A restaurant order system manages the full lifecycle of a diner's experience: from browsing the menu to paying the bill. Before jumping into classes and patterns, you need to understand the workflow that drives every design decision. Think about what happens when you sit down at a restaurant. A waiter hands you a menu organized into categories: appetizers, mains, desserts. You pick items, possibly customizing them (no onions, extra cheese). The waiter writes down your order and sends it to the kitchen. The kitchen prepares items in the order they arrive, notifies the waiter when each is ready, and the waiter brings your food. After eating, you ask for the bill: and maybe split it three ways because you are dining with friends. Every step in that workflow maps to a class, a state transition, or a design pattern. The menu is a tree structure (Composite). The order moves through a strict sequence of states (State pattern). The kitchen is a queue that notifies observers when items are ready (Observer). Bill splitting is a strategy that varies at runtime (Strategy). And payment commands can be reversed for refunds (Command).

The functional requirements break down into four subsystems. First, menu management: creating categories and items with modifiers and prices. Second, order management: placing orders for tables, adding items, and tracking the order through its lifecycle. Third, kitchen management: receiving orders, processing them in queue order, and signaling completion. Fourth, billing: calculating totals, splitting bills, and processing payments. For non-functional considerations, think about what happens when a waiter adds an item to an order that is already being prepared. The system should allow modifications only in valid states: you cannot add items to an order that has already been served. State validation prevents impossible transitions and keeps the data consistent.