Lesson 35 of 40 Architecture Expert 60 min

Event Sourcing & CQRS

Implement Event Sourcing with Marten, the Aggregate pattern, projections, and combine it with CQRS for a complete event-driven architecture.

Part 1: Event Store with Marten

builder.Services.AddMarten(opts =>
{
  opts.Connection(connectionString);
  opts.Projections.Add<OrderProjection>(ProjectionLifecycle.Inline);
}).UseLightweightSessions();

// Append events to stream
session.Events.Append(orderId, new OrderPlaced(orderId, items));
await session.SaveChangesAsync();

Part 2: Aggregate Pattern

public class Order
{
  public Guid Id { get; private set; }
  public OrderStatus Status { get; private set; }

  public void Apply(OrderPlaced e) => Status = OrderStatus.Placed;
  public void Apply(OrderShipped e) => Status = OrderStatus.Shipped;
}

Part 3: Projections for Read Models

public class OrderSummaryProjection : SingleStreamProjection<OrderSummary>
{
  public OrderSummary Create(OrderPlaced e) =>
    new() { Id = e.OrderId, Status = "Placed" };

  public void Apply(OrderShipped e, OrderSummary view)
    => view.Status = "Shipped";
}

Part 4: Event Replay & Debugging

One of Event Sourcing's biggest advantages is full auditability. In VS 2026's Marten integration:

View event stream history in the Diagnostics panel
Replay events to rebuild projections
Time-travel queries: rebuild state at any point in time
Debugging: see exact sequence of events that led to current state