Here's a neat little video from the Erlang User Conference in Stockholm about Riak. You don't need to know Erlang to learn a lot from this:
Riak Pipe is most simply described as “UNIX pipes for Riak.” In much the same way you would pipe the output of one program to another on the command line, Riak Pipe allows you to pipe the output of a function on one vnode to the input of a function on another. This talk covers the basic structure of Riak Pipe, with an emphasis on the structures and practices used to prevent overload. An analysis of the strengths and weaknesses of the approaches chosen, and potentials for future improvement, will also be presented.