The processing layer is where you will find code that manages the components dealing with processing the request/response cycle. It is split into two parts: the sitemap components and logicsheets.
Sitemap components are called by the pipeline mechanism as it handles the processing of a request. The sitemap components include the various generators, transformers, and serializers that are needed to deal with XML streams. They also include logical components such as selectors, matchers, and actions. All of these components are dealt with in detail in the next chapter.
As part of Cocoon's implementation of the model-view-controller (MVC) pattern, libraries of code have been developed that provide access to typical functions that the Web application developer needs. These can be called from the application code and are called logicsheets. Future chapters on how to create content with Cocoon cover the exact nature of logicsheets in more detail; however, their existence provides a highly useful bed of reusable functionality for the developer.