I refactor an existing APP to use WFE 4.5. The APP runs in an endless loop and fetches new workitems via polling from a third party host via webservice. The app is multi tenant, where each tenant can have its own workflow.
I read so far that there is a WorkflowInvoker, which is designed to unit test workflows. I am not sure if this does fit my requirements.
There are two options that you can use: WorkflowInvoker and WorkflowApplication.
WorkflowInvoker provides a simple way for invoking a workflow as if it were a method call and can be used only for workflows that do not use persistence. WorkflowApplication provides a richer model for executing workflows that includes notification of lifecycle events, execution control, bookmark resumption, and persistence.
If you don't need features from WorkflowApplication it is easier to use WorkflowInvoker.