Design a Distributed File Locking System

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 Google, I wouldn't build a custom consensus algorithm but instead leverage an existing coordination service like ZooKeeper or etcd. The core strategy involves treating each lock request as creating an ephemeral, sequential z-node in a specific path, say /locks/fileA. When a client wants to lock a file, it creates its node with a unique sequence number. The client then watches the node with the lowest sequence number; if it is itself, it holds the lock. This ensures fairness and prevents starvation. For fault tolerance, the ephemeral nature of these nodes is critical. If a client holding the lock crashes or loses connectivity, the session expires, and the node is automatically deleted by the coordinator. This immediately releases the lock without requiring a timeout-based heartbeat mechanism from the client side, which could lead to false positives. However, we must consider the 'thundering herd' problem where many clients watch the same node. To mitigate this, the winning client should only notify the next immediate successor in the sequence chain rather than broadcasting to everyone. Finally, we handle read-write conflicts by maintaining separate namespaces for read and write locks, ensuring multiple readers can proceed concurrently while writers remain mutually exclusive.