System requirements

Functional:

List functional requirements for the system (Ask the chat bot for hints if stuck.)...

1. Product Selection:
   • Users should be able to view all available products in the UI.
   • Users should be able to select one or more products from the interface.
2. Payment Processing:
   • The system should support multiple payment methods (e.g., cash, credit/debit cards, mobile payments).
   • Upon product selection, the system must prompt the user for payment.
   • The system should securely process payments and provide confirmation after transaction completion.
3. Inventory Management:
   • The system should track product availability in real-time.
   • Users should be notified if a selected product is out of stock.
4. User Interaction:
   • The interface should be user-friendly and intuitive.
   • The system should provide clear instructions and feedback (e.g., "Please insert cash," "Select a payment method," etc.).
5. Refund Mechanism:
   • The system should have the capability to process refunds in case of errors or cancellations.
6. Admin Management:
   • Admin should be able to manage inventory (add/remove products) and view sales reports.

Non-Functional:

List non-functional requirements for the system...

1. Performance:
   • The system should process transactions within 3 seconds.
2. Security:
   • The system must ensure secure transactions and protect user data.
   • Sessions should be managed securely to prevent unauthorized access.
3. Scalability:
   • The system should be designed to easily add products and payment methods in the future.
   • The system architecture must horizontally scale to support a fleet of $10,000+$ geographically distributed vending machines without degrading central server performance.
   • The backend should handle high-concurrency telemetry and transaction data streaming from multiple machines simultaneously, anticipating peaks during high-traffic hours (e.g., lunch rushes in office buildings)
   • The system must include a centralized management plane capable of monitoring machine health, hardware faults, and inventory levels in near real-time across all regions.
   • Operators must be able to push Over-The-Air (OTA) firmware updates, pricing changes, and new product catalogs to specific geographic zones or the entire fleet simultaneously.
   • Machines must be partition-tolerant. In the event of a network failure, the machine should degrade gracefully—retaining cash transaction capabilities and queuing audit logs locally to sync with the central database once connectivity is restored (Eventual Consistency).
4. Reliability:
   • The system should be operational 24/7 with minimal downtime.
5. Compliance:
   • The system must comply with regulations related to payment processing and data protection.

API design

Define what APIs are expected from the system...

GET /items

Response: a list of items object

Item:

id: String, display_name: String, price: String (for accurate number), stock: int

POST /makePayment

request: itemId: String, payment_method: enum, amount: String

Response: status (enum).

POST /dispenseItem

request: itemId: String

Get /stockLevels

Response: a list of items object, similar to Get /items

Item:

id: String, display_name: String, price: String (for accurate number), stock: int

POST /restock

Request: itemId: String, stockNumber: Int

Response: status (enum)

GET /health

Response:

machineId: String

status: enum

// This is for monitoring the status of the machine

GET /logs/{machineId}

Response: list of strings.

// This is for grabbing logs of a machine.

High-level design

You should identify enough components that are needed to solve the actual problem from end to end. Also remember to draw a block diagram using the diagramming tool to augment your design. If you are unfamiliar with the tool, you can simply describe your design to the chat bot and ask it to generate a starter diagram for you to modify...

There are four high level components in the system:

UI: This is the component where user interacts with the system. It provides a way for user to select the product, make payment, etc.

Transaction Processing Unit: This is the main orchestration unit, it fetches information from database, serves requests from UI, and forward payment information to gateway.

Database: This database contains the two tables listed above.

Payment Gateway: The Payment Gateway is a gateway to the outside payment system.

Inventory Management Component: This is the brain of the stock operations. After the Transaction Processing Unit successfully clears a payment with the Payment Gateway, it signals this component to decrement the purchased item from the machine's virtual inventory.

Alert & Notification Service: Every time the inventory drops, the Inventory Management Component checks if the new total falls below a predefined threshold (e.g., fewer than 3 sodas left). If it does, it triggers this service to send out an SMS, email, or push notification.

Operator Dashboard: This represents the staff or management system receiving the alerts so they know exactly which machine needs a refill and what products to bring.

Fleet Management Service: A dedicated cloud service that monitors the "health" of the machines. Is Machine #402 online? What temperature is it running at? Does its software need an update?

API Gateway / IoT Message Broker (MQTT): This is the front door for cloud. Machines send lightweight messages (heartbeats, sales events, inventory drops) to a central broker.

Local Cache: The system should use a local cache, which holds all the stock / sale / price information locally. In a normal process, this cache is updated after database update. When there is a network disruption, where database is not available, the vending machine can still operates based on the local cache.

Detailed component design

Dig deeper into 2-3 components and explain in detail how they work. For example, how well does each component scale? Any relevant algorithm or data structure you like to use for a component? Also you could draw a diagram using the diagramming tool to enhance your design...

1. The payment gateway should interface with the user's credit / debit card bank, and have them process the payment in the system's stead. It needs to handle a few security concerns.
   1. Encryption of data, for privacy protection
   2. Certificate verification of the other party.
2. UI: the UI of a vending machine should be designed in a way that's easy to understand and handle. For example, when it is time to process payment, some light can highlight the card inserting or cash inserting machine. Every product should be listed and have its inventory information and price information shown clearly. Besides, when dealing with cards, it would be great if the UI will ask the customer to get the card first before dispensing the item.
3. To ensure transactional integrity, the system governs operations through a Finite State Machine (FSM) with explicit states (IDLE -> PAYMENT_PENDING -> DISPENSING -> FULFILLED / REFUNDING). Payment authorization is verified and held synchronously with the payment gateway before any physical hardware is triggered. Once verified, the motor rotates and infrared drop sensors in the delivery chute wait to confirm physical product release. If the sensor detects a successful drop, the transaction commits (FULFILLED) and the payment is finalized. If no drop is detected within a defined timeout (e.g., due to a mechanical jam), an automatic rollback mechanism cancels the hold, triggers an instant user refund, logs the error for maintenance, and transitions the system back to IDLE.
4. Idempotency key: Payment requests should contain an idempotency key to avoid double charging.
5. Atomic decrement: Decrement of inventory size of items should use atomic decrement, so that when two users attempt to get the last item, only one of them gets it.
6. The cache and TTL strategy handles the read-heavy product catalog to ensure low latency for the user browsing the machine. The rollback mechanism handles the write-heavy transactional state—ensuring that if a user's payment fails or a motor jams, the hold on the inventory is released immediately, bypassing the cache entirely to maintain strict data consistency.

Database design

Defining the system data model early on will clarify how data will flow among different components of the system. Also you could draw an ER diagram using the diagramming tool to enhance your design...

The only database tables we need here would probably be the inventory table and transaction history.

Inventory

item_id PK, Varchar

item_display_name Varchar

item_price Varchar

item_count INT

Transactions

transaction_id PK, Varchar

item_id FK, Varchar

status Varchar (inProgress, succeeded, failed)

started_at INT

completed_at INT

paymentMethod Varchar

cardNumber Varchar