Design Coffee Vending Machine
What is a Coffee Vending Machine?
A coffee vending machine is an automated machine that prepares and dispenses coffee and related beverages (like espresso, cappuccino, or tea) to users with minimal human intervention.
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Users typically interact with the machine by:
- Selecting a drink from a menu
- Customizing options (e.g., sugar level, milk type, size)
- Making payment (coins, cash, card, or digital wallet)
- Receiving the prepared beverage
Internally, the machine manages ingredient inventory (coffee powder, water, milk, sugar), tracks user inputs, processes payments, and maintains operational states (e.g., idle, dispensing, out of service).
In this chapter, we will explore the low-level design of a coffee vending machine in detail.
Lets start by clarifying the requirements:
1. Clarifying Requirements
Before starting the design, it's important to ask thoughtful questions to uncover hidden assumptions and better define the scope of the system.
Here is an example of how a conversation between the candidate and the interviewer might unfold:
Candidate: Should the machine support multiple beverage types?
Interviewer: Yes, the machine should support a configurable menu of beverage types like espresso, cappuccino, latte, black coffee, etc.
Candidate: Does the system need to support payment functionality?
Interviewer: Yes, the machine should accept money for the selected beverage, dispense the drink once enough has been inserted, and return any change. Keep it simple, a single integer amount is enough, you do not need to model individual coins or notes.
Candidate: Should the machine allow customization, such as choosing sugar level or milk quantity?
Interviewer: Yes, users should be able to make basic customizations such as choosing sugar and milk levels per beverage.
Candidate: Should the machine track inventory of ingredients like coffee, milk, and sugar?
Interviewer: Yes, it should maintain an internal inventory for all ingredients.
Candidate: How should the system handle situations where some ingredients are unavailable?
Interviewer: The system should notify the user with an appropriate message.
Candidate: Does the machine need to support concurrent user interactions?
Interviewer: A single customer interacts with the machine at a time, so you do not need to coordinate multiple simultaneous orders. The shared inventory can still be touched concurrently though, for example an admin restocking while a drink is being dispensed, so it should stay consistent under concurrent access.
Candidate: Should the machine support an admin interface for restocking and maintenance?
Interviewer: Yes, an admin should be able to view current inventory levels and refill or reset the machine as needed.
With the scope clarified, we can now summarize the core system requirements.
- Support a configurable list of beverage types (e.g., espresso, cappuccino, latte)
- Allow an admin to view current inventory levels, refill ingredient stock, and reset the machine
- Allow users to select a beverage and customize basic options such as sugar and milk levels
- Accept payment for the selected beverage and return change when the inserted amount exceeds the price
- Allow the user to cancel an in-progress order and get a refund
- Deduct the appropriate amount of ingredients from inventory after each successful order
- Display an appropriate message if a selected beverage cannot be prepared due to insufficient ingredients
- Serve one customer at a time
- Modularity: The system should have clear separation of concerns
- Extensibility: The design should support adding new drink types, and customizing ingredient mixes
- Maintainability: The codebase should follow object-oriented principles and allow easy modification or extension
- User Feedback: The machine should provide clear instructions and status updates to the user throughout the interaction
- Thread Safety: Shared state such as the ingredient inventory should stay consistent when accessed concurrently