Design a BLE Command Handler
Company: Whoop
Role: iOS Engineer
Category: System Design
Difficulty: hard
Interview Round: Technical Screen
Design a Bluetooth Low Energy (BLE) command handler for an iOS application.
The app needs to send commands to a wearable device over BLE. Commands must be processed strictly one at a time:
- A command may not be sent until all earlier commands have completed.
- After sending a command, the handler must wait for either:
- the expected response from the device, or
- a timeout.
- If a command times out, the handler should unblock the queue and continue handling subsequent commands according to a clearly defined failure policy.
- Multiple parts of the app may enqueue commands concurrently.
- The design should be suitable for production use in a mobile app, including lifecycle, cancellation, error handling, and testability considerations.
Explain the architecture, key data structures, concurrency model, timeout handling, response matching, and how you would validate the design.
Overview: This question evaluates a candidate's understanding of BLE communication, concurrency control, command queuing and sequencing, timeout and failure policies, lifecycle and cancellation handling, and testability for production-grade mobile applications.
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