Reducers

Reducers are classes that holds methods that react to certain events and produce a read model as a result. It is intended only for creating read models and not meant as a way to react and perform tasks, which is what a reactor is for.

By implementing the interface IReducerFor, it will automatically be discovered at startup and the system will figure out which methods will be handling what events by convention.

using Cratis.Chronicle.Reactors;

public class MyReducer : IReducerFor<MyReadModel>
{
}

The IReducerFor<> takes a generic argument, which is the type of the read model the reducer works with. A read model is basically a class or record that represents the state events are reduced to.

public record MyReadModel(Guid Id, string Something);

The methods you add onto the class will be discovered by convention and the system recognizes the following signatures, name of the method(s) can be anything:

public MyReadModel SynchronousMethodWithoutContext(MyEvent @event, MyReadModel? currentInstance);
public MyReadModel SynchronousMethodWithContext(MyEvent @event, MyReadModel? currentInstance, EventContext context);
public Task<MyReadModel> AsynchronousMethodWithoutContext(MyEvent @event, MyReadModel? currentInstance);
public Task<MyReadModel> AsynchronousMethodWithContext(MyEvent @event, MyReadModel? currentInstance, EventContext context);

Note: Only public methods are supported without any return types. Also worth noting is that you can't have two methods handling the same event as the system would not know how to recover if one of them fails.

The currentInstance argument will hold the current instance from the underlying database. If there is no instance, this value will be null.

Note: Deletion is currently not supported, but will be implemented as the following issue describes.

A concrete example of a reducer:

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Reducers;

namespace Quickstart.Common;

public class BooksReducer : IReducerFor<Book>
{
    public Task<Book> Added(BookAddedToInventory @event, Book? initialState, EventContext context) =>
         Task.FromResult(new Book(Guid.Parse(@context.EventSourceId), @event.Title, @event.Author, @event.ISBN));
}

Snippet source

Table of Contents

Reducers