Flume is only related to MapReduce in that it is able to write to HDFS. All Flume is is a transport mechanism to send log-like data from one place to another. It can be made fancy by adding decorators to manipulate the data along the way, but at its core, it just moves bytes around. Unfortunately, at this stage, it's highly unreliable with its fault tolerant design causing more problems than it solves. Hopefully FlumeNG will improve upon this. I speak as someone who runs Flume in production and has to deal with it constantly failing on me, usually silently.

It's a great technology, but it just isn't there yet.