I may be wrong but I am guessing that Hadoop 1 is old now and Hadoop 2 with Yarn is in place.
Professional Hadoop Solutions - is this book worth reading? or it too old?
At one place I see that it describes HDFS Java API, I mostly do not see anybody talking about this API, who uses this API? the hype is around Pig, Oozie and other methods of using hadoop, is it worth reading that stuff? or is it an outdated method of using hadoop?