YarnScheduler is the TaskScheduler for Spark on YARN in client deploy mode.
It is a custom TaskSchedulerImpl with ability to compute racks per hosts, i.e. it comes with a specialized getRackForHost.
It also sets org.apache.hadoop.yarn.util.RackResolver logger to WARN if not set already.
getRackForHost attempts to compute the rack for a host.
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Note
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getRackForHost overrides the parent TaskSchedulerImpl’s getRackForHost
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It simply uses Hadoop’s org.apache.hadoop.yarn.util.RackResolver to resolve a hostname to its network location, i.e. a rack.