt = @event.OccurredAt;
74            await _readRepository.UpdateAsync(product);
75        }
76    }
77}

4. Saga Pattern (Process Manager)

Manages long-running business processes that span multiple services.

1// Saga state
2public class OrderProcessingSaga
3{
4    public Guid OrderId { get; set; }
5    public Guid CustomerId { get; set; }
6    public decimal TotalAmount { get; set; }
7    public bool PaymentProcessed { get; set; }
8    public bool InventoryReserved { get; set; }
9    public bool ShippingScheduled { get; set; }
10    public SagaStatus Status { get; set; } = SagaStatus.Started;
11}
12
13public enum SagaStatus
14{
15    Started,
16    PaymentProcessing,
17    InventoryReserving,
18    Shipping,
19    Completed,
20    Failed,
21    Compensating
22}
23
24// Saga implementation
25public class OrderProcessingSagaHandler : 
26    IConsumer<OrderCreatedEvent>,
27    IConsumer<PaymentProcessedEvent>,
28    IConsumer<PaymentFailedEvent>,
29    IConsumer<InventoryReservedEvent>,
30    IConsumer<InventoryReservationFailedEvent>,
31    IConsumer<ShippingScheduledEvent>
32{
33    private readonly ISagaRepository _sagaRepository;
34    private readonly IMessagePublisher _publisher;
35    
36    public async Task ConsumeAsync(OrderCreatedEvent @event)
37    {
38        var saga = new OrderProcessingSaga
39        {
40            OrderId = @event.OrderId,
41            CustomerId = @event.CustomerId,
42            TotalAmount = @event.TotalAmount,
43            Status = SagaStatus.PaymentProcessing
44        };
45        
46        await _sagaRepository.SaveAsync(saga);
47        
48        // Start the saga by requesting payment
49        await _publisher.PublishAsync(new ProcessPaymentCommand(
50            @event.OrderId, 
51            @event.CustomerId, 
52            @event.TotalAmount));
53    }
54    
55    public async Task ConsumeAsync(PaymentProcessedEvent @event)
56    {
57        var saga = await _sagaRepository.GetByOrderIdAsync(@event.OrderId);
58        saga.PaymentProcessed = true;
59        saga.Status = SagaStatus.InventoryReserving;
60        
61        await _sagaRepository.SaveAsync(saga);
62        
63        // Next step: reserve inventory
64        await _publisher.PublishAsync(new ReserveInventoryCommand(@event.OrderId));
65    }
66    
67    public async Task ConsumeAsync(PaymentFailedEvent @event)
68    {
69        var saga = await _sagaRepository.GetByOrderIdAsync(@event.OrderId);
70        saga.Status = SagaStatus.Failed;
71        
72        await _sagaRepository.SaveAsync(saga);
73        
74        // Publish order failed event
75        await _publisher.PublishAsync(new OrderFailedEvent(@event.OrderId, "Payment failed"));
76    }
77    
78    public async Task ConsumeAsync(InventoryReservedEvent @event)
79    {
80        var saga = await _sagaRepository.GetByOrderIdAsync(@event.OrderId);
81        saga.InventoryReserved = true;
82        saga.Status = SagaStatus.Shipping;
83        
84        await _sagaRepository.SaveAsync(saga);
85        
86        // Final step: schedule shipping
87        await _publisher.PublishAsync(new ScheduleShippingCommand(@event.OrderId));
88    }
89    
90    public async Task ConsumeAsync(InventoryReservationFailedEvent @event)
91    {
92        var saga = await _sagaRepository.GetByOrderIdAsync(@event.OrderId);
93        saga.Status = SagaStatus.Compensating;
94        
95        await _sagaRepository.SaveAsync(saga);
96        
97        // Compensate: refund payment
98        await _publisher.PublishAsync(new RefundPaymentCommand(@event.OrderId, saga.TotalAmount));
99    }
100    
101    public async Task ConsumeAsync(ShippingScheduledEvent @event)
102    {
103        var saga = await _sagaRepository.GetByOrderIdAsync(@event.OrderId);
104        saga.ShippingScheduled = true;
105        saga.Status = SagaStatus.Completed;
106        
107        await _sagaRepository.SaveAsync(saga);
108        
109        // Order processing complete
110        await _publisher.PublishAsync(new OrderCompletedEvent(@event.OrderId));
111    }
112}

