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ROS nodes do not have a direct parallel in DDS. DDS deals only with topics, it has no externally-visible awareness of the entities that are publishing or subscribing to those topics. Internally, DDS has a concept of participants, which are objects that publish and subscribe to topics, but there is no equivalence relationship between a DDS participant and a ROS node any more (it used to be a 1-to-1 mapping, but now there is only one DDS participant per process and it is possible to have multiple nodes in a single process).