Design a Distributed File Locking System
Design a system that provides exclusive read/write access to shared files across a distributed cluster. Discuss using ZooKeeper or a dedicated lock service.
Why Interviewers Ask This
Interviewers ask this to evaluate your ability to handle distributed consistency, race conditions, and fault tolerance in high-concurrency environments. They specifically want to see if you understand the CAP theorem trade-offs when designing coordination services like ZooKeeper. The question tests your capacity to translate theoretical consensus algorithms into a practical, production-grade locking mechanism that prevents data corruption across a cluster.
How to Answer This Question
1. Clarify requirements immediately: define if locks are exclusive or shared, synchronous vs asynchronous, and the expected failure modes like network partitions or node crashes.
2. Propose a centralized coordinator using an external service like ZooKeeper or etcd rather than building a custom consensus layer from scratch, as this aligns with Google's preference for leveraging mature infrastructure.
3. Detail the lock acquisition flow: explain how clients create ephemeral sequential nodes to request access and how the system determines the winner based on sequence numbers.
4. Discuss release mechanisms: describe how ephemeral nodes automatically vanish on client death to prevent deadlocks, ensuring liveness.
5. Address edge cases explicitly: cover split-brain scenarios, leader election failures, and how to handle clock skew or network delays during lock contention.
Key Points to Cover
- Explicitly choosing a proven coordination service like ZooKeeper over building a custom consensus layer
- Using ephemeral sequential nodes to handle automatic lock release on client failure
- Explaining the logic of comparing sequence numbers to determine lock ownership fairly
- Addressing the thundering herd problem through targeted notifications rather than broadcasts
- Demonstrating understanding of CAP theorem trade-offs regarding availability versus strict consistency
Sample Answer
To design a distributed file locking system, I would first clarify that we need exclusive write locks and potentially shared read locks, with a strong guarantee of safety even under network partitions. Given the scale at…
Common Mistakes to Avoid
- Suggesting a naive timeout-based approach where clients check if a lock is stale, which fails during network partitions
- Ignoring the difference between exclusive and shared locks, leading to potential data corruption scenarios
- Failing to mention ephemeral nodes, resulting in deadlocks if the client process crashes unexpectedly
- Overlooking the performance cost of having every waiting client watch the lock holder instead of chaining notifications
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