Implementation with MassTransit and RabbitMQ

MassTransit with RabbitMQ is a popular choice for implementing event-driven architecture in .NET applications. Here's a practical setup:

1// Program.cs - Service configuration
2public class Program
3{
4    public static void Main(string[] args)
5    {
6        var builder = WebApplication.CreateBuilder(args);
7        
8        // Add MassTransit with RabbitMQ
9        builder.Services.AddMassTransit(x =>
10        {
11            // Register consumers
12            x.AddConsumer<OrderCreatedEventConsumer>();
13            x.AddConsumer<InventoryConsumer>();
14            x.AddConsumer<EmailConsumer>();
15            x.AddConsumer<OrderProcessingSagaHandler>();
16            
17            x.UsingRabbitMq((context, cfg) =>
18            {
19                cfg.Host("localhost", "/", h =>
20                {
21                    h.Username("guest");
22                    h.Password("guest");
23                });
24                
25                // Configure endpoints
26                cfg.ReceiveEndpoint("order-events", e =>
27                {
28                    e.ConfigureConsumer<OrderCreatedEventConsumer>(context);
29                });
30                
31                cfg.ReceiveEndpoint("inventory-events", e =>
32                {
33                    e.ConfigureConsumer<InventoryConsumer>(context);
34                });
35                
36                cfg.ReceiveEndpoint("email-events", e =>
37                {
38                    e.ConfigureConsumer<EmailConsumer>(context);
39                });
40                
41                cfg.ReceiveEndpoint("order-saga", e =>
42                {
43                    e.ConfigureConsumer<OrderProcessingSagaHandler>(context);
44                });
45            });
46        });
47        
48        var app = builder.Build();
49        app.Run();
50    }
51}
52
53// Message publisher interface
54public interface IMessagePublisher
55{
56    Task PublishAsync<T>(T message) where T : class;
57}
58
59// MassTransit implementation
60public class MassTransitMessagePublisher : IMessagePublisher
61{
62    private readonly IPublishEndpoint _publishEndpoint;
63    
64    public MassTransitMessagePublisher(IPublishEndpoint publishEndpoint)
65    {
66        _publishEndpoint = publishEndpoint;
67    }
68    
69    public async Task PublishAsync<T>(T message) where T : class
70    {
71        await _publishEndpoint.Publish(message);
72    }
73}

Event-Driven Architecture with Azure Service Bus

For cloud-native applications, Azure Service Bus provides enterprise-grade messaging:

1// Azure Service Bus configuration
2public static class ServiceBusExtensions
3{
4    public static IServiceCollection AddAzureServiceBus(
5        this IServiceCollection services, 
6        string connectionString)
7    {
8        services.AddMassTransit(x =>
9        {
10            x.AddConsumer<OrderCreatedEventConsumer>();
11            x.AddConsumer<PaymentConsumer>();
12            
13            x.UsingAzureServiceBus((context, cfg) =>
14            {
15                cfg.Host(connectionString);
16                
17                cfg.SubscriptionEndpoint<OrderCreatedEvent>("order-processing", e =>
18                {
19                    e.ConfigureConsumer<OrderCreatedEventConsumer>(context);
20                });
21                
22                cfg.SubscriptionEndpoint<PaymentProcessedEvent>("payment-processing", e =>
23                {
24                    e.ConfigureConsumer<PaymentConsumer>(context);
25                });
26            });
27        });
28        
29        return services;
30    }
31}
32
33// Usage in Program.cs
34builder.Services.AddAzureServiceBus(builder.Configuration.GetConnectionString("ServiceBus"));

Handling Event Processing Challenges

The Outbox Pattern

The Outbox Pattern ensures reliable event publishing by storing events in the same database transaction as business data:

