Projections

[woksin]

Summary

Adds Projections, that are a special type of event handler dealing with read models. Projections can be defined either inline in the client build steps, or declaratively with [Projection()] attribute.

Example of writing a Projection inline and registering a declared one:

var client = Client
    .ForMicroservice("f39b1f61-d360-4675-b859-53c05c87c0e6")
    .WithEventTypes(eventTypes =>
    {
        eventTypes.Register<DishPrepared>();
        eventTypes.Register<ChefFired>();
    })
    .WithProjections(projections =>
    {
        projections.CreateProjection("4a4c5b13-d4dd-4665-a9df-27b8e9b2054c")
            .ForReadModel<Chef>()
            .On<DishPrepared>(_ => _.KeyFromProperty(_ => _.Chef), (chef, @event, ctx) =>
            {
                chef.Name = @event.Chef;
                chef.Dishes.Add(@event.Dish);
                return chef;
            })
            .On<ChefFired>(_ => _.KeyFromProperty(_ => _.Chef), (chef, @event, ctx) =>
            {
                return ProjectionResult<Chef>.Delete;
            });
        projections.RegisterProjection<Menu>();
    })
    .Build();

Example of a declared projection:

[Projection("0405b93f-1461-472c-bdc2-f89e0afd4dfe")]
public class Menu
{
    public List<string> Dishes = new List<string>();

    [KeyFromEventSource]
    public void On(DishPrepared @event, ProjectionContext context)
    {
        if (!Dishes.Contains(@event.Dish)) Dishes.Add(@event.Dish);
    }
}

Example of getting projections:

var menu = await client.Projections
    .ForTenant(TenantId.Development)
    .Get<Menu>("bfe6f6e4-ada2-4344-8a3b-65a3e1fe16e9")
    .ConfigureAwait(false);

System.Console.WriteLine($"Menu consists of: {string.Join(", ", menu.State.Dishes)}");

var allChefs = await client.Projections
    .ForTenant(TenantId.Development)
    .GetAll<Chef>()
    .ConfigureAwait(false);

foreach (var chef in allChefs)
{
    System.Console.WriteLine($"Chef name: {chef.State.Name} and prepared dishes: {string.Join(",", chef.State.Dishes)}");
}

Added

• New client.WithProjections() to build Projections inline in the clients build steps.
• Classes can be attributed with[Projection('projectionId')] to declare them as Projections (just like you can do with EventHandlers). The class itself becomes the readmodel for the projection.
• On() methods are the handlers for a Projection. They can be decorated with different attributes to declare the key to the projection.
• Get the state of a Projection with client.Projections.Get<ReadModel>(key) and client.Projections.GetAll<ReadModel>() (+ other overloads).
• Sample for how to use Projections in Samples/Tutorials/Projections.

Changed

• Sample directory structure and moved the tutorials around