By David Dossot and John D’Emic
Mule is a workhorse whose sole purpose in life is to move your messages around. It actually does way more than just moving messages but is also able to transform, enrich, and smartly route them. In this article based on chapter 2 of Mule in Action, Second Edition, the authors look at details of message interactions Mule supports and the set of abstractions that enables these interactions.

Interacting with Messages (PDF)