1public class OutboxEvent
2{
3    public Guid Id { get; set; }
4    public string EventType { get; set; } = string.Empty;
5    public string EventData { get; set; } = string.Empty;
6    public DateTime CreatedAt { get; set; }
7    public bool Published { get; set; }
8}
9
10public class OrderService
11{
12    private readonly IDbContext _context;
13    
14    public async Task<Order> CreateOrderAsync(CreateOrderRequest request)
15    {
16        using var transaction = await _context.Database.BeginTransactionAsync();
17        
18        try
19        {
20            // Save business data
21            var order = new Order { /* ... */ };
22            _context.Orders.Add(order);
23            
24            // Save event to outbox (same transaction)
25            var eventData = new OrderCreatedEvent(order.Id, order.CustomerId, order.TotalAmount, DateTime.UtcNow);
26            var outboxEvent = new OutboxEvent
27            {
28                Id = Guid.NewGuid(),
29                EventType = nameof(OrderCreatedEvent),
30                EventData = JsonSerializer.Serialize(eventData),
31                CreatedAt = DateTime.UtcNow,
32                Published = false
33            };
34            
35            _context.OutboxEvents.Add(outboxEvent);
36            
37            await _context.SaveChangesAsync();
38            await transaction.CommitAsync();
39            
40            return order;
41        }
42        catch
43        {
44            await transaction.RollbackAsync();
45            throw;
46        }
47    }
48}
49
50// Background service to publish outbox events
51public class OutboxPublisher : BackgroundService
52{
53    private readonly IServiceProvider _serviceProvider;
54    
55    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
56    {
57        while (!stoppingToken.IsCancellationRequested)
58        {
59            using var scope = _serviceProvider.CreateScope();
60            var context = scope.ServiceProvider.GetRequiredService<IDbContext>();
61            var publisher = scope.ServiceProvider.GetRequiredService<IMessagePublisher>();
62            
63            var unpublishedEvents = await context.OutboxEvents
64                .Where(e => !e.Published)
65                .OrderBy(e => e.CreatedAt)
66                .Take(100)
67                .ToListAsync(stoppingToken);
68            
69            foreach (var outboxEvent in unpublishedEvents)
70            {
71                try
72                {
73                    var eventData = DeserializeEvent(outboxEvent);
74                    await publisher.PublishAsync(eventData);
75                    
76                    outboxEvent.Published = true;
77                    await context.SaveChangesAsync(stoppingToken);
78                }
79                catch (Exception ex)
80                {
81                    // Log error and continue with next event
82                    // Consider implementing retry logic with exponential backoff
83                }
84            }
85            
86            await Task.Delay(TimeSpan.FromSeconds(5), stoppingToken);
87        }
88    }
89}

Idempotency and Deduplication

1public class IdempotentConsumer<T> : IConsumer<T> where T : class, IEventWithId
2{
3    private readonly IProcessedEventStore _processedEventStore;
4    private readonly IConsumer<T> _innerConsumer;
5    
6    public async Task ConsumeAsync(T @event)
7    {
8        // Check if already processed
9        if (await _processedEventStore.IsProcessedAsync(@event.EventId))
10        {
11            // Event already processed, skip
12            return;
13        }
14        
15        try
16        {
17            // Process the event
18            await _innerConsumer.ConsumeAsync(@event);
19            
20            // Mark as processed
21            await _processedEventStore.MarkAsProcessedAsync(@event.EventId);
22        }
23        catch (Exception)
24        {
25            // Don't mark as processed on failure
26            throw;
27        }
28    }
29}
30
31public interface IEventWithId
32{
33    Guid EventId { get; }
34}
35
36public record OrderCreatedEvent(
37    Guid EventId, // Add event ID for idempotency
38    Guid OrderId, 
39    Guid CustomerId, 
40    decimal TotalAmount, 
41    DateTime CreatedAt) : IEventWithId;

Real-World Industry Applications

E-Commerce Platform

Modern e-commerce platforms like Amazon use event-driven architectures extensively:

  • Order Processing: Order created → Payment processed → Inventory reserved → Shipping scheduled
  • Recommendation Engine: User behavior events → Real-time personalization
  • Inventory Management: Stock level changes → Automatic reordering

Financial Services

Banks and fintech companies rely on event-driven patterns:

  • Transaction Processing: Account debited → Real-time balance updates → Fraud detection
  • Risk Management: Market events → Portfolio rebalancing → Compliance reporting
  • Customer Onboarding: KYC completed → Account activation → Welcome campaign

Streaming Platforms

Netflix, Spotify, and similar platforms use events for:

  • Content Recommendations: View completed → Update user preferences → Generate recommendations
  • Analytics: User interactions → Real-time dashboards → Content optimization
  • Content Distribution: New content uploaded → Global CDN distribution → User notifications

