5.3 The tracks

A held order stops with a message for the customer. The junction that does it has nothing to return, so it returns Unit, the type for "no value".

Trains/PlaceOrder/Junctions/RefuseOrderJunction.cs:

using Shopfront.Fraud;
using Trax.Core.Exceptions;
using Trax.Core.Functional;
using Trax.Effect.Services.EffectJunction;
 
namespace Shopfront.Trains.PlaceOrder.Junctions;
 
internal class RefuseOrderJunction(ILogger<RefuseOrderJunction> logger)
    : EffectJunction<OrderRisk, Unit>
{
    public override Task<Unit> Run(OrderRisk risk)
    {
        logger.LogWarning("Held a first order of {Total} from {CustomerId}", risk.Total, risk.CustomerId);
        throw new TrainException(
            "We could not take this order without a check. You have not been charged."
        );
    }
}

The gate goes after pricing and the risk, and before anything is reserved or charged, so a held order has nothing to undo.

Trains/PlaceOrder/PlaceOrderTrain.cs:

using Shopfront.Fraud;
using Shopfront.Trains.PlaceOrder.Junctions;
using Trax.Core.Functional;
using Trax.Effect.Services.ServiceTrain;
 
namespace Shopfront.Trains.PlaceOrder;
 
public class PlaceOrderTrain : ServiceTrain<PlaceOrderInput, PlaceOrderOutput>, IPlaceOrderTrain
{
    protected override Task<Either<Exception, PlaceOrderOutput>> Junctions() =>
        Chain<PriceCartJunction>()
            .Chain<AssessRiskJunction>()
            .Gate<OrderRisk, LooksFraudulent>(gate =>
                gate.Yes(hold => hold.Chain<RefuseOrderJunction>())
                    .No(take => take)
                    .Shadow<ICandidateFraudDecider>()
                    .WaitForShadows(TimeSpan.FromSeconds(1))
            )
            .Chain<ReserveStockJunction>()
            .Chain<TakePaymentJunction>()
            .Chain<ConfirmOrderJunction>()
            .Resolve();
}

Gate<OrderRisk, LooksFraudulent> asks the decider how likely the answer is yes, about the OrderRisk in Memory. At 0.5 or more the train takes the Yes track, which refuses the order; below it, the No track, which adds nothing and carries on to ReserveStockJunction. Both tracks are part of the chain, so the startup check that refused a misordered chain in chapter 1 checks both: whichever is taken, ReserveStockJunction still finds its PricedCart.

The last two lines are the shadow, which comes later in the chapter.

A rule decides

The rule the shop uses is policy, not judgement: hold any first order of 100 or more. Policy is written as a RuleDecider, which maps the value asked about to an answer and is always certain of it.

Fraud/FraudRules.cs:

using Trax.Core.Decisions;
 
namespace Shopfront.Fraud;
 
public static class FraudRules
{
    // A stolen card's first use is usually its largest: hold first orders of 100 or more.
    public static RuleDecider Live() =>
        new RuleDecider().YesNo<OrderRisk, LooksFraudulent>(risk =>
            risk.FirstOrder && risk.Total >= 100m
        );
}

The rule we might move to

Someone thinks 100 is too high and wants to hold first orders from 60. Switching the live rule would show what it catches only by stopping real customers. A shadow decider is asked every question the live one is, beside it, and its answer is recorded with whether it agreed. It is never acted on: a shadow that fails, disagrees or is slow changes nothing about the order.

A shadow is named by type, so the candidate gets a type of its own.

Fraud/ICandidateFraudDecider.cs:

using Trax.Core.Decisions;
 
namespace Shopfront.Fraud;
 
public interface ICandidateFraudDecider : IDecider;

Fraud/StricterFraudRules.cs:

using Trax.Core.Decisions;
 
namespace Shopfront.Fraud;
 
// Holds first orders from 60. Asked beside the live rule, never acted on.
public sealed class StricterFraudRules : ICandidateFraudDecider
{
    private readonly RuleDecider rules = new RuleDecider().YesNo<OrderRisk, LooksFraudulent>(risk =>
        risk.FirstOrder && risk.Total >= 60m
    );
 
    public Task<DecisionResult> Decide(DecisionRequest request, CancellationToken cancellationToken) =>
        rules.Decide(request, cancellationToken);
}

.Shadow<ICandidateFraudDecider>() on the gate puts every question to it as well. .WaitForShadows(TimeSpan.FromSeconds(1)) bounds how long a run waits for it once the live answer is in. The default is five seconds, which every order would pay for a slow candidate.

SDK Reference

Gate | Decide | AddDecisionRecording | Write a decider | Try a decider in shadow | Test a train's decisions

Next: 5.4 Register Shopfront in one place