Once you are satisfied with the designed workflow schema and workflow and have it configured, click the Run workflow button on the toolbar (alternatively, you can select the Actions ‣ Run workflow item in the main menu or launch it by pressing Ctrl+R). The workflow gets verified and scheduled for background execution. If you continue editing the workflow, this will not affect the launched execution. You can control the workflow execution via the Task View: watch progress, cancel it, etc.
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Note, that you can see the progress of a schema execution workflow execution in a Workflow Designer window by checking the Track running progress on diagram option in the Applcation Settings dialog:
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