Architecture Deployment Patterns

Centralized Event Hub

Centralized event hub patterns help ensure scalability and fault tolerance in event-driven systems:

1// Centralized event hub configuration
2public class CentralizedEventHub
3{
4    private readonly IServiceBusClient _serviceBusClient;
5    private readonly Dictionary<Type, string> _eventTopics;
6    
7    public async Task PublishAsync<T>(T @event) where T : class
8    {
9        var topicName = _eventTopics[typeof(T)];
10        await _serviceBusClient.PublishAsync(topicName, @event);
11    }
12    
13    public async Task SubscribeAsync<T>(string subscriptionName, Func<T, Task> handler) where T : class
14    {
15        var topicName = _eventTopics[typeof(T)];
16        await _serviceBusClient.SubscribeAsync(topicName, subscriptionName, handler);
17    }
18}

Decentralized Event Mesh

For large organizations, event mesh provides distributed event routing:

1// Event mesh configuration
2public class EventMeshConfiguration
3{
4    public Dictionary<string, ServiceEndpoint> Services { get; set; } = new();
5    public Dictionary<string, List<string>> EventRouting { get; set; } = new();
6}
7
8public class EventMeshRouter
9{
10    private readonly EventMeshConfiguration _config;
11    
12    public async Task RouteEventAsync<T>(T @event, string sourceService) where T : class
13    {
14        var eventType = typeof(T).Name;
15        
16        if (_config.EventRouting.TryGetValue(eventType, out var targetServices))
17        {
18            var tasks = targetServices.Select(service => 
19                ForwardEventToServiceAsync(@event, service));
20                
21            await Task.WhenAll(tasks);
22        }
23    }
24}

Best Practices and Common Pitfalls

Do's

  1. Design Events as Business Facts: Events should represent meaningful business occurrences
  2. Make Events Immutable: Once published, events should never change
  3. Include Event Metadata: Timestamps, correlation IDs, event versions
  4. Plan for Schema Evolution: Use backward-compatible event schemas
  5. Implement Proper Error Handling: Dead letter queues, retry policies, circuit breakers

Don'ts

  1. Don't Include Sensitive Data: Events may be logged or replicated
  2. Don't Make Events Too Granular: Avoid event spam
  3. Don't Ignore Ordering: Some business processes require event ordering
  4. Don't Forget About Monitoring: Track event flow and processing times

Performance Considerations

1// Batch processing for high-throughput scenarios
2public class BatchEventProcessor<T> : IConsumer<T> where T : class
3{
4    private readonly Channel<T> _channel = Channel.CreateUnbounded<T>();
5    private readonly Timer _batchTimer;
6    private readonly List<T> _batch = new();
7    private const int BatchSize = 100;
8    private const int BatchTimeoutMs = 5000;
9    
10    public async Task ConsumeAsync(T @event)
11    {
12        await _channel.Writer.WriteAsync(@event);
13    }
14    
15    private async Task ProcessBatch()
16    {
17        while (await _channel.Reader.WaitToReadAsync())
18        {
19            while (_batch.Count < BatchSize && _channel.Reader.TryRead(out var item))
20            {
21                _batch.Add(item);
22            }
23            
24            if (_batch.Any())
25            {
26                await ProcessBatchAsync(_batch.ToList());
27                _batch.Clear();
28            }
29        }
30    }
31    
32    private async Task ProcessBatchAsync(List<T> events)
33    {
34        // Process events in batch for better performance
35        // Database bulk operations, batch API calls, etc.
36    }
37}

