10.3 The restock

using Trax.Effect.Models.Manifest;
 
namespace Shopfront.Trains.Restock;
 
public record RestockInput : IManifestProperties
{
    public int Threshold { get; init; } = 3;
}

namespace Shopfront.Trains.Restock.Models;
 
public record Delivery(string Sku, int Units);

namespace Shopfront.Trains.Restock.Models;
 
internal record ShortProducts(List<string> Skus);

using Shopfront.Trains.Restock.Models;
 
namespace Shopfront.Trains.Restock;
 
public record RestockOutput(List<Delivery> Delivered);

using Trax.Effect.Services.ServiceTrain;
 
namespace Shopfront.Trains.Restock;
 
public interface IRestockTrain : IServiceTrain<RestockInput, RestockOutput>;

using Microsoft.EntityFrameworkCore;
using Shopfront.Data;
using Shopfront.Trains.Restock.Models;
using Trax.Effect.Services.EffectJunction;
 
namespace Shopfront.Trains.Restock.Junctions;
 
internal class FindShortProductsJunction(IShopfrontDbContext db)
    : EffectJunction<RestockInput, ShortProducts>
{
    public override async Task<ShortProducts> Run(RestockInput input) =>
        new(
            await db
                .Products.Where(p => p.Stock <= input.Threshold)
                .OrderBy(p => p.Sku)
                .Select(p => p.Sku)
                .ToListAsync(CancellationToken)
        );
}

using Microsoft.EntityFrameworkCore;
using Shopfront.Data;
using Shopfront.Suppliers;
using Shopfront.Trains.Restock.Models;
using Trax.Effect.Services.EffectJunction;
 
namespace Shopfront.Trains.Restock.Junctions;
 
internal class ReceiveDeliveryJunction(
    ISupplier supplier,
    IShopfrontDbContext db,
    ILogger<ReceiveDeliveryJunction> logger
) : EffectJunction<ShortProducts, RestockOutput>
{
    public override async Task<RestockOutput> Run(ShortProducts shortProducts)
    {
        var delivered = await supplier.Deliver(shortProducts.Skus, CancellationToken);
 
        foreach (var (sku, units) in delivered)
            await db
                .Products.Where(p => p.Sku == sku)
                .ExecuteUpdateAsync(
                    set => set.SetProperty(p => p.Stock, p => p.Stock + units),
                    CancellationToken
                );
 
        logger.LogInformation("Received {Count} deliveries", delivered.Count);
        return new RestockOutput([.. delivered.Select(d => new Delivery(d.Sku, d.Units))]);
    }
}

using Shopfront.Trains.Restock.Junctions;
using Trax.Core.Functional;
using Trax.Effect.Services.ServiceTrain;
 
namespace Shopfront.Trains.Restock;
 
public class RestockTrain : ServiceTrain<RestockInput, RestockOutput>, IRestockTrain
{
    protected override Task<Either<Exception, RestockOutput>> Junctions() =>
        Chain<FindShortProductsJunction>().Chain<ReceiveDeliveryJunction>().Resolve();
}

using Microsoft.EntityFrameworkCore;
using Shopfront.Suppliers;
using Trax.Core.Exceptions;
using Trax.Effect.Services.FailureClassifier;
 
namespace Shopfront.Failures;
 
public class ShopfrontFailureClassifier : IFailureClassifier
{
    public FailureClass? Classify(Exception exception) =>
        exception switch
        {
            // A message for the customer: the same order fails the same way however often it runs.
            TrainException => FailureClass.Permanent,
            // The provider did not answer this time; it may next time.
            TimeoutException => FailureClass.Transient,
            // Another order changed the stock first; trying again reads the new stock.
            DbUpdateConcurrencyException => FailureClass.Transient,
            // The supplier's feed is down; it comes back.
            SupplierUnavailableException => FailureClass.Transient,
            _ => null,
        };
}