Control links enable you to set dependencies between services in a workflow that do not directly share data (i.e. that are not otherwise linked by passing data from one to the other directly or indirectly). A control link allows you to delay the invocation of a service until another has finished.
You might also need to use control links when using asynchronous services, to avoid calling
checkStatus says the service has finished. (See the section on Looping.)
Taverna tutorial 2014: Advanced Taverna features includes an exercise on Control links.
Watch the Control links video.
To add a control links
You may also delete control links from the diagram, but as they are small, they can be difficult to select unless you zoom in.