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### Output of ros2 node info: what are these publishers/subscribers?

I ran the following command, trying to find out more about my /gazebo node:

 user@ubuntu:~\$ ros2 node info /gazebo
/gazebo
Subscribers:
/parameter_events: rcl_interfaces/ParameterEvent
Publishers:
/clock: rosgraph_msgs/Clock
/parameter_events: rcl_interfaces/ParameterEvent
/rosout: rcl_interfaces/Log
Services:
/gazebo/describe_parameters: rcl_interfaces/DescribeParameters
/gazebo/get_parameter_types: rcl_interfaces/GetParameterTypes
/gazebo/get_parameters: rcl_interfaces/GetParameters
/gazebo/list_parameters: rcl_interfaces/ListParameters
/gazebo/set_parameters: rcl_interfaces/SetParameters
/gazebo/set_parameters_atomically: rcl_interfaces/SetParametersAtomically
/pause_physics: std_srvs/Empty
/reset_simulation: std_srvs/Empty
/reset_world: std_srvs/Empty
/unpause_physics: std_srvs/Empty


I don't really understand the output. I understand that /gazebo is subscribing to the topic /parameter_events, but what is rcl_interfaces/ParameterEvent? I thought it was a node that was publishing to /parameter_events, but it doesn't show up when I run ros2 node list and it also doesn't appear as a node on rqt_graph.

I also thought that it might be the location of a file, so I went to search in my ros2 workspace, but I didn't find anything with the same name.

What is it, really?