You can clone with
HTTPS or Subversion.
The middleware onion was very enlightening and it is also use elsewhere to illustrate the idea of a middleware. The image seems to be licensed CC-NC-SA but we could also create another image. Images and diagrams often help more than many words to actually help people to understand concepts and architectures, not only the idea of a middleware.
I created another onion picture as freely licensed SVG, so it can be used for the book.
Great, thank you! Where's the most appropriate place in the text to put this? Where would you find it most useful?