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== Apache ZooKeeper ==
Apache ZooKeeper is an open-source, centralized service designed to manage configuration, naming, synchronization, and group services in distributed systems. It simplifies coordination and state management in large-scale distributed applications.

=== Overview ===
ZooKeeper provides a high-performance coordination service for distributed applications. It offers a simple and reliable mechanism to store shared configuration and state information.

=== Key Features ===
* '''Centralized Metadata Management''': Stores and manages configuration data and metadata for distributed applications.
* '''Simple API''': Intuitive and minimalistic API for easy interaction.
* '''High Availability''': Ensures service continuity through data replication across multiple servers.
* '''Sequential Consistency''': Guarantees that all clients see updates in the same order.
* '''Scalability''': Efficiently handles large-scale systems.

=== Architecture ===
ZooKeeper’s architecture consists of the following:
* **ZooKeeper Ensemble:** A cluster of ZooKeeper servers that work together to provide high availability and reliability.
* **ZNode:** Hierarchical nodes used to store data. Each ZNode can hold data and child nodes.
* **Watcher:** A mechanism that allows clients to register for notifications when a specific ZNode changes.

=== Use Cases ===
1. **Distributed Lock Management:** Prevents race conditions by managing locks across distributed systems.
2. **Leader Election:** Simplifies leader election processes in distributed systems.
3. **Configuration Management:** Dynamically manages and distributes configuration data.
4. **Service Discovery:** Tracks and provides information about active services.
5. **Cluster Management:** Monitors and coordinates the state of nodes in a cluster.

=== Advantages ===
* Reduces complexity in distributed system coordination.
* Strong consistency model for reliable operations.
* Supports dynamic reconfiguration.
* Fault-tolerant and resilient to node failures.

=== Disadvantages ===
* Requires careful configuration and tuning for optimal performance.
* Potential bottleneck if overused as a central hub.
* Learning curve for implementation and operation.

=== Tools and Ecosystem ===
ZooKeeper is a critical component in many distributed system frameworks:
* **Hadoop:** Used for coordination and configuration management.
* **Apache Kafka:** Handles broker coordination and metadata storage.
* **HBase:** Provides high availability and fault tolerance.
* **Apache Storm:** Manages distributed task assignments and states.

=== Example Usage ===
Below is an example of how ZooKeeper can be used to monitor a distributed lock:

<syntaxhighlight lang="bash">
# Start ZooKeeper server
bin/zkServer.sh start

# Create a ZNode
zkCli.sh -server localhost:2181 create /my_lock "lock_data"

# Watch for changes to the ZNode
zkCli.sh -server localhost:2181 get /my_lock true
</syntaxhighlight>

=== Community and Contributions ===
Apache ZooKeeper is actively maintained by the Apache Software Foundation. It has a vibrant community contributing to its development and improving its ecosystem.

=== References ===
<references/>

=== External Links ===
* [Official Website](https://zookeeper.apache.org)
* [ZooKeeper Documentation](https://zookeeper.apache.org/doc)
* [Apache ZooKeeper GitHub](https://github.com/apache/zookeeper)

=== See Also ===
* [[Apache Kafka]]
* [[Apache Hadoop]]
* [[Distributed Systems]]