title: what are the sampling methods in spark? Why not reservoir sampling?
body: <p>I know reservoir sampling can be applied in parallel, but spark seems use the other sampling methods I have no idea about. could someone describe them briefly?</p>
  
  <p>According to @Tristan answer, I guess the purpose of not using reservoir sampling is to keep the balance of classes. But I go though the source code and found noting about labels.</p>