title: how to distinguish an operation in spark is a transformation or an action?
body: <p>I'm learning spark recently and confused about the transformation and action operation. I read the spark document and some books about spark, and I know action will cause a spark job to be executed in the cluster while transformation will not. But the operations of rdd listed in spark's api doc are not stated whether it is a transformation or an action operation. </p>
  
  <p>For example, reduce is an action, on the other hand reduceByKey is a transformation! Why could this be.</p>