Gazebo Components

There are six components involved in running an instance of a Gazebo simulation:

1. Gazebo Server
2. Gazebo Client
3. World Files
4. Model Files
5. Environment Variables
6. Plugins

1- Gazebo Server

The first main component involved in running an instance of a Gazebo simulation is the Gazebo Server or also known by gzserver .

gzserver performs most of the heavy-lifting for Gazebo. It is responsible for parsing the description files related to the scene we are trying to simulate, as well as the objects within. It then simulates the complete scene using a physics and sensor engine.

While the server can be launched independently by using the following command in a terminal:

$ gzserver

It does not have any GUI component. Running gzserver in a so-called headless mode can come in handy in certain situations, but we will talk more about that in future lessons.

2- Gazebo Client

The second main component involved in running an instance of a Gazebo simulation is the Gazebo Client or also known by gzclient .

gzclient on the other hand provides the very essential Graphical Client that connects to the gzserver and renders the simulation scene along with useful interactive tools. While you can technically run gzclient by itself using the following command:

$ gzclient

it does nothing at all (except consume your compute resources), since it does not have a gzserver to connect to and receive instructions from.

Combining Gazebo Server and Gazebo Client

It is a common practice to run gzserver first, followed by gzclient , allowing some time to initialize the simulation scene, objects within, and associated parameters before rendering it. To make our lives easier, there is a single intuitive command that necessarily launches both the components sequentially:

$ gazebo

3- World Files

A world file in Gazebo contains all the elements in the simulated environment. These elements are your robot model, its environment, lighting, sensors, and other objects. You have the ability to save your simulation to a world file that usually has a .world extension.

Gazebo can also read the content of a world file from your disk to generate the simulation. To launch the simulation from a world file, type:

$ gazebo <yourworld>.world

The world file is formatted using the Simulation Description Format or SDF for short. Here’s the basic format of an SDF world file:

<?xml version="1.0" ?>
<sdf version="1.5">
  <world name="default">
    <physics type="ode">
      ...
    </physics>

    <scene>
      ...
    </scene>

    <model name="box">
      ...
    </model>

    <model name="sphere">
      ...
    </model>

    <light name="spotlight">
      ...
    </light>

  </world>
</sdf>

4- Model Files

For simplification, you must create a separate SDF file of your robot with exactly the same format as your world file. This model file should only represent a single model (ex: a robot) and can be imported by your world file. The reason why you need to keep your model in a separate file is to use it in other projects. To include a model file of a robot or any other model inside your world file, you can add the following code to the world’s SDF file:

<include>
  <uri>model://model_file_name</uri>
</include>

5- Environment Variables

There are many environment variables that Gazebo uses, primarily to locate files (world, model, …) and set up communications between gzserver and gzclient . While working on a robotic project, you’ll leave these variables as default.
Here’s an example of a variable that Gazebo uses:

GAZEBO_MODEL_PATH: List of directories where Gazebo looks to populate a model file.

6- Plugins

To interact with a world, model, or sensor in Gazebo, you can write plugins. These plugins can be either loaded from the command line or added to your SDF world file. You’ll learn about World Plugins later in the lesson.