thread safety of ManualWorkflowSchedulerService.RunWorkflow

Question

I was wonder about the proper usage of ManualWorkflowSchedulerService.RunWorkflow(). Do I need to synchronize calls to the same workflowInstanceId across threads when I call ManualWorkflowSchedulerService.RunWorkflow?

Answer 1

No, I don't think so. Following is a relevant excerpt from the book 'Essential Windows Workflow Foundation'

Although operations on WorkflowInstance can be invoked on arbitrary threads, the WF scheduler hosted within the program instance is serviced by a single thread. The WF runtime guarantees that no other thread can interfere or service the scheduler while its dispatcher loop is actively processing work items. To be clear, the hosting application can invoke methods of WorkflowInstance on separate threads concurrently - this does not affect the scheduler executing the activities on a dedicated thread (for an episode of execution).

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EDIT: In order to further investigate the issue, I created a wf with a ParallelActivity which contains two HandleExternalEvent activities. The invoked handler of each activity simply puts its thread to sleep for 3 seconds. In the host program, I created two threads and trigger the two events via the service. Moreover, I subclass the ManualWorkflowSchedulerService in order to track its Schedule method. Here are the results (the time is in 10ths of ms):

Src    Time Thread
HOST   7616      1 CreateWorkflow
MWSS   7642      1 Schedule workflow
HOST   8297     12 Trigger event 1 and wait for RunWorkflow
MWSS   8316     12 Schedule workflow
  WF   8327     12 Handler 1 Invoked...wait 3 sec
HOST   8327      1 Press any key to exit...
HOST   8767     13 Trigger event 2 and wait for RunWorkflow
MWSS   8784     13 Schedule workflow
  WF  38319     12 Handler 1 Completed
  WF  38406     12 Handler 2 Invoked...wait 3 sec
  WF  68396     12 Handler 2 Completed
HOST  68573     13 RunWorkflow for event 2 completed in 5,98 sec
HOST  68794     12 WorkflowCompleted
HOST  68795     12 RunWorkflow for event 1 completed in 6,05 sec

Some remarks:

1. The scheduler always use the thread of the host to schedule the workitem.
2. The workflow instance does not always use the thread of the host to execute the activities. If another activity is already executing in a thread, then this thread is used for executing all scheduled activities.
3. The execution of the handlers is thread-safe, but both threads wait both handlers to finish!

If the latter is your concern, I would suggest the following posts:

• Understanding ParallelActivity in Windows Workflow
• Use Workflow to Invoke Web Services in Parallel

BTW, can you share some info about the scenario that you are facing?