Note that a service is said to have _completed_ even if it returned an error in one or more of its iterations.
Taverna 2 requires all parts of the workflow to return a value, even if it is an error document. Downstream services receiving an error should normally return a new, derived error directly - but error documents are not delivered to control links.
This is different from Taverna 1 behaviour, where control links were only activated if the full iteration was completed. In some Taverna 1 workflows control links were used together with local workers like
There are plans to develop native branching for Taverna 2.
To remove control links