Launching multiple nodes in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you launch two nodes with the same name in ROS?

Consider you have a ROS launch file that tries to start two nodes with the exact same name attribute. What will be the behavior of the ROS system?

ROS

<launch>
  <node pkg="demo_pkg" type="node_a" name="shared_node" />
  <node pkg="demo_pkg" type="node_b" name="shared_node" />
</launch>

A ROS will throw a syntax error and refuse to launch.

B Both nodes will run independently without any conflict.

C The second node will override the first node, causing the first to shut down.

D Only the first node will run; the second will be ignored silently.

💡 Hint

Think about how ROS manages node names and uniqueness in the system.

In ROS, node names must be unique. Launching two nodes with the same name causes the second node to kick out the first one, shutting it down.

📝 Syntax

intermediate

Identify the syntax error in this ROS launch file snippet

Which option correctly identifies the syntax error in the following ROS launch snippet?

ROS

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

A The <launch> tag is missing a closing tag.

B The first <node> tag is not closed properly.

C The attribute 'output' is invalid in <node> tags.

D The 'type' attribute should be 'executable' instead.

💡 Hint

Check if all XML tags are properly closed.

The first tag is opened but not closed. It should be self-closed or have a closing tag.

❓ state_output

advanced

What is the output when launching nodes with remapped topics?

Given the following launch file remapping the topic /chatter to /talk, what topic will the listener node subscribe to?

ROS

<launch>
  <node pkg="demo_pkg" type="talker" name="talker" output="screen">
    <remap from="/chatter" to="/talk" />
  </node>
  <node pkg="demo_pkg" type="listener" name="listener" output="screen">
    <remap from="/chatter" to="/talk" />
  </node>
</launch>

A The listener fails to subscribe to any topic due to remap conflict.

B The listener subscribes to the topic '/chatter'.

C The listener subscribes to both '/chatter' and '/talk'.

D The listener subscribes to the topic '/talk'.

💡 Hint

Remapping changes the topic names nodes use internally.

Both nodes remap '/chatter' to '/talk', so the listener subscribes to '/talk'.

🔧 Debug

advanced

Why does this launch file fail to start the second node?

Examine the launch file below. The first node starts fine, but the second node never launches. What is the most likely cause?

ROS

<launch>
  <node pkg="demo_pkg" type="node_a" name="node_a" output="screen" />
  <node pkg="demo_pkg" type="node_b" args="--verbose" output="screen" />
</launch>

A The second node is missing the 'name' attribute, causing a launch failure.

B The 'args' attribute is invalid and causes a syntax error.

C The second node's type 'node_b' does not exist in the package.

D The first node blocks the launch process, preventing the second from starting.

💡 Hint

Check required attributes for each node in a launch file.

Each node should have a unique 'name' attribute. Missing it can cause the node not to launch properly.

🧠 Conceptual

expert

How does ROS handle node lifecycle when launching multiple nodes with dependencies?

In a complex ROS system, you launch multiple nodes where some nodes depend on others to be fully initialized before starting. Which ROS feature or approach best manages this dependency and lifecycle?

A Using the ROS 2 Lifecycle Node feature to explicitly manage states like configuring, activating, and shutting down nodes.

B Using <code>roslaunch</code> with <code>respawn</code> and <code>required</code> attributes to control node restarts and failures.

C Manually adding delays in the launch file between node starts to ensure dependencies are ready.

D Relying on topic subscription callbacks to delay node processing until data arrives.

💡 Hint

Think about built-in ROS features designed for node state management.

ROS 2 Lifecycle Nodes provide a formal way to manage node states and dependencies, allowing controlled startup and shutdown sequences.

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