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Apache ZooKeeper – Guardian of Distributed Systems

Apache ZooKeeper – Guardian of Distributed Systems


Apache ZooKeeper, a volunteer project under the Apache Software Foundation, is an open-source coordination service for distributed computing environments. Furthermore, Zookeeper is designed to be highly reliable and fault-tolerant and can manage high levels of read and write data.

Companies including Yelp, RackSpace, Yahoo!, Reddit, Facebook, and Twitter employ ZooKeeper. In order to ensure data consistency and availability, the distributed system provides a simple, tree-structured data model, a simple API, and a distributed protocol.

How Does Apache ZooKeeper Work?

Apache ZooKeeper is a simple, replicated coordination service that allows distributed processes to coordinate through a shared hierarchical namespace similar to a file system. The data registers, known as znodes, reside in memory for low latency.

ZooKeeper runs on an ensemble of replicated servers that maintain an in-memory state image along with logs and snapshots. As long as most servers are available, ZooKeeper service is available.

Furthermore, clients connect to ZooKeeper servers, make requests, and send heartbeats to mark their presence. If a machine fails, ZooKeeper notifies components like YARN so actions can be taken for high resilience. Ultimately, ZooKeeper acts as a monitoring tool holding cluster configuration data.

History of Apache ZooKeeper:

ZooKeeper originally started as a subproject of Hadoop at Yahoo! to streamline processes running on big-data clusters and fix bugs that occurred while deploying distributed big-data applications. 2008, ZooKeeper became an independent, standalone project under the Apache Foundation.

Features of Apache ZooKeeper:

Since we know how the Apache standalone project works, let’s discuss the features it carries:

  1. High Availability: In case of server failures, ZooKeeper replicates across multiple servers to ensure data consistency and availability.
  2. Distributed Coordination: The system provides primitives like locks, leader election, and distributed queues for coordinating activities across multiple nodes.
  3. Hierarchical Data Storage: Just like the file system, configured data is stored in a hierarchical tree structure.
  4. Ephemeral Nodes: Nodes that expire automatically after a configurable time, useful for temporary tasks or leader election.
  5. Watch Mechanism: Allows applications to be notified of changes to specific data paths in the ZooKeeper tree.
  6. Performance: Provides fast read and write operations for managing coordination data.
  7. Open-Source & Community-Driven: Freely available and backed by an active community.

Use Cases of Apache ZooKeeper:

The following are the use cases of the distributed system:

  • Configuration Management
  • Naming services
  • Choosing the leader
  • Message Queuing
  • Managing the notification system
  • Synchronization
  • Managing Distributed Cluster

Companies using Apache ZooKeeper:








Challenges of Apache ZooKeeper:

Just like the advantages it provides, users must be aware of the challenges they can face when using Zookeeper.

No migration is allowed for users as it does not offer rack placement and awareness support. The number of pods cannot be reduced to prevent accidental data loss, and switching to host networking requires complete reinstallation if initially deployed on a virtual network.

Volume requirements cannot be changed after initial deployment. With large node numbers, there could be multiple points of failure. Special software is required to restore lost messages in the communication network. Lastly, adding new Zookeeper servers can jeopardize data loss.


In conclusion, Apache ZooKeeper is a distributed coordination service managing large sets of hosts in a distributed environment. As coordinating and managing a service in a distributed environment is complicated, the distributed system solves this issue with its simple architecture, APIs including configuration management and naming services.

With its focus on reliability, coordination, and ease of use, Apache ZooKeeper plays a vital role in ensuring the smooth functioning of many large-scale distributed systems.

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