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ZooKeeper is an open source distributed coordination service, originally developed by Yahoo and now maintained by the Apache Software Foundation. It is mainly used for coordination services in distributed applications, such as configuration management, naming services, distributed synchronization, and cluster management. ZooKeeper solves many complex problems in distributed systems by providing reliable data storage, simple APIs, and high-performance distributed locks and synchronization mechanisms.

1. Key Features

• Centralized management: ZooKeeper provides a centralized naming registry that simplifies the configuration and management of distributed systems.
• High Availability: Through multiple replica nodes and election mechanisms, ZooKeeper ensures high availability and fault recovery capabilities of the system.
• Strict sequential consistency: ZooKeeper ensures that all updates to data by the client are performed in a strict order, ensuring data consistency.
• Quick response: Thanks to the in-memory data structure and optimized communication protocol, ZooKeeper can provide fast read and write responses.
• Scalability: ZooKeeper can be expanded horizontally by adding more nodes to increase the processing capacity of the system.

2. Core Components

• ZNode：The basic data unit in ZooKeeper, similar to the node in the file system. Each ZNode has a path and can store data and child nodes.
• Server: A ZooKeeper cluster consists of multiple server nodes, one of which serves as the leader and the rest as followers.
• Client: An application or service that uses the ZooKeeper API to communicate with server nodes.

3. working principle

1. Cluster composition: A ZooKeeper cluster usually consists of several server nodes, and the nodes communicate and synchronize data through a consistency protocol (such as the ZAB protocol).
2. Election Mechanism: When the cluster starts or the leader node fails, ZooKeeper will select a new leader through the election mechanism to ensure the normal operation of the system.
3. data storage：Data is stored in memory in the form of ZNodes, and is regularly snapshotted and logged on disk. Each ZNode contains data and paths to child nodes.
4. Client communication：The client communicates with a server node in the cluster through the ZooKeeper API to read and write data. The server node is responsible for processing client requests and synchronizing data to other nodes in the cluster.
5. Session Management: ZooKeeper uses sessions to track the client's connection status and supports ephemeral ZNode and watcher mechanisms.

4. Common usage scenarios

• Configuration Management: In a distributed system, ZooKeeper can be used to centrally store and manage configuration information, and clients can dynamically obtain and update configuration information.
• Naming Service: ZooKeeper can be used as a distributed naming service, providing a globally unique namespace for registering and searching resources.
• Distributed Locks: Through ZooKeeper's sequential consistency and temporary node mechanism, efficient distributed locking and synchronization control can be achieved.
• Cluster Management: ZooKeeper can be used for node management in distributed systems, such as service discovery, load balancing, fault detection and recovery, etc.

5. ecosystem

As a general coordination service, ZooKeeper is widely used in various distributed systems and big data ecosystems. Many open source projects, such as Hadoop, HBase, Kafka, Dubbo, etc., rely on the coordination services provided by ZooKeeper.

In short, ZooKeeper greatly simplifies the design and implementation of distributed systems by providing highly available, reliable, and sequentially consistent distributed coordination services. It is an important basic component for building reliable distributed applications.