9.3 The low-stock check

namespace Shopfront.Staff;
 
public interface IStaffNotifier
{
    Task Notify(string message, CancellationToken cancellationToken);
}

namespace Shopfront.Staff;
 
// Writes each message to the log. A real shop sends it to a channel staff read.
public class LoggingStaffNotifier(ILogger<LoggingStaffNotifier> logger) : IStaffNotifier
{
    public Task Notify(string message, CancellationToken cancellationToken)
    {
        logger.LogWarning("For staff: {Message}", message);
        return Task.CompletedTask;
    }
}

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

namespace Shopfront.Trains.LowStockCheck;
 
public record LowStockCheckOutput(List<string> LowSkus);

using Trax.Effect.Services.ServiceTrain;
 
namespace Shopfront.Trains.LowStockCheck;
 
public interface ILowStockCheckTrain : IServiceTrain<LowStockCheckInput, LowStockCheckOutput>;

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

using Shopfront.Staff;
using Trax.Core.Functional;
using Trax.Effect.Services.EffectJunction;
 
namespace Shopfront.Trains.LowStockCheck.Junctions;
 
internal class NotifyStaffJunction(IStaffNotifier staff) : EffectJunction<LowStockCheckOutput, Unit>
{
    public override async Task<Unit> Run(LowStockCheckOutput low)
    {
        if (low.LowSkus.Count > 0)
            await staff.Notify($"Low on {string.Join(", ", low.LowSkus)}.", CancellationToken);
        return Unit.Default;
    }
}

using Shopfront.Trains.LowStockCheck.Junctions;
using Trax.Core.Functional;
using Trax.Effect.Services.ServiceTrain;
 
namespace Shopfront.Trains.LowStockCheck;
 
public class LowStockCheckTrain
    : ServiceTrain<LowStockCheckInput, LowStockCheckOutput>,
        ILowStockCheckTrain
{
    protected override Task<Either<Exception, LowStockCheckOutput>> Junctions() =>
        Chain<FindLowStockJunction>().Chain<NotifyStaffJunction>().Resolve();
}

using Microsoft.EntityFrameworkCore;
using Shopfront.Auth;
using Shopfront.Data;
using Shopfront.Failures;
using Shopfront.Fraud;
using Shopfront.Inventory;
using Shopfront.Payments;
using Shopfront.Staff;
using Trax.Core.Decisions;
using Trax.Effect.Services.FailureClassifier;
 
namespace Shopfront.Extensions;
 
public static class ShopfrontServiceCollectionExtensions
{
    // Everything Shopfront's trains need, apart from Trax itself.
    public static IServiceCollection AddShopfront(
        this IServiceCollection services,
        string connectionString
    ) =>
        services
            // One context per scope, built for that scope's caller: a pool would share it.
            .AddDbContextFactory<ShopfrontDbContext>(
                options => options.UseNpgsql(connectionString),
                ServiceLifetime.Scoped
            )
            .AddScoped<IShopfrontDbContext>(sp =>
                sp.GetRequiredService<IDbContextFactory<ShopfrontDbContext>>().CreateDbContext()
            )
            .AddScoped<IShopfrontCaller, TraxShopfrontCaller>()
            .AddScoped<Catalogue>()
            .AddSingleton<IPaymentProvider, FakePaymentProvider>()
            .AddSingleton<IFailureClassifier, ShopfrontFailureClassifier>()
            .AddSingleton<IDecider>(FraudRules.Live())
            .AddSingleton<ICandidateFraudDecider, StricterFraudRules>()
            .AddSingleton<IStaffNotifier, LoggingStaffNotifier>();
}