---
title: Event sourcing in .NET
description: "Do event sourcing in .NET with Cratis Chronicle: an open-source (MIT) event-sourcing database with a first-class .NET SDK — typed events, projections, reducers, reactors, and ASP.NET Core integration."
---

import { CardGrid, LinkCard } from '@astrojs/starlight/components';

Chronicle is an open-source (MIT) event-sourcing database and processing runtime, and .NET is its first-class client platform. The `Cratis.Chronicle` SDK gives you typed access to event stores, event logs, read models, projections, reducers, reactors, constraints, migrations, namespaces, and hosting integration — including a one-line [.NET Aspire resource](/chronicle/hosting/aspire/). You define events as C# records, append them to an event log, and Chronicle takes care of storage, processing, and delivering derived read models.

Instead of storing only the current state, every state change in your system is captured as an immutable event. That gives you a full history to audit, replay, and build new views from — while the SDK keeps the developer experience convention-based: events, reducers, reactors, and projections are discovered automatically and registered with the kernel when your application connects.

Chronicle's kernel runs on Microsoft Orleans and stores events in pluggable storage — MongoDB by default, with PostgreSQL, SQL Server, SQLite, and in-memory providers implemented in the kernel. The same store is also reachable from [TypeScript, Kotlin/Java, and Elixir clients](/chronicle-clients/) over Chronicle's language-agnostic gRPC contract, so a .NET service and services in other languages can share one event store.

## Install

```bash
dotnet add package Cratis.Chronicle
```

For ASP.NET Core applications, use the `Cratis.Chronicle.AspNetCore` package instead.

## A taste

Hook Chronicle into an ASP.NET Core application, define an event, and append it:

```csharp
var builder = WebApplication.CreateBuilder(args)
    .AddCratisChronicle(options => options.EventStore = "MyApp");

var app = builder.Build();
app.UseCratisChronicle();
app.Run();
```

```csharp
[EventType]
public record UserOnboarded(string Name, string Email);
```

```csharp
// Inject IEventLog or grab it from the event store
await eventLog.Append(Guid.NewGuid(), new UserOnboarded("Jane Doe", "jane@example.com"));
```

From there, reducers fold events into read models, reactors run side effects when events arrive, and declarative projections describe read models without writing the fold by hand.

## Where to go next

<CardGrid>
  <LinkCard title="Get started with Chronicle" description="Run Chronicle locally and build your first event-sourced feature." href="/chronicle/get-started/" />
  <LinkCard title=".NET client docs" description="Installation, connection setup, hosting, and the full .NET API surface." href="/chronicle/clients/dotnet/" />
  <LinkCard title="Chronicle on GitHub" description="The Chronicle kernel and .NET SDK — open source, MIT licensed." href="https://github.com/Cratis/Chronicle" />
  <LinkCard title="Cratis.Chronicle on NuGet" description="The .NET client package for Chronicle." href="https://www.nuget.org/packages/Cratis.Chronicle" />
</CardGrid>

Looking for another language? See [Chronicle in your language](/chronicle-clients/) for the TypeScript, Kotlin/Java, and Elixir clients.