What is Hadoop Zookeeper?
Coursera Data Science

Hadoop Zookeeper is an open source Apache™ project that provides a centralized infrastructure and services that enable synchronization across a cluster. ZooKeeper maintains common objects needed in large cluster environments. Examples of these objects include configuration information, hierarchical naming space, etc. Applications can leverage these services to coordinate distributed processing across large clusters. Name services, group services, synchronization services, configuration management, and more, are available in Zookeeper, which means that each of these projects can embed ZooKeeper without having to build synchronization services from scratch into each project. Interaction with ZooKeeper occurs via Java or C interfaces time. Within ZooKeeper, an application can create what is called a znode (a file that persists in memory on the ZooKeeper servers). The znode can be updated by any node in the cluster, and any node in the cluster can register to be informed of changes to that znode (in ZooKeeper parlance, a server can be set up to “watch” a specific znode). Using this znode infrastructure, applications can synchronize their tasks across the distributed cluster by updating their status in a ZooKeeper znode. This cluster-wide status centralization service is essential for management and serialization tasks across a large distributed set of servers.

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