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00
Guided beta caseProbeginnerQoS reliability

The topic exists but no messages arrive

Restore the obstacle pipeline while preserving the sensor driver's best-effort profile.

Pairs with QoS policies in ROS 2 Systems Debugging. Read the concept first, then diagnose it here.

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Operator report

`/scan` is visible in the graph, but the reliable subscriber receives zero messages.

Ubuntu 24.04, ROS 2 Jazzy, Python and C++ nodes, browser telemetry workspace

01

System boundary

Trace only the relevant path.

3 nodes
  1. 01LiDAR publisher→
  2. 02/scan→
  3. 03obstacle subscriberOUT
02

How this lab works

You diagnose it. No command list.

Open the repair workspace and gather your own evidence in a real terminal. No diagnostic commands are handed to you - finding the fault is the exercise. Stuck? Progressive hints unlock inside the workspace.

03

Verification

Pass more than the visible symptom.

4 checks
  • Subscriber receives at least 9 Hz
  • Graph retains one publisher
  • Hidden regression behavior
  • Root cause explanation