The On Start action is an activity callback action that allows you to implement business rules to validate whether an instance of a process activity in the process flow can start executing its outgoing flow. An On Start action can have local variables.
If you want to stop the flow execution from being started, raise an exception using the Raise Exception element. This will prevent the process activity from starting the flow execution. Learn more about how to Explicitly Raise an Exception.
As an example, think of a conditional start to cancel the request of an IT resource. If there are no resources available at the moment of the request and there is budget, the resource can be leased to satisfy the request. Once the order is placed it is not possible to cancel the request.
To prevent users to cancel a request after an order has been submitted, the On Start action is used. If an order has been placed to rent the resource, the Abort Activity Change Exception is thrown, and the process is not cancelled. Instead, if an order has not been submitted the request is cancelled and the process instance terminated.
In these Scenarios you should use the Abort Activity Change Exception. Once this exception is raised, it is logged in Service Center under the general log. All other exceptions are logged under the error tab.
Process Activities Callback Actions | Process Activities Life Cycle | Process Callback Actions | Explicitly Raise an Exception | Designing Actions | Designing Processes | Process Flow Toolbox