System requirements

Functional:

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

Users can choose the product and its number in the vending machine.

Users can pay for the product with a bank card, cash, Paypal, the cryptocurrency and get the choosen product.

If users choose a product number that isn't available, the vending machine shows an error.

If users choose the product and there is a blackout, the vending machine shows an error.

If users choose the product and there is an internet connection issue, the user can pay with cash.

Non-Functional:

• Performance: The service should serve the users with minimal latency.
• Reliability: The service should be highly available and fault-tolerant.
• Scalability: Should scale dynamically based on user load.
• Security: Ensure secure processing of the user's payment transactions.
• Maintainability: Code and system architecture should be easy to maintain and extend.

Capacity estimation

Estimate the scale of the system you are going to design...

Assumptions

1. Total Number of Users: Assume the system needs to support 10 million active users, and the system serves 10 million vending machines.
2. Concurrency Level: Assume at any given time, 10% of vending machines might be active .
3. Product Selection Frequency: On average, an active user selects a product in the vending machine once every 5 minutes.

Calculations

• Write Operations: Each user selects and buys a product in a vending machine every 5 minutes. For 10 million vending machines, that's (10,000,000 users×1 update)/5 minutes=2,000,000 buys/hour

Resource Estimation

• Data Storage: Each product might need about 100 bytes (product ID, status, timestamp, name, price, count). Each vending machine contains 100 different products and one vending machine requires 10 kbytes,

and 10 millions vending machines need about 100 GBytes.

• Network Bandwidth: With frequent buying product and high concurrency, substantial bandwidth will be needed, especially for service synchronization across different regions if the service is globally distributed.
• Memory and CPU: To ensure real-time performance, a significant portion of operations should be handled in-memory. High CPU resources will be required to process the continuous flow of reads and writes.

Considerations

• Database Choices: Given the high write and read requirements, a NoSQL database like Cassandra or Redis, which can handle high throughput and horizontal scaling, might be appropriate.
• Caching: Implementing caching mechanisms to reduce database load, using solutions like Redis to cache frequent queries or active user statuses.
• Load Balancers: To manage traffic and ensure even distribution of requests across servers.

API design

Define what APIs are expected from the system...

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...

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...

Request flows

Explain how the request flows from end to end in your high level design. Also you could draw a sequence diagram using the diagramming tool to enhance your explanation...

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...

Trade offs/Tech choices

Explain any trade offs you have made and why you made certain tech choices...

Failure scenarios/bottlenecks

Try to discuss as many failure scenarios/bottlenecks as possible.

Future improvements

What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?