Testing Event-Driven Systems

1[TestClass]
2public class EventDrivenSystemTests
3{
4    [TestMethod]
5    public async Task OrderCreated_ShouldTriggerInventoryReservation()
6    {
7        // Arrange
8        var eventBus = new InMemoryEventBus();
9        var inventoryService = new InventoryService(eventBus);
10        var orderService = new OrderService(eventBus);
11        
12        var orderRequest = new CreateOrderRequest
13        {
14            CustomerId = Guid.NewGuid(),
15            Items = new[] { new OrderItem { ProductId = Guid.NewGuid(), Quantity = 1 } }
16        };
17        
18        // Act
19        await orderService.CreateOrderAsync(orderRequest);
20        
21        // Assert
22        var publishedEvents = eventBus.GetPublishedEvents<InventoryReservationRequestedEvent>();
23        Assert.AreEqual(1, publishedEvents.Count());
24    }
25    
26    [TestMethod]
27    public async Task EventProcessing_ShouldBeIdempotent()
28    {
29        // Arrange
30        var consumer = new OrderCreatedEventConsumer();
31        var orderEvent = new OrderCreatedEvent(Guid.NewGuid(), Guid.NewGuid(), Guid.NewGuid(), 100m, DateTime.UtcNow);
32        
33        // Act - Process same event twice
34        await consumer.ConsumeAsync(orderEvent);
35        await consumer.ConsumeAsync(orderEvent);
36        
37        // Assert - Should only process once
38        Assert.AreEqual(1, consumer.ProcessedOrdersCount);
39    }
40}
41
42// Test doubles for event bus
43public class InMemoryEventBus : IMessagePublisher
44{
45    private readonly List<object> _publishedEvents = new();
46    
47    public async Task PublishAsync<T>(T message) where T : class
48    {
49        _publishedEvents.Add(message);
50        await Task.CompletedTask;
51    }
52    
53    public IEnumerable<T> GetPublishedEvents<T>() where T : class
54    {
55        return _publishedEvents.OfType<T>();
56    }
57}

Monitoring and Observability

1// Event tracing and metrics
2public class EventMetrics
3{
4    private readonly IMetricsLogger _metrics;
5    
6    public async Task TrackEventProcessed<T>(T @event, TimeSpan processingTime)
7    {
8        _metrics.Counter("events_processed_total")
9            .WithTag("event_type", typeof(T).Name)
10            .Increment();
11            
12        _metrics.Histogram("event_processing_duration_ms")
13            .WithTag("event_type", typeof(T).Name)
14            .Record(processingTime.TotalMilliseconds);
15    }
16    
17    public async Task TrackEventFailed<T>(T @event, Exception ex)
18    {
19        _metrics.Counter("events_failed_total")
20            .WithTag("event_type", typeof(T).Name)
21            .WithTag("error_type", ex.GetType().Name)
22            .Increment();
23    }
24}
25
26// Distributed tracing with correlation IDs
27public record EventMetadata(
28    string CorrelationId,
29    string CausationId,
30    DateTime Timestamp,
31    string Source);
32
33public abstract record BaseEvent(EventMetadata Metadata);
34
35public record OrderCreatedEvent(
36    EventMetadata Metadata,
37    Guid OrderId,
38    Guid CustomerId,
39    decimal TotalAmount) : BaseEvent(Metadata);
40
41// Correlation tracking middleware
42public class EventCorrelationMiddleware<T> : IConsumer<T> where T : BaseEvent
43{
44    private readonly IConsumer<T> _next;
45    private readonly ILogger<EventCorrelationMiddleware<T>> _logger;
46    
47    public async Task ConsumeAsync(T @event)
48    {
49        using var scope = _logger.BeginScope(new Dictionary<string, object>
50        {
51            ["CorrelationId"] = @event.Metadata.CorrelationId,
52            ["CausationId"] = @event.Metadata.CausationId,
53            ["EventType"] = typeof(T).Name,
54            ["Source"] = @event.Metadata.Source
55        });
56        
57        _logger.LogInformation("Processing event {EventType} with correlation {CorrelationId}", 
58            typeof(T).Name, @event.Metadata.CorrelationId);
59        
60        await _next.ConsumeAsync(@event);
61        
62        _logger.LogInformation("Completed processing event {EventType}", typeof(T).Name);
63    }
64}

Migration Strategies

Moving from monolithic to event-driven architecture requires careful planning:

