Design a vending machine system

Problem

Design an object-oriented system for a network of automated coffee kiosks spread across multiple locations. Customers can place orders through a mobile app or web interface, selecting their preferred kiosk location for pickup. Each kiosk can prepare various beverages using available ingredients. The system must track ingredient quantities at each location and automatically mark beverages as unavailable when ingredients run low. Staff should only need to physically refill ingredients when inventory alerts trigger.

Your design should support the following operations:

• Registering new kiosks at different locations
• Adding beverage recipes with required ingredients and quantities
• Checking ingredient availability at specific kiosks
• Placing orders for beverages at chosen locations
• Preparing beverages and deducting ingredient inventory
• Alerting when ingredients need refilling
• Handling concurrent orders safely

Requirements

Core Components

1. Ingredient Management

   • Each ingredient has a name, unit of measurement, and current quantity
   • Track minimum threshold levels for refill alerts
   • Support adding and consuming ingredient quantities

2. Recipe System

   • Recipes define beverage names and required ingredients with amounts
   • Calculate if a recipe can be prepared with current inventory
   • Support multiple size options (small, medium, large) with different ingredient ratios

3. Kiosk Operations

   • Each kiosk has a unique identifier and location
   • Maintains its own ingredient inventory
   • Can check if specific beverages are available
   • Processes orders and dispenses beverages

4. Order Management

   • Orders contain customer details, beverage selection, size, and kiosk location
   • Track order status (placed, preparing, ready, completed, cancelled)
   • Generate unique order IDs
   • Support order history tracking

5. Central Coordination

   • Central system manages all kiosks
   • Routes orders to appropriate kiosks
   • Provides inventory visibility across all locations
   • Generates refill alerts when ingredients drop below thresholds

Design Considerations

Object-Oriented Principles

• Encapsulation: Hide internal inventory details and expose clean interfaces
• Single Responsibility: Each class should have one clear purpose
• Abstraction: Define clear contracts between components
• Composition: Build complex behaviors from simpler objects

Scalability

• Support adding new kiosks without code changes
• Allow easy addition of new beverage recipes
• Handle multiple simultaneous orders per kiosk

Reliability

• Prevent overselling when inventory is low
• Handle race conditions when multiple orders deplete the same ingredient
• Validate orders before accepting them

Extensibility

• Design for future features like loyalty programs
• Support custom beverage modifications
• Allow for seasonal menu items

Class Design Guide

Consider these classes in your design:

Ingredient: Represents a single ingredient with quantity tracking and consumption logic

Recipe: Defines how to make a beverage with required ingredients and proportions

Beverage: Represents a drink that can be ordered, linking to its recipe

InventoryManager: Manages ingredient stock at a single location with thread-safe operations

Order: Captures customer order details with status tracking

Kiosk: Represents a physical kiosk location with its inventory and order processing capability

CentralSystem: Coordinates multiple kiosks and provides system-wide views

Example Usage Scenario

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Hint 1: Core Classes Start by identifying the key entities in the system:

• Ingredient: The fundamental building block with quantity and unit
• Recipe: A blueprint mapping ingredient names to required amounts
• InventoryManager: Handles stock levels with thread-safe operations
• Kiosk: Owns an inventory manager and processes orders
• Order: Contains all information about a customer request
• CentralSystem: Maintains the registry of all kiosks and recipes

Think about the relationships: A kiosk HAS-A inventory manager, recipes USE ingredients, orders REFERENCE both beverages and kiosks.

Hint 2: Inventory Management The inventory management is critical for correctness:

1. Use a dictionary to map ingredient names to their current quantities
2. Implement atomic operations for checking and consuming ingredients
3. Consider using locks or other synchronization primitives to prevent race conditions
4. When checking if a beverage can be made, verify ALL ingredients are sufficient
5. Only deduct inventory after confirming the entire recipe can be fulfilled
6. Track minimum thresholds separately from current quantities

The check-and-consume operation should be atomic to prevent two orders from depleting the same ingredient below zero.

Hint 3: Order Processing Flow Design the order lifecycle carefully:

1. Validation: Check if the requested kiosk exists and the beverage recipe is registered
2. Availability Check: Verify the kiosk has sufficient ingredients
3. Reservation: Optionally lock ingredients for the order
4. Preparation: Deduct ingredients and mark order as preparing
5. Completion: Mark order ready for pickup

Consider what happens if an order fails at any stage. Should ingredients be rolled back? How do you track order status?

