Design Vending Machine
What is a Vending Machine?
A vending machine is a self-service automated device that dispenses items like snacks, beverages, or other products to users without the need for human assistance. These machines are commonly found in places like offices, schools, airports, and train stations.
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Users typically interact with the machine by:
- Selecting an item (using a code, button, or touchscreen),
- Inserting payment (coins, bills, cards, or digital wallets),
- And then receiving the item from a dispensing slot.
In this chapter, we will explore the low level design of a 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 discussion between the candidate and the interviewer might unfold:
Candidate: Should the machine allow multiple transactions to run concurrently?
Interviewer: No, only one transaction should be allowed at a time.
Candidate: Should the vending machine support multiple payment methods, such as coins, notes, and digital payments?
Interviewer: For this version, let’s support only coin-based transactions. The machine should accept fixed denominations such as $1, $5, and $10.
Candidate: Should the machine be able to return change if the inserted amount exceeds the item price?
Interviewer: Yes, returning the correct change is an important part of the system.
Candidate: Can users cancel a transaction midway and get their money back?
Interviewer: Yes, users should be able to cancel a transaction any time before an item is dispensed. The full amount inserted should be refunded.
Candidate: Do we need to support admin operations such as restocking items or updating item prices?
Interviewer: The system should support restocking items or adding new items with a specified quantity. For simplicity, let's skip price updates for now.
Candidate: Do we need to track and log transaction history or sales data?
Interviewer: No need to maintain detailed transaction history. However, we can display intermediate states during a transaction
Candidate: Should the machine generate a receipt after the purchase?
Interviewer: No, receipt generation is not required for this version.
After gathering the details, we can summarize the key system requirements.
1.1 Functional Requirements
- Accept coin-based payments using fixed denominations (e.g., $1, $5, $10)
- Allow adding new items or restocking existing items in the vending machine
- Allow users to view available items along with their price
- Users can select an item by entering the item code
- The machine should dispense the selected item if sufficient money is inserted
- Return change if the inserted amount exceeds the item’s price
- Allow users to cancel a transaction before item dispensing and receive a full refund
- Display intermediate states such as inserted amount, selected item, or refund messages
1.2 Non-Functional Requirements
- Maintainability: The system should follow object-oriented principles, ensuring modularity, testability, and ease of extension
- Atomicity: The purchase operation must be atomic. A user either receives the item and correct change, or receives a full refund
- Concurrency Control: The machine must handle only one transaction at a time. While a transaction is in progress, the system should remain locked to other inputs
- Extensibility: The system should be designed in a way that future features (e.g., digital payments) can be added with minimal changes
After the requirements are clear, lets identify the core entities/objects we will have in our system.