Implementing Distributed Locks with Zookeeper: Exclusive and Read‑Write Locks

When first learning Zookeeper, many developers struggle to see its role beyond a "Unix‑like file system" abstraction; this article bridges that gap by showing how Zookeeper can provide distributed coordination primitives such as locks.

Exclusive lock implementation : the resource to be locked is represented by a temporary Zookeeper node (the lock node). Clients compete to create this node; the one that succeeds holds the lock, while others set a watcher on the node. When the lock holder deletes the node (or crashes), Zookeeper removes it and notifies the waiting clients, which then retry the acquisition process. The article lists the five concrete steps and includes a flow diagram.

Read‑write lock implementation : a read‑write lock consists of separate read and write lock nodes under a shared parent. All clients first create their own lock node. A client can acquire a read lock if its node is the smallest among all nodes and there is no preceding write‑lock node; a write lock can be acquired if its node is the smallest overall. Two implementation strategies are described:

1. First approach : set a watcher on the parent /share_lock node; when any child node is deleted, all waiting clients are notified and re‑evaluate their eligibility. This method is simple but can generate unnecessary notifications.

2. Second approach : each client watches only the specific predecessor node that matters to it (the nearest earlier write node for readers, or the immediate predecessor for writers). This reduces spurious notifications and improves performance. The article outlines the five steps for this optimized workflow and provides corresponding diagrams.

Finally, the author notes that a demo implementation is available on GitHub, acknowledges that the code is a simple reference implementation, and invites feedback and discussion.