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1 What is the difference between Spark UI and Spark history server.

SparkLinsenling Bus

A Listener Bus asynchronously passes listener events to registered Spark listeners. An instance of Listener Bus runs on a driver as an instance of LiveListenerBus (as listenerBus). It is created and started when a Spark context starts. A Listener Bus is a daemon thread called SparkListenerBus that asynchronously processes events from a java.util.concurrent.LinkedBlockingQueue capped at 10000 events.