A graph-based program specification includes components, at least one having at least one input port for receiving a collection of data elements, or at least one collection type output port for providing a collection of data elements. Executing a program specified by the graph-based program specification at a computing node, includes: receiving data elements of a first collection into a first storage in a first order via a link connected to a collection type output port of a first component and an input port of a second component, and invoking a plurality of instances of a task corresponding to the second component to process data elements of the first collection, including retrieving the data elements from the first storage in a second order, without blocking invocation of any of the instances until after any particular instance completes processing one or more data elements.