title: How to speed up RDD recomputation?
body: <p>Suppose I have two RDDs, A and B. B depends on A. </p>
  
  <p>If A changes to a new RDD, A2, which only differs from A in a small number of elements (A2 can be computed by using Spark Streaming, for example).</p>
  
  <p>If I understand it correctly, Spark always recompute every elements B since the parent RDD has changed from A to A2, even though only a few elements in B should change. Is there a way to recompute B without touching every elements?</p>