Before a graph can be evaluated, one or more of its nodes must be activated. Activation introduces the concepts of an active node and an invertible edge. An active node is one that is designated by an application to participate in the current evaluation. Activation designates a node as a starting point for traversing the graph.
An invertible edge is initially an undirected edge. An invertible edge may be activated from either end, based on which node currently generates data and which receives data. Its direction is determined during the activation phase and depends on the active topology. When the direction is determined, the invertible edge is converted to a directed edge, and its active potential in the reverse direction is suppressed. Therefore, a suppressed edge represents a directed edge whose generating and receiving ends are reversed. Suppressed edges cannot be evaluated under the prevailing topology.
The activation phase is a breadth-first traversal, beginning at the initially activated nodes. Activation establishes the subgraph that will participate in the evaluation.