ROS 2 Migration: Entity states

Overview

In ROS 1, services for querying and setting the state (position and velocity) of entities in the world were provided by the gazebo_ros_api_plugin. In ROS 2, this feature is provided by the gazebo_ros_state plugin.

Read more about the migration of gazebo_ros_api_plugin at ROS 2 Migration: gazebo_ros_api_plugin.

⌛ TODO: The following still haven't been migrated:

• WorldState message (the message type exists but doesn't seem to be used anywhere)

Summary

• Services have been renamed to better reflect the functionality, by including the word "entity", which encompasses models, links and lights in Gazebo.

• There are no longer separate services to get / set link and model states. Both are handled by the same service, which now also handles lights, collisions, etc.

• There used to be a duplicate topic and service for each entity type to set state, for example set_model_state was both a topic and a service. Now there are only services.

• Note that the header sequence field (header.seq) has been deprecated in ROS 1 for a while and is not present in ROS 2, so it's not being populated anymore.

• The Get and Set services had inconsistent fields for the name of the relative entity (relative_entity_name / reference_frame). This has been standardized to reference_frame.

• The Get services' responses had duplicate information from Set. To reduce duplication, the same message is now used on both services. This means that now some fields of the Get response are one level deeper, that is, what used to be res.pose is now res.state.pose.

• On ROS 1 the plugins used to accept the string map to refer to the world inertial frame. On ROS 2, this has been removed since map doesn't have a special meaning in Gazebo.

	ROS 1	ROS 2
Service names	get_model_state	get_entity_state
	get_link_state	get_entity_state
	set_model_state	set_entity_state
	set_link_state	set_entity_state
Service types	gazebo_msgs/GetModelState	gazebo_msgs/GetEntityState
	gazebo_msgs/GetLinkState	gazebo_msgs/GetEntityState
	gazebo_msgs/SetModelState	gazebo_msgs/SetEntityState
	gazebo_msgs/SetLinkState	gazebo_msgs/SetEntityState
Topic names	model_states	model_states
	link_states	link_states
Message types	gazebo_msgs/ModelState	gazebo_msgs/EntityState
	gazebo_msgs/LinkState	gazebo_msgs/EntityState
	gazebo_msgs/ModelStates	gazebo_msgs/ModelStates
	gazebo_msgs/LinkStates	gazebo_msgs/LinkStates
Fields	model_name	name
	link_name	name
	relative_entity_name	reference_frame