std::deque<Node*> ready;
  ready.push_back(src_node);
  // ready_enters_per_frame and ready_exits serve as a staging area to buffer
  // the ready enters/exits before they are moved to the `ready` queue for
  // controlling the start and end of a processing frame.
  absl::flat_hash_map<absl::string_view, std::vector<Node*>>
      ready_enters_per_frame;
  // Exit nodes shall all be from the same frame, as we process a frame at a