Real Time in a Hadoop cluster is one of the many things possible when you use MapR Enterprise Grade Distribution compared to Apache Hadoop or other distributions thanks to the many enhancements and features of MapR-FS, which is a read/write file system among others.
See this use case documented here
http://www.mapr.com/blog/twitter-feed-fuels-real-time-hadoop-with-storm-and-maprr-at-the-strata-conference.
If you want to accomplish this using other Hadoop distributions at least you would need 2 clusters and a lot of more hardware, therefore way more expensive.
HTH,
Carlos.