Information representative of a graph-based program specification has a plurality of components, each of which corresponds to a task, and directed links between ports of said components. A program corresponding to said graph-based program specification is executed. A first component includes a first data port, a first control port, and a second control port. Said first data port is configured to receive data to be processed by a first task corresponding to said first component, or configured to provide data that was processed by said first task corresponding to said first component. Executing a program corresponding to said graph-based program specification includes: receiving said first control information at said first control port, in response to receiving said first control information, determining whether or not to invoke said first task, and after receiving said first control information, providing said second control information from said second control port.