Launching multiple nodes in ROS

Introduction

Launching multiple nodes lets you start several parts of a robot program at once. This helps different robot functions work together smoothly.

You want to run a sensor node and a motor control node together.

You need to start a navigation node and a mapping node at the same time.

You want to test multiple robot behaviors in one go.

You have separate nodes for data processing and user interface and want them running together.

Syntax

ROS

ros2 launch package_name file.launch.py

The ros2 launch command runs a launch file that can start many nodes.

Launch files are Python scripts that list nodes and their settings.

Examples

This launch file starts two nodes: the turtle simulator and the keyboard teleoperation.

ROS

<launch>
  <node pkg="turtlesim" type="turtlesim_node" name="sim"/>
  <node pkg="turtlesim" type="turtle_teleop_key" name="teleop"/>
</launch>

This launch file runs sensor and motor nodes from the my_robot package, showing their output on the screen.

ROS

<launch>
  <node pkg="my_robot" type="sensor_node" name="sensor" output="screen"/>
  <node pkg="my_robot" type="motor_node" name="motor" output="screen"/>
</launch>

Sample Program

This launch file starts two demo nodes: one that sends messages (talker) and one that receives them (listener). Both print output to the screen.

ROS

<launch>
  <node pkg="demo_nodes_cpp" type="talker" name="talker" output="screen"/>
  <node pkg="demo_nodes_cpp" type="listener" name="listener" output="screen"/>
</launch>

Important Notes

Make sure all nodes you launch are installed and built correctly.

You can add parameters and remappings inside the launch file for more control.

The output="screen" attribute shows node outputs in your terminal.

Summary

Launch files let you start many ROS nodes together easily.

Use ros2 launch with a launch file to run multiple nodes.

Launch files are XML and list nodes with their package, type, and name.

Practice

1. What is the main purpose of a ROS launch file when working with multiple nodes?

easy

A. To debug a single ROS node

B. To write the source code of ROS nodes

C. To start several ROS nodes together with one command

D. To stop all running ROS nodes automatically

Solution

1. Step 1: Understand the role of launch files and compare options

   Launch files are used to start multiple ROS nodes at once, simplifying the process. Only "To start several ROS nodes together with one command" correctly describes starting multiple nodes together; others describe unrelated tasks.

2. Final Answer:

   To start several ROS nodes together with one command -> Option C

3. Quick Check:

   Launch file = start multiple nodes [OK]

Hint: Launch files group nodes to run with one command [OK]

Common Mistakes:

• Thinking launch files write node code
• Confusing launch files with debugging tools
• Assuming launch files stop nodes

2. Which of the following is the correct XML tag to define a node in a ROS launch file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall ROS launch file syntax and check options

   The correct tag to start a node is <node> with attributes pkg, type, and name. Only <node pkg="package_name" type="node_type" name="node_name" /> uses the correct <node> tag; others use invalid tags like <launch>, <start>, <run>.

2. Final Answer:

   <node pkg="package_name" type="node_type" name="node_name" /> -> Option A

3. Quick Check:

   Node tag = <node> [OK]

Hint: Use <node> tag with pkg, type, and name attributes [OK]

Common Mistakes:

• Using <launch> or <start> instead of <node>
• Forgetting to close the tag properly
• Mixing up attribute names

3. Given this launch file snippet:

<launch>
  <node pkg="turtlesim" type="turtlesim_node" name="sim1" />
  <node pkg="turtlesim" type="turtlesim_node" name="sim2" />
</launch>

What happens when you run roslaunch with this file?

medium

A. The launch file causes a syntax error and no nodes start

B. Only one turtlesim node starts named sim1

C. Two nodes start but both have the same name sim1 causing a conflict

D. Two turtlesim nodes named sim1 and sim2 start simultaneously

Solution

1. Step 1: Analyze node definitions and ROS naming rules

   Two nodes are defined with different names: sim1 and sim2, both from turtlesim package. Nodes with unique names can run simultaneously without conflict.

2. Final Answer:

   Two turtlesim nodes named sim1 and sim2 start simultaneously -> Option D

3. Quick Check:

   Unique node names = both run [OK]

Hint: Different node names allow multiple nodes to run [OK]

Common Mistakes:

• Assuming same package means one node only
• Thinking duplicate names cause silent errors
• Believing launch files always cause syntax errors

4. You wrote this launch file snippet:

<launch>
  <node pkg="demo_pkg" type="demo_node" name="node1">
</launch>

What is the error and how to fix it?

medium

A. Missing closing tag for <node>, fix by adding </node>

B. Wrong attribute name 'type', fix by changing to 'exec'

C. Missing package name, fix by adding pkg attribute

D. Extra closing tag needed for <launch>, fix by adding </launch>

Solution

1. Step 1: Check XML tag completeness and verify attributes

   The <node> tag is opened but not closed properly; it needs a closing </node> tag. Attributes pkg and type are correct; <launch> tag is closed properly.

2. Final Answer:

   Missing closing tag for <node>, fix by adding </node> -> Option A

3. Quick Check:

   Unclosed node tag = add </node> [OK]

Hint: Every opened tag must be closed properly [OK]

Common Mistakes:

• Confusing 'type' with 'exec' attribute
• Ignoring missing closing tags
• Assuming launch tag is not closed

5. You want to launch three nodes from different packages in one launch file:

<launch>
  <node pkg="pkg1" type="node1" name="node1" />
  <node pkg="pkg2" type="node2" name="node2" />
  <node pkg="pkg3" type="node3" name="node3" />
</launch>

How can you modify this launch file to add a parameter use_sim_time set to true for all nodes?

hard

A. Add <param name="use_sim_time" value="true" /> inside each <node> tag

B. Add <param name="use_sim_time" value="true" /> directly inside <launch> tag

C. Add <arg name="use_sim_time" default="true" /> inside each <node> tag

D. Add <env name="use_sim_time" value="true" /> inside each <node> tag

Solution

1. Step 1: Understand parameter scope in launch files and compare options

   Parameters placed directly inside <launch> apply globally to all nodes. Adding <param name="use_sim_time" value="true" /> directly inside <launch> sets it once for all; inside each <node> repeats unnecessarily; options B and C use wrong tags (<env>, <arg>).

2. Final Answer:

   Add <param name="use_sim_time" value="true" /> directly inside <launch> tag -> Option B

3. Quick Check:

   Global param inside <launch> = all nodes get it [OK]

Hint: Put common params inside <launch> for all nodes [OK]

Common Mistakes:

• Adding param inside each node causing repetition
• Using <arg> or <env> instead of <param>
• Placing param outside launch tag