title: What is a glom?. How it is different from mapPartitions?
body: <p>I've come across the <code>glom()</code> method on RDD. As per the documentation </p>
  
  <blockquote>
    <p>Return an RDD created by coalescing all elements within each partition into an array</p>
  </blockquote>
  
  <p>Does <code>glom</code> shuffle the data across the partitions or does it only return the partition data as an array? In the latter case, I believe that the same can be achieved using <code>mapPartitions</code>.</p>
  
  <p>I would also like to know if there are any use cases that benefit from <code>glom</code>.</p>