Strangler Fig Pattern

1// Legacy service adapter
2public class LegacyOrderServiceAdapter : IOrderService
3{
4    private readonly ILegacyOrderSystem _legacySystem;
5    private readonly IModernEventBus _eventBus;
6    private readonly IConfiguration _config;
7    
8    public async Task<Order> CreateOrderAsync(CreateOrderRequest request)
9    {
10        // Determine routing based on feature flags
11        if (_config.GetValue<bool>("UseModernOrderProcessing"))
12        {
13            return await CreateOrderModernAsync(request);
14        }
15        
16        // Fallback to legacy system
17        var legacyOrder = await _legacySystem.CreateOrderAsync(request);
18        
19        // Publish event for new consumers while maintaining legacy flow
20        await _eventBus.PublishAsync(new OrderCreatedEvent(
21            CreateEventMetadata(),
22            legacyOrder.Id,
23            legacyOrder.CustomerId,
24            legacyOrder.TotalAmount));
25            
26        return legacyOrder;
27    }
28    
29    private async Task<Order> CreateOrderModernAsync(CreateOrderRequest request)
30    {
31        // New event-driven implementation
32        var order = new Order { /* ... */ };
33        await _eventBus.PublishAsync(new OrderCreatedEvent(
34            CreateEventMetadata(),
35            order.Id,
36            order.CustomerId,
37            order.TotalAmount));
38            
39        return order;
40    }
41}

Event Bridge Pattern

1// Bridge between legacy and modern systems
2public class LegacyEventBridge : IConsumer<OrderCreatedEvent>
3{
4    private readonly ILegacyNotificationSystem _legacyNotifications;
5    private readonly ILegacyInventorySystem _legacyInventory;
6    
7    public async Task ConsumeAsync(OrderCreatedEvent @event)
8    {
9        // Translate modern events to legacy system calls
10        await _legacyNotifications.SendOrderConfirmationAsync(
11            @event.CustomerId, 
12            @event.OrderId);
13            
14        await _legacyInventory.ReserveInventoryAsync(@event.OrderId);
15    }
16}

Security Considerations

Event-driven systems require specific security measures:

1// Event encryption and signing
2public class SecureEventPublisher : IMessagePublisher
3{
4    private readonly IMessagePublisher _innerPublisher;
5    private readonly IEventEncryption _encryption;
6    private readonly IEventSigning _signing;
7    
8    public async Task PublishAsync<T>(T message) where T : class
9    {
10        // Sign the event for integrity
11        var signedMessage = await _signing.SignAsync(message);
12        
13        // Encrypt sensitive data
14        var encryptedMessage = await _encryption.EncryptAsync(signedMessage);
15        
16        await _innerPublisher.PublishAsync(encryptedMessage);
17    }
18}
19
20// Event authorization
21public class AuthorizedEventConsumer<T> : IConsumer<T> where T : class
22{
23    private readonly IConsumer<T> _innerConsumer;
24    private readonly IEventAuthorizationService _authService;
25    
26    public async Task ConsumeAsync(T @event)
27    {
28        // Verify consumer has permission to process this event type
29        if (!await _authService.CanConsumeAsync<T>(GetCurrentServiceIdentity()))
30        {
31            throw new UnauthorizedAccessException($"Service not authorized to consume {typeof(T).Name}");
32        }
33        
34        await _innerConsumer.ConsumeAsync(@event);
35    }
36}

Advanced Patterns

Event Sourcing with Snapshots

1public class SnapshotEventStore : IEventStore
2{
3    private readonly IEventStore _eventStore;
4    private readonly ISnapshotStore _snapshotStore;
5    
6    public async Task<T> GetAggregateAsync<T>(Guid aggregateId) where T : AggregateRoot, new()
7    {
8        // Try to load from snapshot first
9        var snapshot = await _snapshotStore.GetSnapshotAsync<T>(aggregateId);
10        var fromVersion = 0;
11        T aggregate;
12        
13        if (snapshot != null)
14        {
15            aggregate = snapshot.Aggregate;
16            fromVersion = snapshot.Version;
17        }
18        else
19        {
20            aggregate = new T();
21        }
22        
23        // Load events since snapshot
24        var events = await _eventStore.GetEventsAsync(aggregateId, fromVersion);
25        aggregate.LoadFromHistory(events);
26        
27        return aggregate;
28    }
29    
30    public async Task SaveAggregateAsync<T>(T aggregate) where T : AggregateRoot
31    {
32        await _eventStore.SaveEventsAsync(aggregate.Id, aggregate.GetUncommittedEvents());
33        
34        // Create snapshot every N events
35        if (aggregate.Version % 100 == 0)
36        {
37            var snapshot = new AggregateSnapshot<T>
38            {
39                AggregateId = aggregate.Id,
40                Aggregate = aggregate,
41                Version = aggregate.Version,
42                CreatedAt = DateTime.UtcNow
43            };
44            
45            await _snapshotStore.SaveSnapshotAsync(snapshot);
46        }
47        
48        aggregate.MarkEventsAsCommitted();
49    }
50}

