In Chapter 2, Organizing Your Nodes and Data, we showed you how to organize your nodes using an ENC or hiera, and ideally both. At that point, we hadn't covered the forge modules or writing your own modules as we did in Chapter 4, Public Modules and Chapter 5, Custom Facts and Modules. In this chapter, we will cover a popular design concept in large installations of Puppet. The idea was originally made popular by Craig Dunn in his blog, which can be found at http://www.craigdunn.org/2012/05/239/.