Running services: when starting, stopping and restarting CDH components, always use the service (8) command rather than running /etc/init.d scripts directly. This is important because service sets the current working directory to / and removes most environment variables (passing only LANG and TERM) so as to create a predictable environment in which to administer the service. If you run the /etc/init.d scripts directly, any environment variables you have set remain in force, and could produce unpredictable results. (If you install CDH from packages, service will be installed as part of the Linux Standard Base (LSB).)
Apache ZooKeeper is a highly reliable and available service that provides coordination between distributed processes.
For More Information
From the Apache ZooKeeper site:
"ZooKeeper is a high-performance coordination service for distributed applications. It exposes common services — such as naming, configuration management, synchronization, and group services - in a simple interface so you don't have to write them from scratch. You can use it off-the-shelf to implement consensus, group management, leader election, and presence protocols. And you can build on it for your own, specific needs."
To learn more about Apache ZooKeeper, visit http://zookeeper.apache.org/.