Parallelize the lock BFS safely evaluates requirements, scale assumptions, API/data design, architecture, trade-offs, failure modes, and rollout in a realistic interview setting. A strong answer states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.
Follow-up: If multiple threads are available, how would you parallelize the BFS for the rotating-lock problem? Describe how to partition work across threads, maintain BFS level ordering if required, implement a thread-safe visited set and queues, prevent duplicate exploration, and evaluate trade-offs such as contention, memory overhead, and determinism.
Overview: Parallelize the lock BFS safely evaluates requirements, scale assumptions, API/data design, architecture, trade-offs, failure modes, and rollout in a realistic interview setting. A strong answer states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.
You are given the classic rotating-lock problem (e.g., a 4-wheel lock from "0000" to a target like "0202" with deadends). In the single-threaded solution, we use BFS over the implicit state graph where each state has up to 8 neighbors (turn each wheel +1 or −1 mod 10). The goal is to find the minimum number of moves.
Assume:
State space size is at most 10,000 (for a 4-digit lock with digits 0–9).
There is a set of deadend states to avoid.
Task
If multiple threads are available, design a parallel BFS for this problem. Describe:
How to partition work across threads.
How to maintain BFS level ordering (if required for shortest-path correctness).
How to implement a thread-safe visited set and queues.
How to prevent duplicate exploration across threads.
Trade-offs: contention, memory overhead, determinism, and when to prefer one approach over another.
Clarifying Questions to Ask Guidance
Clarify users, core use cases, read/write patterns, scale, latency, availability, and data retention.
State explicit assumptions before making sizing or architecture decisions.
Prioritize the functional path first, then address reliability, security, observability, and rollout.
What a Strong Answer Covers Guidance
A scoped requirements summary with concrete non-goals and success metrics.
API, data model, architecture, consistency, capacity, and operations.
Reasoned trade-offs among simple and scalable designs, including bottlenecks and failure modes.
A validation, monitoring, migration, and launch plan appropriate for the risk level.
Follow-up Questions Guidance
What breaks first at 10x traffic or data volume?
How would you degrade gracefully during dependency failures?
What metrics and alerts would prove the design is healthy after launch?