split cart and checkout and checkout vs payments

pkg/checkout/mutation_order_created.go

@@ -0,0 +1,49 @@
+package checkout
+
+import (
+	"fmt"
+
+	messages "git.k6n.net/go-cart-actor/proto/checkout"
+)
+
+// mutation_order_created.go
+//
+// Registers the OrderCreated mutation.
+//
+// This mutation represents the completion (or state transition) of an order
+// initiated earlier via InitializeCheckout / external processing.
+// It finalizes (or updates) the checkout's order metadata.
+//
+// Behavior:
+//   - Validates payload non-nil and OrderId not empty.
+//   - Sets Status from payload.Status.
+//   - Sets OrderId if not already set.
+//   - Does NOT adjust monetary totals.
+//
+// Notes / Future Extensions:
+//   - If multiple order completion events can arrive (e.g., retries / webhook
+//     replays), this handler is idempotent: it simply overwrites fields.
+//   - If you need to guard against conflicting order IDs, add a check:
+//     if g.OrderId != "" && g.OrderId != m.OrderId { ... }
+//   - Add audit logging or metrics here if required.
+//
+// Concurrency:
+//   - Relies on the higher-level guarantee that Apply() calls are serialized
+//     per grain. If out-of-order events are possible, embed versioning or
+//     timestamps in the mutation and compare before applying changes.
+
+func HandleOrderCreated(g *CheckoutGrain, m *messages.OrderCreated) error {
+	if m == nil {
+		return fmt.Errorf("HandleOrderCreated: nil payload")
+	}
+	if m.OrderId == "" {
+		return fmt.Errorf("OrderCreated: missing orderId")
+	}
+	if g.OrderId == nil {
+		g.OrderId = &m.OrderId
+	} else if *g.OrderId != m.OrderId {
+		return fmt.Errorf("OrderCreated: conflicting order ID")
+	}
+
+	return nil
+}