Implementing Event Sourcing in .NET

Introduction

Event Sourcing is a pattern where the state of an application is determined by a sequence of events. In this tutorial, we will explore how to implement Event Sourcing in .NET applications to achieve auditability, scalability, and resilience.

Prerequisites

Before we begin, ensure you have the following:

• .NET SDK installed
• Visual Studio or Visual Studio Code (optional)
• Basic understanding of C# and ASP.NET Core

Setting Up the Project

Create a new ASP.NET Core project to implement Event Sourcing.

dotnet new webapi -n EventSourcingApp
cd EventSourcingApp

Defining Events

Define events that represent changes in application state.

Example Event

// Events/ProductCreatedEvent.cs
public class ProductCreatedEvent
{
    public Guid ProductId { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
}

Implementing Event Handlers

Create event handlers to process events and update the application state.

Example Event Handler

// Handlers/ProductCreatedEventHandler.cs
public class ProductCreatedEventHandler : IEventHandler<ProductCreatedEvent>
{
    public async Task Handle(ProductCreatedEvent @event)
    {
        // Logic to handle product creation event
    }
}

Implementing Event Store

Create an event store to persist events.

Example Event Store

// EventStore.cs
public class EventStore
{
    public async Task SaveEventAsync(Event @event)
    {
        // Logic to save event to storage
    }

    public async Task

// Aggregates/ProductAggregate.cs
public class ProductAggregate
{
    public Guid Id { get; private set; }
    public string Name { get; private set; }
    public decimal Price { get; private set; }

    private List<Event> _changes = new List<Event>();

    public void Apply(ProductCreatedEvent @event)
    {
        Id = @event.ProductId;
        Name = @event.Name;
        Price = @event.Price;
    }

    public List<Event> GetChanges()
    {
        return _changes;
    }

    public void ClearChanges()
    {
        _changes.Clear();
    }
}

// Startup.cs
public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<IEventStore, EventStore>();
    services.AddTransient<IEventHandler<ProductCreatedEvent>, ProductCreatedEventHandler>();
    services.AddSingleton<ProductAggregate>();
}