Use enums or constants for order states: PLACED, PREPARING, READY, COMPLETED, CANCELLED, FAILED.

Hint 4: Scalability Patterns For a production-ready system, consider these patterns:

Observer Pattern: Notify subscribers when inventory drops below thresholds Factory Pattern: Create different beverage types with a common interface Strategy Pattern: Different pricing strategies for different locations or times Singleton Pattern: Ensure only one central system instance exists

For thread safety in a multi-threaded environment:

• Use threading locks around inventory modifications
• Consider using queues for order processing
• Implement retry logic for transient failures

Full Solution ``

Solution Explanation

Architecture Overview:

The solution follows object-oriented design principles with clear separation of concerns:

1. Ingredient Class: Encapsulates quantity tracking and threshold monitoring for a single ingredient. Provides methods to check sufficiency, consume amounts, and detect when refills are needed.
2. Recipe Class: Acts as a blueprint defining ingredient requirements. Supports size scaling through multipliers, making it easy to offer small, medium, and large options without duplicating recipe data.
3. Beverage Class: Links a recipe to a product name and pricing. Separates the "what to make" (recipe) from the "what to sell" (beverage) concerns.
4. InventoryManager Class: Provides thread-safe inventory operations using locks. The critical consume_for_recipe method atomically checks and consumes ingredients, preventing race conditions where two orders might both think they can use the last of an ingredient.
5. Order Class: Tracks the complete lifecycle of a customer order with status updates and timestamps. Uses UUID for unique identification.
6. Kiosk Class: Represents a physical location with its own inventory. Processes orders locally and maintains order history. Delegates inventory concerns to InventoryManager.
7. CentralSystem Class: Provides the coordination layer managing multiple kiosks, global recipe catalog, and system-wide queries. Acts as the entry point for order placement and status tracking.

Key Design Decisions:

• Thread Safety: Used locks in InventoryManager to handle concurrent access safely
• Atomic Operations: Check-and-consume happens atomically to prevent overselling
• Status Tracking: Enum-based order states provide type safety and clarity
• Composition Over Inheritance: Kiosk HAS-A InventoryManager rather than extending it
• Separation of Concerns: Each class has a single, well-defined responsibility

Time and Space Complexity:

• Order placement: O(1) - just creates an order object
• Order processing: O(I) where I is ingredients in recipe - must check each ingredient
• Availability check: O(B × I) where B is beverages and I is ingredients per recipe
• Refill alerts: O(K × I) where K is kiosks and I is ingredients per kiosk
• Space: O(K × I + R × I + O) for kiosks, recipes, and orders

Extensions for Production:

1. Add database persistence for orders and inventory
2. Implement message queues for asynchronous order processing
3. Add payment processing integration
4. Implement customer notification system (SMS/email when order ready)
5. Add analytics for popular drinks and usage patterns
6. Implement predictive restocking based on historical data
7. Add user authentication and loyalty programs
8. Support customizations (extra shot, sugar-free, etc.)

# Initialize central system
central = CentralSystem()

# Register kiosks
kiosk1 = central.register_kiosk("K001", "Downtown Plaza")
kiosk2 = central.register_kiosk("K002", "Airport Terminal")

# Add initial inventory to kiosk1
kiosk1.add_ingredient("coffee_beans", 1000, "grams", min_threshold=100)
kiosk1.add_ingredient("milk", 2000, "ml", min_threshold=200)
kiosk1.add_ingredient("sugar", 500, "grams", min_threshold=50)
kiosk1.add_ingredient("water", 5000, "ml", min_threshold=500)

# Create beverage recipes
latte_recipe = Recipe("Latte", {
    "coffee_beans": 18,
    "milk": 200,
    "water": 30
})
central.add_recipe(latte_recipe)

cappuccino_recipe = Recipe("Cappuccino", {
    "coffee_beans": 18,
    "milk": 120,
    "water": 30
})
central.add_recipe(cappuccino_recipe)

# Customer places order
order1 = central.place_order(
    customer_id="C12345",
    beverage_name="Latte",
    size="medium",
    kiosk_id="K001"
)

# Process the order
if kiosk1.can_fulfill(order1):
    kiosk1.prepare_beverage(order1)
    print(f"Order {order1.id} ready for pickup")
else:
    print("Insufficient ingredients")

# Check for refill alerts
alerts = central.get_refill_alerts()
for alert in alerts:
    print(f"Kiosk {alert.kiosk_id}: {alert.ingredient} needs refill")