If you have used any of the Flume 0.9 releases, you'll notice that the
TailSource is no longer a part of Flume.
TailSource provided a mechanism to "tail" (http://en.wikipedia.org/wiki/Tail_(Unix)) any file on the system and create Flume events for each line of the file. It could also handle file rotations, so many used the filesystem as a handoff point between the application creating the data (for instance, log4j) and the mechanism responsible for moving those files someplace else (for instance, syslog).
As is the case with both channels and sinks, events are added and removed from a channel as part of a transaction. When you are tailing a file, there is no way to participate properly in a transaction. If failure to write successfully to a channel occurred, or if the channel was simply full (a more likely event than failure), the data couldn't be "put back" as rollback semantics dictate.
Furthermore, if the rate of data written to a file exceeds the rate Flume...