ROS 2 overview


ROS 2 integration overview

For ROS 1, see ROS integration overview.

Gazebo is a stand-alone application which can be used independently of ROS or ROS 2. The integration of Gazebo with either ROS version is done through a set of packages called gazebo_ros_pkgs. These packages provide a bridge between Gazebo's C++ API and transport system, and ROS 2 messages and services.

Not all functionality from ROS 1 has been ported to ROS 2 yet. You can check the progress on this wiki page.

The ROS 2 package gazebo_ros_pkgs is a metapackage which contains the following packages:

  • gazebo_dev: Provides a cmake config for the default version of Gazebo for the ROS distribution. So downstream packages can just depend on gazebo_dev instead of needing to find Gazebo by themselves.

  • gazebo_msgs: Message and service data structures for interacting with Gazebo from ROS 2.

  • gazebo_ros: Provides convenient C++ classes and functions which can be used by other plugins, such as gazebo_ros::Node and conversion and testing utilities. It also provides some generally useful plugins.

  • gazebo_plugins: A series of Gazebo plugins exposing sensors and other features to ROS 2. For example, gazebo_ros_camera publishes ROS 2 images, and gazebo_ros_diff_drive provides an interface for controlling and instrospecting differential drive robots through ROS 2.

Target versions

The ROS 2 port of gazebo_ros_pkgs will have debian packages released with the Crystal Clemmys release, scheduled for December 2018. The code can also be built from source using the ros2 branch against ROS 2 master branches.

The targeted Gazebo version is Gazebo 9.

Differences from ROS 1

Several changes and refactoring were done to gazebo_ros_pkgs when migrating from ROS 1 to ROS 2.

Some goals of the refactoring were:

  • Take advantage of new ROS 2 features, such as masterless discovery.
  • Remove code which duplicates functionality already present in Gazebo.
  • Reduce duplication by standardizing common functionality, such as how to set ROS namespaces, parameters and topic remapping.
  • Modernize the codebase, making use of the latest SDFormat, Gazebo and Ignition APIs, as well as ROS 2's style guidelines and linters.
  • Add tests to all ported functionality.

Detailed migration guides for each plugin can be found on the gazebo_ros_pkgs wiki. See some general highlights below.

Init

The ROS 1 integration required that Gazebo be launched with the gazebo_ros_api_plugin system plugin, which would initialize ROS.

There's no such requirement with ROS 2. Gazebo can be started without any plugins and ROS-2-enabled plugins can be added at runtime.

Node

In ROS 1, each plugin typically had one or more ros::NodeHandle instances to interact with ROS.

In ROS 2, plugins use gazebo_ros::Node instead, which allows each plugin to exist as its own node in the ROS graph, with its own parameters, namespace, loggers, etc. Plugins also don't need to worry about spinning the node or keeping callback queues - gazebo_ros handles all that internally.

SDF parsing

There are several configurations which Gazebo ROS plugins commonly want to set through SDF, and in the ROS 1 implementation, there was a lot of duplicate code on plugins parsing the same things, sometimes following loose conventions.

In ROS 2, common configurations like namespace, ROS parameters and topic remapping rules are parsed by gazebo_ros::Node, so there's no need for plugins to reimplement them every time.

gazebo_ros_api_plugin

In ROS 1, gazebo_ros_api_plugin was a large plugin which offered a lot of functionality in a bundle, giving users little flexibility to opt-in/out of features.

In ROS 2, this plugin is being split into smaller, more focused plugins. You can read the migration details on ROS 2 Migration: gazebo_ros_api_plugin.