Event Replay and Recovery

1public class EventReplayService
2{
3    private readonly IEventStore _eventStore;
4    private readonly IMessagePublisher _publisher;
5    
6    public async Task ReplayEventsAsync(DateTime fromDate, DateTime? toDate = null)
7    {
8        var endDate = toDate ?? DateTime.UtcNow;
9        var events = await _eventStore.GetEventsByDateRangeAsync(fromDate, endDate);
10        
11        foreach (var @event in events)
12        {
13            try
14            {
15                await _publisher.PublishAsync(@event);
16                await Task.Delay(10); // Small delay to prevent overwhelming consumers
17            }
18            catch (Exception ex)
19            {
20                // Log error but continue with other events
21                Console.WriteLine($"Failed to replay event {@event.GetType().Name}: {ex.Message}");
22            }
23        }
24    }
25    
26    public async Task RecoverFromEventStoreAsync<T>(Guid aggregateId) where T : AggregateRoot, new()
27    {
28        var events = await _eventStore.GetEventsAsync(aggregateId);
29        var aggregate = new T();
30        aggregate.LoadFromHistory(events);
31        
32        // Republish domain events for any missing projections
33        foreach (var domainEvent in aggregate.GetUncommittedEvents())
34        {
35            await _publisher.PublishAsync(domainEvent);
36        }
37    }
38}

Performance Optimization

Event Streaming and Batching

1public class HighThroughputEventProcessor
2{
3    private readonly IEventProcessor _processor;
4    private readonly SemaphoreSlim _semaphore;
5    
6    public HighThroughputEventProcessor(int maxConcurrency = 100)
7    {
8        _semaphore = new SemaphoreSlim(maxConcurrency, maxConcurrency);
9    }
10    
11    public async Task ProcessEventStreamAsync(IAsyncEnumerable<DomainEvent> eventStream)
12    {
13        var tasks = new List<Task>();
14        
15        await foreach (var @event in eventStream)
16        {
17            await _semaphore.WaitAsync();
18            
19            tasks.Add(ProcessEventAsync(@event));
20            
21            // Clean up completed tasks
22            if (tasks.Count > 1000)
23            {
24                var completedTasks = tasks.Where(t => t.IsCompleted).ToList();
25                foreach (var completed in completedTasks)
26                {
27                    tasks.Remove(completed);
28                }
29            }
30        }
31        
32        // Wait for all remaining tasks
33        await Task.WhenAll(tasks);
34    }
35    
36    private async Task ProcessEventAsync(DomainEvent @event)
37    {
38        try
39        {
40            await _processor.ProcessAsync(@event);
41        }
42        finally
43        {
44            _semaphore.Release();
45        }
46    }
47}

Event-Driven Architecture Governance

Event Schema Registry

1public interface IEventSchemaRegistry
2{
3    Task<EventSchema> GetSchemaAsync(string eventType, int version);
4    Task RegisterSchemaAsync(EventSchema schema);
5    Task<bool> IsCompatibleAsync(EventSchema newSchema, EventSchema oldSchema);
6}
7
8public class EventSchema
9{
10    public string EventType { get; set; } = string.Empty;
11    public int Version { get; set; }
12    public string JsonSchema { get; set; } = string.Empty;
13    public CompatibilityLevel Compatibility { get; set; }
14    public DateTime CreatedAt { get; set; }
15}
16
17public enum CompatibilityLevel
18{
19    None,
20    Backward,
21    Forward,
22    Full
23}
24
25// Event versioning example
26public abstract record BaseOrderEvent(EventMetadata Metadata) : BaseEvent(Metadata);
27
28public record OrderCreatedEventV1(
29    EventMetadata Metadata,
30    Guid OrderId,
31    Guid CustomerId,
32    decimal TotalAmount
33) : BaseOrderEvent(Metadata);
34
35public record OrderCreatedEventV2(
36    EventMetadata Metadata,
37    Guid OrderId,
38    Guid CustomerId,
39    decimal TotalAmount,
40    string Currency,
41    List<OrderItem> Items
42) : BaseOrderEvent(Metadata);
43
44// Event version handler
45public class VersionedOrderEventConsumer : 
46    IConsumer<OrderCreatedEventV1>, 
47    IConsumer<OrderCreatedEventV2>
48{
49    public async Task ConsumeAsync(OrderCreatedEventV1 @event)
50    {
51        // Handle V1 events
52        await ProcessOrderAsync(@event.OrderId, @event.TotalAmount, "USD", new List<OrderItem>());
53    }
54    
55    public async Task ConsumeAsync(OrderCreatedEventV2 @event)
56    {
57        // Handle V2 events with full data
58        await ProcessOrderAsync(@event.OrderId, @event.TotalAmount, @event.Currency, @event.Items);
59    }
60    
61    private async Task ProcessOrderAsync(Guid orderId, decimal amount, string currency, List<OrderItem> items)
62    {
63        // Common processing logic
64    }
65}

