When a scenario calls an action that takes an acknowledgement() action parameter the scenario expects to receive a timely acknowledgement.

This means that the acknowledgement must be made within the chain of routed events that are currently being processed, starting with the event that is the immediate cause of the operation being performed. This constraint exists because the scenario is in a state of limbo while it is waiting for an acknowledgement. If another event comes into the scenario, either a control event or one that comes into one of its blocks, while the scenario is waiting for an acknowledgement then the scenario can get into an inconsistent state. For example, during a block operation, the scenario expects updates only from the block that the operation is called on.

This constraint is usually easily met. If an operation routes a request event that it expects a routed response to then the block can simply wait for that response before returning the acknowledgement to the scenario. Alternatively, the block can set up a completed listener for the request event. If the block does not expect a response with interesting data that it wants to reflect to output feeds then the block can immediately return the acknowledgement even if there are still routed events to be processed. It is especially important to ensure that all operations are acknowledged for all paths through the code because unacknowledged operations will cause the scenario to hang.