Tutorial

1.9 When the chain does not fit

Swap the first two junctions in PlaceOrderTrain, so stock is reserved before the cart is priced:

Chain<ReserveStockJunction>()
    .Chain<PriceCartJunction>()
    .Chain<TakePaymentJunction>()
    .Chain<ConfirmOrderJunction>()
    .Resolve();

It builds. Run it, and the application refuses to start:

Unhandled exception. Trax.Core.Exceptions.TrainException: 1 of 1 registered trains cannot run:
  - IPlaceOrderTrain: step 1 (ReserveStockJunction) needs 'Shopfront.Trains.PlaceOrder.Models.PricedCart' in Memory and nothing before it puts one there. Chain a junction that produces it first.

When the application starts, Trax reads every train's chain and checks that each junction's input is in Memory by the time it runs. ReserveStockJunction needs a PricedCart, and at step 1 Memory holds only the PlaceOrderInput. A chain that cannot run stops the application before it takes a single request, rather than failing the first order that reaches it.

Swap the junctions back before going on.

What we have

Shopfront takes an order, prices it, reserves stock, charges the customer and confirms it, as four small junctions that each do one thing. A junction that fails stops the order where it is, and a chain in the wrong order never starts.

What we cannot do yet is look back. The runs are recorded, but in memory, and they are gone when the application stops. In chapter 2 we test the train, and in chapter 3 we move the runs to Postgres, make the payment provider fail now and then, and read back what happened to each order.

SDK Reference

Junctions | Chain | Run / RunEither | TrainException | AddTrax / AddEffects | UseInMemory | SaveTrainParameters | AddMediator

Next: 2. Testing PlaceOrder