Start by clarifying requirements and constraints, then outline a high-level design that addresses each challenge (concurrency, cancellation, partial failures, stale results, ordering). Dive into key components like a task manager, cache, and cancellation tokens, and discuss trade-offs in your choices.
Pro tip: Emphasize how you would handle cancellation and stale results using generation tokens or request IDs, and mention the importance of testing edge cases like rapid scrolling in a list.
Ask questions to understand the expected scale, platform (iOS/macOS), caching needs, and whether ordering is per-request or global. Confirm if partial failures should be retried or surfaced to the user.
Outline a task manager to control concurrency (e.g., using OperationQueue or async/await with TaskGroup), a cache layer (memory/disk), and a mechanism for cancellation (e.g., CancellationToken or Task cancellation).
Use request IDs or generation counters to discard outdated results. For ordering, ensure that results are delivered in the order requested, possibly using a serial queue or by tracking sequence numbers.
Implement error handling that distinguishes between retryable and non-retryable errors. Consider exponential backoff for retries and how to aggregate failures without blocking successful loads.
Compare different concurrency models (GCD vs. OperationQueue vs. async/await) and caching strategies. Mention how you would test scenarios like rapid scrolling, network flakiness, and cancellation.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Overcomplicated it at the start, then had to walk it back, which ate most of my time.
Start by clarifying requirements and scope, then design a clean object-oriented model with clear separation of concerns (inventory, payment, state machine). Implement core logic with testable interfaces, and write unit tests covering normal flows, edge cases, and failure modes.
Pro tip: Emphasize testability by designing interfaces that allow dependency injection for payment and inventory, and discuss how you'd handle concurrency and hardware failures—shows production maturity.
Ask questions to define supported items, payment methods, change handling, and whether it's a physical or software simulation. Confirm expected behaviors like refunds, sold-out handling, and exact change.
Identify core entities: VendingMachine, Inventory, Item, PaymentProcessor, and State (Idle, HasMoney, Dispensing). Define clear responsibilities and interactions, using a state machine for vending flow.
Code the main flow: select item, validate availability, accept payment, dispense item, return change. Use dependency injection for payment and inventory to enable unit testing and future extensions.
Cover happy paths (successful purchase), edge cases (exact change, insufficient funds, sold out), and failure modes (payment failure, dispensing error). Use mocks for external dependencies.
Talk about design choices: state pattern vs. conditionals, concurrency handling, and how to extend for new payment methods or item types. Mention potential refactoring for scalability.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.