11.3 Fulfilment is a train

namespace Shopfront.Warehouse;
 
public interface IWarehouse
{
    // Packs the order's items and hands back the parcel's tracking number.
    Task<string> Pack(Guid orderId, int items, CancellationToken cancellationToken);
}

namespace Shopfront.Warehouse;
 
// Takes ten seconds to pack anything, long enough to kill the worker while it does.
public class FakeWarehouse : IWarehouse
{
    public TimeSpan PackingTime { get; init; } = TimeSpan.FromSeconds(10);
 
    public async Task<string> Pack(Guid orderId, int items, CancellationToken cancellationToken)
    {
        await Task.Delay(PackingTime, cancellationToken);
        return $"PKG-{orderId.ToString("N")[..8].ToUpperInvariant()}";
    }
}

namespace Shopfront.Trains.FulfilOrder;
 
public record FulfilOrderInput
{
    public required Guid OrderId { get; init; }
    public required int Items { get; init; }
}

namespace Shopfront.Trains.FulfilOrder;
 
public record FulfilOrderOutput(Guid OrderId, string TrackingNumber);

using Trax.Effect.Services.ServiceTrain;
 
namespace Shopfront.Trains.FulfilOrder;
 
public interface IFulfilOrderTrain : IServiceTrain<FulfilOrderInput, FulfilOrderOutput>;

using Shopfront.Warehouse;
using Trax.Effect.Services.EffectJunction;
 
namespace Shopfront.Trains.FulfilOrder.Junctions;
 
internal class PackOrderJunction(IWarehouse warehouse, ILogger<PackOrderJunction> logger)
    : EffectJunction<FulfilOrderInput, FulfilOrderOutput>
{
    public override async Task<FulfilOrderOutput> Run(FulfilOrderInput input)
    {
        logger.LogInformation("Packing order {OrderId}", input.OrderId);
        var tracking = await warehouse.Pack(input.OrderId, input.Items, CancellationToken);
        logger.LogInformation("Packed order {OrderId} as {Tracking}", input.OrderId, tracking);
        return new FulfilOrderOutput(input.OrderId, tracking);
    }
}

using Shopfront.Trains.FulfilOrder.Junctions;
using Trax.Core.Functional;
using Trax.Effect.Models.Metadata;
using Trax.Effect.Services.ServiceTrain;
 
namespace Shopfront.Trains.FulfilOrder;
 
public class FulfilOrderTrain : ServiceTrain<FulfilOrderInput, FulfilOrderOutput>, IFulfilOrderTrain
{
    protected override Task<Either<Exception, FulfilOrderOutput>> Junctions() =>
        Chain<PackOrderJunction>().Resolve();
 
    // One order is fulfilled one queued run at a time, however often it is queued.
    protected override string? QueueSubjectKey(Metadata metadata) =>
        $"order:{TrainInput.OrderId}";
}