Maybe the better question to ask is, "What kinds of problems are these technologies intended to solve? And how can I go about understanding those problems, and what other approaches might be used to solve them? Are Spark/Hadoop really the answer, or is that just what people are talking about because... it's what they keep hearing about?"

Remembering that, like everything, Hadoop, Spark and whatnot are means to the end -- and not even necessarily the best solutions for the problems everyone talks about using them for. And that understanding business problems end-to-end (and how they drive technology choices) is infinitely more valuable than being able to play back what you learning in some online course somewhere.

And in the meantime -- don't worry about the rejection slips; just be awesome in the skills you do have, + continually seek to become awesome in whatever new areas you may be exposed to (which might be something relatively "unsexy" like Postgres or Solr) --

-- and eventually people will come to see that it is your awesomeness, not your online coursework or having this or that keyword show up in your most recent job description, that is your main selling point, and will be happy to bring you on board and throw you at new problems, and newer, even hotter technologies (way more interesting than whatever the online courses are pushing these days).