Design cart management lifecycle service

## Scenario You are designing the backend for an on-demand delivery app (restaurants and grocery). Users can create a cart, modify items from multiple devices, and then checkout to create an order that will be fulfilled. The prompt is intentionally vague. Drive the requirements, scope, and assumptions through clarifying questions. ## Core workflow (cart lifecycle) 1. Create cart 2. Add / remove items 3. Update quantity / options 4. View cart 5. Checkout (convert cart → order) with strong correctness guarantees 6. Fulfillment happens on the order 7. Cart closes after checkout, or expires if abandoned ## Functional requirements - Create an active cart for a user and a merchant/store. - Mutations: - Add item - Remove item - Update quantity - Update selected options (e.g., size, toppings) - Read: - Fetch active cart (commonly by user + merchant) - Fetch cart details (header + full list of items) - Checkout: - Convert an active cart into an order exactly once. - Validate correctness (prices/options, cart not stale, cart not already checked out, etc.). ## Non-functional requirements - Correctness/consistency is the top priority, especially at checkout. - Low latency and high availability are important, but cannot compromise correctness at checkout. - Concurrency: multiple devices may update the same cart at the same time. ## Scale assumptions - Up to **10M concurrent orders** (use this to derive rough QPS and storage assumptions). ## Data modeling expectations Propose a data model and indexing strategy that supports typical queries and correctness. Examples of typical queries to support: - Get a user’s active cart quickly (often by `(user_id, merchant_id)` or `(user_id, status)`). - Get all items by `cart_id`. - At checkout, read cart + items efficiently and validate consistency. Also discuss how you would handle: - Optimistic concurrency control (e.g., `version` on cart header and conditional writes). - If asked: finer-grained conflicts (e.g., can two devices modify different items without conflict?).