.NET Event Sourcing Pattern with EventStore

In modern .NET applications, maintaining a reliable and scalable event-driven architecture is crucial. The Event Sourcing pattern provides a powerful way to store state changes as a sequence of events. When combined with EventStore, a purpose-built event database, it enables robust auditing, easy state reconstruction, and scalability.

This article explores the fundamentals of Event Sourcing in .NET, its advantages, how to implement it using EventStore, and best practices for ensuring optimal performance and security.

What is Event Sourcing?

Event Sourcing is a design pattern where application state changes are stored as a sequence of events rather than modifying the current state directly. Instead of persisting only the latest state of an entity, every state-changing action is recorded as an immutable event.

Benefits of Event Sourcing

• Auditability: Every state change is logged, making it easier to track application behavior.
• Scalability: Event-driven systems can scale horizontally.
• Time Travel & Debugging: Past states can be reconstructed at any point in time.
• Event-Driven Integration: Seamless integration with CQRS (Command Query Responsibility Segregation) and microservices architectures.

Setting Up EventStore for .NET Applications

Prerequisites

To work with EventStore in a .NET application, ensure you have:

• .NET 6 or later installed
• Docker (optional, for running EventStore locally)
• EventStore NuGet package

Installing EventStore

To install the EventStore client library in a .NET application, use the following command:

 dotnet add package EventStore.Client

Alternatively, run EventStore locally using Docker:

docker run --name eventstore-node -it -p 2113:2113 -p 1113:1113 eventstore/eventstore:latest

Connecting to EventStore in .NET

To interact with EventStore from a .NET application, establish a connection:

using EventStore.Client;

var settings = EventStoreClientSettings.Create("esdb://localhost:2113?tls=false");
var eventStoreClient = new EventStoreClient(settings);

Implementing Event Sourcing in .NET

Defining Event Models

Create event models representing state changes:

public record UserCreatedEvent(string UserId, string Name, string Email);
public record UserUpdatedEvent(string UserId, string Name);

Storing Events

Use the EventStoreClient to persist events:

var userCreated = new UserCreatedEvent("123", "John Doe", "john@example.com");
var eventData = new EventData(
    Uuid.NewUuid(),
    "UserCreated",
    JsonSerializer.SerializeToUtf8Bytes(userCreated)
);
await eventStoreClient.AppendToStreamAsync("user-stream-123", StreamState.Any, new[] { eventData });

Retrieving Events

To read events from EventStore:

var result = eventStoreClient.ReadStreamAsync(Direction.Forwards, "user-stream-123", StreamPosition.Start);
await foreach (var resolvedEvent in result)
{
    var eventJson = Encoding.UTF8.GetString(resolvedEvent.Event.Data.ToArray());
    Console.WriteLine(eventJson);
}

Best Practices for Using Event Sourcing

• Ensure Event Idempotency: Handle duplicate events gracefully.
• Use Snapshots: Improve performance by storing snapshots of the application state.
• Implement Event Versioning: Ensure backward compatibility for future changes.
• Monitor EventStore Health: Use logging and monitoring tools to track event store performance.

Conclusion

Event Sourcing with EventStore in .NET provides a scalable and reliable approach to managing application state changes. By leveraging immutable event logs, developers can build applications that are auditable, scalable, and resilient. Whether you're working on microservices or domain-driven design, Event Sourcing can significantly enhance your architecture.

FAQs

1. When should I use Event Sourcing in .NET applications?

Use Event Sourcing when you need strong auditing, traceability, and an event-driven architecture.

2. Can I combine Event Sourcing with a relational database?

Yes, many applications use a hybrid approach, storing events in EventStore and using SQL databases for querying.

3. Is EventStore free to use?

EventStore offers an open-source edition as well as a commercial cloud-hosted version.

4. How do I handle event schema changes?

Use event versioning techniques such as upcasting and maintaining backward compatibility.

5. What are alternatives to EventStore in .NET?

Alternatives include Kafka, Azure Event Hubs, and RabbitMQ for event streaming solutions.