Apache ZooKeeper is an open-source system used to manage and coordinate distributed applications. It helps multiple nodes in a cluster stay in sync and make consistent decisions. ZooKeeper was a sub-project of Hadoop but is now a top-level project in its own right.
ZooKeeper's architecture supports high availability through redundant services. The clients can thus ask another ZooKeeper leader if the first fails to answer. ZooKeeper nodes store their data in a hierarchical name space, much like a file system or a tree data structure. Clients can read from and write to the nodes and in this way have a shared configuration service. Updates are totally ordered.
ZooKeeper is a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services. All of these kinds of services are used in some form or another by distributed applications. Each time they are implemented there is a lot of work that goes into fixing the bugs and race conditions that are inevitable. Because of the difficulty of implementing these kinds of services, applications initially usually skimp on them ,which make them brittle in the presence of change and difficult to manage. Even when done correctly, different implementations of these services lead to management complexity when the applications are deployed.
Why we use ZooKeeper?
• To manage distributed system coordination
• To handle leader election (which node is in charge)
• To maintain configuration data centrally
• To provide synchronization and naming services
• To ensure consistency across nodes
When should you use ZooKeeper?
ZooKeeper is a good fit when:
• You are building or running a distributed system
• You need coordination between services
• You require leader election or locking mechanisms
• You want a reliable metadata store
Not ideal when:
• You just need a regular database
• Your system is simple or single-node
• You need high-throughput data storage (ZooKeeper is for metadata, not bulk data)
Common use cases of Apache ZooKeeper
• Leader election (e.g., deciding a master node)
• Configuration management
• Service discovery
• Distributed locking
• Metadata management
• Naming service
• Synchronization
• Leader election
• Message Queue
• Notification system
Many systems rely on ZooKeeper, including:
• Apache Kafka (older versions)
• Apache HBase
• Apache Solr
Key features of Apache ZooKeeper
• Strong consistency (CP system in CAP theorem)
• Hierarchical namespace (like a filesystem)
• Atomic operations
• Watch mechanism (notifications on changes)
• Fault tolerance via replication
• Leader election support
Key components of ZooKeeper
• ZNode (ZooKeeper node): Basic data unit (like a file in a filesystem)
• Ensemble: Cluster of ZooKeeper servers
• Leader: Handles write requests
• Followers: Serve read requests and replicate data
• Observers: Optional nodes that don’t vote but help scale reads
• Sessions: Client connections to ZooKeeper
• Watches: Event notifications when data changes
Advantages
• Reliable coordination for distributed systems
• Guarantees strong consistency
• Simple data model (hierarchical structure)
• Widely used and battle-tested
• Supports leader election and distributed locks
Disadvantages
• Not designed for large data storage
• Can become a bottleneck if misused
• Requires careful setup and maintenance
• Limited write scalability (leader-based)
• Operational complexity in large clusters
Alternatives
etcd
Popular in Kubernetes ecosystems
Consul
Service discovery and configuration
Eureka
Used in microservices (Netflix ecosystem)