Industry Adoption and Future Trends

Event-driven architecture has become the de facto standard for modern distributed systems. Major technology companies have shared their adoption patterns:

  • Netflix uses event-driven patterns for their recommendation engine and content delivery
  • Uber employs event sourcing for their dispatch system and surge pricing
  • Amazon leverages events throughout their entire e-commerce platform
  • Microsoft built Azure's event-driven services like Event Grid and Service Bus

Current industry trends include:

  1. Event Mesh Architectures - Distributed event routing across multiple cloud providers
  2. Real-time Analytics - Stream processing with Apache Kafka and Azure Stream Analytics
  3. Event-driven Microservices - Complete decoupling of service dependencies
  4. Serverless Event Processing - Azure Functions, AWS Lambda for event handling
  5. GraphQL Subscriptions - Real-time data updates in modern web applications

Conclusion

Event-driven architecture isn't just a technical pattern—it's a fundamental shift in how we think about building software systems. After implementing EDA in everything from small startups to enterprise systems, I can confidently say that the investment in understanding these patterns pays dividends in system reliability, scalability, and maintainability.

The key is to start small. Pick a bounded context in your application, identify the business events that matter, and begin implementing event-driven patterns there. As you gain confidence and see the benefits, you can expand the approach to other parts of your system.

Remember that event-driven architecture is not a silver bullet. It introduces complexity in terms of eventual consistency, distributed debugging, and operational overhead. However, for systems that need to scale, integrate multiple services, or respond to real-time events, it's often the most elegant solution available.

The patterns and examples we've covered—from basic pub/sub to advanced saga orchestration—represent the current state of the art in distributed systems design. As you implement these patterns in your own projects, you'll discover that event-driven thinking changes not just how you architect systems, but how you model business processes and think about software design as a whole.

Additional Resources for Mastery

To deepen your understanding of event-driven architecture and stay current with industry practices, explore these valuable resources:

Microsoft Official Documentation

  • Event-driven architecture style - Azure Architecture Center - Comprehensive guide to event-driven patterns in cloud applications
  • Azure Service Bus documentation - Learn Microsoft's enterprise messaging platform
  • Azure Event Grid documentation - Event routing service for building event-driven applications

Industry-Leading Resources

  • Martin Fowler's Event Sourcing articles - Foundational articles on event sourcing patterns from a thought leader
  • Microservices.io Event-driven Architecture - Chris Richardson's comprehensive guide to microservices patterns
  • Event Store documentation - Learn from the creators of a purpose-built event store database

Practical Implementation Guides

  • MassTransit documentation - The most popular .NET service bus framework with extensive examples
  • Building Event-Driven Microservices by Adam Bellemare - O'Reilly book covering practical implementation strategies
  • Kafka: The Definitive Guide - Essential reading for understanding distributed event streaming

Community and Advanced Learning

  • GOTO Conference talks on Event-Driven Architecture - Regular conference presentations from industry experts
  • DDD Community resources - Domain-Driven Design community discussions on event modeling
  • Microsoft's .NET Microservices eBook - Free comprehensive guide with event-driven examples

These resources will help you master both the theoretical foundations and practical implementation details needed to build world-class event-driven systems.

Tags:eventsarchitecturepatternsdistributed-systems
Published inEvent-Driven Architecture