As you seen by now that the Directive interface exposes three methods define(), initialize() and execute(). These methods are invoked by the framework at different stages of processing. They also differ by different context in which they run like in Transform or in Service. So, let's talk about how and when they are called in different context.
Transform Context
A Transform transform is a CDAP plugin that is part of the pipeline configured and UDD or Custom Directives are embedded within such a transform. Each transform implements a few interfaces like configurePipeline(), initialize() and transform().
...