Add Chronicle to a console app

A console app is the smallest place to run Chronicle: no web host, no dependency-injection container, nothing between you and the client. That’s exactly why it’s the clearest place to start — every moving part is something you write explicitly, so nothing is hidden by convention. (The Worker Service and ASP.NET Core guides let the host’s DI container wire the same pieces up for you; start here if you want to see what that wiring actually does.)

We’ll build a small, familiar domain — a library — and by the end you’ll have appended events and projected them into read models you can query in MongoDB.

Before you start

Have the Chronicle kernel running locally. Run Chronicle locally brings it up with a single docker run and lists the prerequisites (.NET 8+, Docker); this guide assumes it’s listening on chronicle://localhost:35000.

You can also find the complete Console quickstart sample on GitHub.

Set up the project

Create a folder for your project, then a .NET console project inside it:

dotnet new console

Add a reference to the Chronicle client package:

dotnet add package Cratis.Chronicle

Connect the client

Everything in Chronicle is reached through a ChronicleClient. From a client you ask for the event store you want to work with — here, one named Quickstart. Because there’s no DI container to do it for you, you create the client yourself:

using Cratis.Chronicle;
using Cratis.Chronicle.Connections;

// ChronicleConnectionString.Development points at the local dev kernel on chronicle://localhost:35000
using var client = new ChronicleClient(ChronicleConnectionString.Development);
var eventStore = await client.GetEventStore("Quickstart");

ChronicleConnectionString.Development is the built-in connection string for the local development kernel — the same one new ChronicleClient() uses when you call it with no arguments. Spelling it out keeps the console version explicit; point it elsewhere with ChronicleConnectionString.Default (no credentials) or your own new ChronicleConnectionString("chronicle://…").

That single eventStore is your handle to everything that follows:

Define some events

Everything in Chronicle starts with a fact. You model facts as record types marked with [EventType] — records because an event, once it happened, never changes. The attribute is how Chronicle discovers the type; it takes no name, the type name is the identity.

Here are the facts of a small library — a book arrives, gets borrowed, and comes back:

using Cratis.Chronicle.Events;

[EventType]
public record BookAdded(string Title, string Isbn);

[EventType]
public record BookBorrowed(string MemberName);

[EventType]
public record BookReturned;

BookReturned carries no data at all — that it happened, on a particular book’s stream, is the whole story. Not every fact needs a payload.

Append them

You record a fact by appending it to an event sequence. Chronicle gives you one by default — the event log, the main sequence you’ll use, much like the main branch of a Git repository. Reach it through the event store:

var eventLog = eventStore.EventLog;

var bookId = Guid.NewGuid();
await eventLog.Append(bookId, new BookAdded("The Pragmatic Programmer", "978-0135957059"));

That first argument is the event source id — the identity of the thing this fact is about, like a primary key. Every event you append against bookId becomes part of that book’s stream of history.

Tip

We use a raw Guid here to keep the quickstart short. In real code you’d wrap it in a strongly-typed BookId so the compiler can’t confuse a book’s id with a member’s — the tutorial shows exactly that.

Run your app, then open the workbench, pick your event store, and select Sequences — your BookAdded is sitting there at sequence number 0, permanent and in order.

Turn events into a read model

Events are the write side — the source of truth. To read current state you don’t query the log directly; you let Chronicle fold the events into a read model for you. The declarative way to do that is a projection: you declare the shape you want and which events feed each field, and Chronicle keeps it in sync — no update code, ever.

using Cratis.Chronicle.Keys;
using Cratis.Chronicle.Projections.ModelBound;

[ReadModel]
public record Book(
    [Key]
    Guid Id,

    [SetFrom<BookAdded>(nameof(BookAdded.Title))]
    string Title,

    [SetFrom<BookAdded>(nameof(BookAdded.Isbn))]
    string Isbn,

    [SetValue<BookAdded>(false)]
    [SetValue<BookBorrowed>(true)]
    [SetValue<BookReturned>(false)]
    bool OnLoan,

    [SetFrom<BookBorrowed>(nameof(BookBorrowed.MemberName))]
    string? BorrowedBy);

Read the attributes as a sentence: a book’s Title and Isbn come from BookAdded; OnLoan is false when it’s added, true when borrowed, false again when returned; BorrowedBy is whoever borrowed it. You’re declaring how facts map onto the view — Chronicle replays the events in order and applies the mapping.

The projection writes Book to a store (MongoDB by default), so reading it is an ordinary query:

public class Books(IMongoCollection<Book> collection)
{
    public IEnumerable<Book> OnLoan() => collection.Find(b => b.OnLoan).ToList();
}

Append a BookBorrowed against the same bookId, query again, and OnLoan is true with BorrowedBy set; append a BookReturned and it flips back. You never wrote an UPDATE.

React when something happens

Projections build state. When you need to do something the moment a fact lands — notify someone, call another system — you write a reactor. IReactor is a marker; you just add a method whose first parameter is the event you care about, and Chronicle routes matching events to it:

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Reactors;

public class BookReturnedNotifier : IReactor
{
    public Task BookReturned(BookReturned @event, EventContext context)
    {
        // context.EventSourceId is the BookId this happened to
        Console.WriteLine($"Book {context.EventSourceId} was returned — notify the next member in line.");
        return Task.CompletedTask;
    }
}

No registration, no wiring — drop the class in and every BookReturned flows to it. Reactors must be idempotent, because the same event may be delivered more than once (during a replay or a recovery). In a real app you’d inject a notification service here — the tutorial and the Reactors guide show that, along with how reactors get their dependencies under a host.

That’s the whole loop — append → project → react. The tutorial builds exactly this library one concept at a time and explains each as you go; the Projections, Reducers, and Reactors guides go deeper on each piece.

Configure the MongoDB client

The Books query above reads documents Chronicle wrote. For your IMongoCollection<Book> to deserialize them, the MongoDB driver needs to match how Chronicle stores documents — register these conventions once at startup:

MongoDB

When leveraging the Reducer and Projection capabilities of Chronicle, your MongoDB Client needs to be configured to match how it produces documents and naming conventions. By adding the following code, you’ll have something that matches:

BsonSerializer
    .RegisterSerializer(new GuidSerializer(GuidRepresentation.Standard));

var pack = new ConventionPack
{
    // We want to ignore extra elements that might be in the documents, Chronicle adds some metadata to the documents
    new IgnoreExtraElementsConvention(true),

    // Chronicle uses camelCase for element names, so we need to use this convention
    new CamelCaseElementNameConvention()
};
ConventionRegistry.Register("conventions", pack, t => true);

Snippet source

With the conventions registered, the Books query reads the projection’s documents exactly as written.

Recap

You wired Chronicle into a bare console app by hand: created a ChronicleClient, opened the Quickstart event store, appended events for a small library domain, projected them into a Book read model with model-bound attributes, and reacted to one with a reactor. Because there was no DI container, every connection was explicit and in plain sight.

Where to go next

• Build the same domain step by step — the tutorial walks the library model one concept at a time, explaining each as you go.
• Let a host wire it up — move the same code into a Worker Service or an ASP.NET Core app and let its DI container register the artifacts for you.
• Understand the pieces — the Concepts section defines events, projections, reducers, and reactors in depth.