Welcome to Apache ZooKeeper™ Apache ZooKeeper is an effort to develop and maintain an open-source server which enables highly reliable distributed coordination. What is ZooKeeper? ZooKeeper is a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services.
ZooKeeper is a distributed, open-source coordination service for distributed applications. It exposes a simple set of primitives that distributed applications can build upon to implement higher level services for synchronization, configuration maintenance, and groups and naming.
ZooKeeper logs messages using log4j -- more detail available in the Logging section of the Programmer's Guide. You will see log messages coming to the console (default) and/or a log file depending on the log4j configuration.
To keep the examples simple, we create a ZooKeeper object the first time we instantiate either a barrier object or a queue object, and we declare a static variable that is a reference to this object. The subsequent instances of Barrier and Queue check whether a ZooKeeper object exists.
ZooKeeper's behavior is governed by the ZooKeeper configuration file. This file is designed so that the exact same file can be used by all the servers that make up a ZooKeeper server assuming the disk layouts are the same.