MRv1 and YARN: The jsvc Program
The jsvc (more info) program is used to start the DataNode listening on low port numbers. Its entry point is the SecureDataNodeStarter class, which implements the Daemon interface that jsvc expects. jsvc is run as root, and calls the SecureDataNodeStarter.init(...) method while running as root. Once the SecureDataNodeStarter class has finished initializing, jsvc sets the effective UID to be the hdfs user, and then calls SecureDataNodeStarter.start(...). SecureDataNodeStarter then calls the regular DataNode entry point, passing in a reference to the privileged resources it previously obtained.
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