Hapnd.Projections.Contracts 0.2.0

Hapnd Projections Contracts

Contracts for building projections and reducers for Hapnd - the event sourcing platform built for engineers.

Installation

dotnet add package Hapnd.Projections.Contracts

Usage

Implement IReducer<TState> to define how events transform into aggregate state:

using Hapnd.Projections.Contracts;

public record OrderState
{
    public decimal Total { get; init; }
    public List<string> Items { get; init; } = [];
    public int ItemCount { get; init; }
}

public class OrderReducer : IReducer<OrderState>
{
    public OrderState Apply(OrderState? state, Event @event)
    {
        state ??= new OrderState();

        return @event.Type switch
        {
            "ItemAdded" => ApplyItemAdded(state, @event.GetData<ItemAddedData>()),
            "ItemRemoved" => ApplyItemRemoved(state, @event.GetData<ItemRemovedData>()),
            _ => state
        };
    }

    object? IProjection.Apply(object? state, Event @event)
        => Apply((OrderState?)state, @event);

    private static OrderState ApplyItemAdded(OrderState state, ItemAddedData data)
    {
        return state with
        {
            Total = state.Total + data.Price,
            Items = state.Items.Append(data.Item).ToList(),
            ItemCount = state.ItemCount + 1
        };
    }

    private static OrderState ApplyItemRemoved(OrderState state, ItemRemovedData data)
    {
        return state with
        {
            Total = state.Total - data.Price,
            Items = state.Items.Where(i => i != data.Item).ToList(),
            ItemCount = Math.Max(0, state.ItemCount - 1)
        };
    }
}

public record ItemAddedData(string Item, decimal Price);
public record ItemRemovedData(string Item, decimal Price);

Strongly-Typed Event Data

Use Event.GetData<T>() to deserialize event data with full type safety:

var data = @event.GetData<ItemAddedData>();
Console.WriteLine($"Added {data.Item} for {data.Price:C}");

Packaging Your Projection

1. Create a class library project referencing this package
2. Implement your projection/reducer
3. Build and zip the output
4. Upload to Hapnd via POST /projections/upload or POST /reducers/upload

Requirements

• .NET 10.0 or later

License

Apache 2.0