Anyone who has used BizTalk 2004 and its orchestrations will know that parallel activities spawn multiple threads. You can see this clear in the various activitiy viewers...so I was wondering if WWF does it the same way - will apprently not. As the Work flow runtime is responsible for starting the threads (it uses one of the CLR threads) its DefaultThreadingService thread is used. The runtime scheduler, instead, schedules execution of all activities and not each parallel branch of the parallel activity. now my thought is this wont be on separate or multiple threads. My thinking has been confirmed by Jon Flanders of the WWF team (see http://www.masteringbiztalk.com/blogs/jon/PermaLink,guid,71130a01-f5a3-49f5-a95c-28f3f1738fdd.aspx) it seems each 1st activity in each brsanch is executed and then on to the second activities and so on.
So I left wondering how performant will multie branch parallelism is going to be...ah well though for another day.