Specifying a dataflow graph includes: rendering, in a first user interface, a representation of a first dataflow graph. At least one component represents a computation associated with at least one of data flowing into an input port or data flowing out of an output port. A link between components is rendered based on user input indicating a flow of data. In a second user interface, a representation of a sub-graph of the first dataflow graph is rendered. A first flow junction represents a connection between: (1) a flow of data outside the sub-graph interface, and (2) a flow of data inside the sub-graph interface. A link is rendered between the first flow junction and a second flow junction based on user input indicating a relationship between a first descriptor and a second descriptor.