Building a componentized workflow model. Each step of the workflow is modeled as an activity that has metadata to describe design time aspects, compile time aspects, and runtime aspects of the workflow step. A user selects and arranges the activities to create the workflow via user interfaces or application programming interfaces. The metadata associated with each of the activities in the workflow is collected to create a persistent representation of the workflow. Users extend the workflow model by authoring custom activities. The workflow may be compiled and executed.