title: Join or Group By operation with Ray
body: <p>I'd like to use ray.io in cluster mode to merge two large datasets (approximately same number of records) that barely fits into memory.
  Apache Spark's <code>join</code> would do this, but my program runs in onprem DC where Spark on k8s is not supported/available yet.
  Ray.io looks more light weight and will be easier to get approval for because it will allow to scale other ML related tasks (i.e. parallelize xgboost etc.)
  So I'm looking how to implement <code>join</code> operation with ray.io. <code>Group By</code> or general shuffle would allow me to implement join too.</p>