flowchart TB
  UI["Web / CLI / API"] --> OC["OrderController
  Controller"]
  OC --> OS["OrderService
  Use-case coordinator"]
  OS --> O["Order
  Domain Entity"]
  O --> LI["LineItem
  Domain Entity"]
  O --> INV["InventoryPolicy
  Information Expert"]
  
  OS --> PR["PricingRules
  Polymorphism"]
  PR --> PRa["StdPricing"]
  PR --> PRb["PromoPricing"]

  OS --> PG["PaymentGateway interface
  Indirection / Protected Variations"]
  PG --> Stripe["StripeAdapter"]
  PG --> Adyen["AdyenAdapter"]

  OS --> OR["OrderRepository
  Pure Fabrication"]
  OR --> DB[("Database")]

Creator

• Definition: The class that owns/contains/aggregates something should create it.

• In the diagram: Order creates LineItem.

• Example: Order.AddItem() constructs LineItem so creation stays near ownership.

Information Expert

• Definition: Put responsibility on the object with the data needed to do it.

• In the diagram: Order computes totals; InventoryPolicy checks stock rules using order data.

• Example: Order.Total() lives on Order, not on OrderController.

Controller

• Definition: One object receives a system event and delegates work.

• In the diagram: OrderController handles “PlaceOrder”.

• Example: UI calls OrderController.PlaceOrder(); controller calls OrderService.

High Cohesion

• Definition: Each class has one clear job.

• In the diagram: OrderService coordinates the use-case; Order holds order rules; PaymentGateway handles payment.

• Example: Controller does not calculate totals; domain does.

Low Coupling

• Definition: Minimize how many things each class depends on.

• In the diagram: OrderService depends on PaymentGateway interface, not Stripe directly.

• Example: swapping Stripe→Adyen doesn’t change OrderService.

Indirection

• Definition: Insert a middle layer to avoid direct dependency.

• In the diagram: PaymentGateway (interface) sits between app and payment provider.

• Example: OrderService calls PaymentGateway.Charge(); adapter translates to Stripe API.

Pure Fabrication

• Definition: Invent a class not in the domain to keep design clean.

• In the diagram: OrderRepository is fabricated to isolate persistence.

• Example: Order doesn’t know SQL; OrderRepository.Save(order) does.

Protected Variations

• Definition: Shield likely-to-change parts behind stable boundaries.

• In the diagram: payment providers are behind PaymentGateway.

• Example: provider API changes only touch StripeAdapter, not domain/app code.

Polymorphism

• Definition: Replace big switch logic with type-specific behavior.

• In the diagram: PricingRules has StdPricing and PromoPricing.

• Example: PricingRules.Calculate(order) varies by implementation, not by if/else.

Low Cohesion

• Definition: One class does too many unrelated things (a “god class”).

• In the diagram: what you avoid by NOT making OrderController do pricing + DB + payment.

• Example: if controller talks to DB + Stripe + pricing + order math, it’s low cohesion.