ROS2 Graph Introspection - Node/Topic Snapshot

Problem Statement

Implement a graph inspector node that periodically queries the ROS2 graph and logs a structured snapshot.

Requirements

Your node must:

• Use create_timer(3.0, ...) to take a graph snapshot every 3 seconds
• In each snapshot, call:
  • self.get_node_names() — returns list of node names
  • self.get_topic_names_and_types() — returns list of (topic_name, [type_strings])
  • self.get_service_names_and_types() — returns list of (service_name, [type_strings])
• Log the counts: f"Graph: {n_nodes} nodes, {n_topics} topics, {n_services} services"
• Publish the snapshot as a JSON std_msgs/String on /graph_snapshot containing:

  { "nodes": [...], "topics": [...], "services": [...] }
  

• Log "Graph Inspector Ready" on startup

Input / Output

Output: std_msgs/String on /graph_snapshot every 3 seconds

🤖 Why This Matters in Real Robots

Health monitoring systems use graph introspection to detect when critical nodes crash and need to be restarted. Integration tests use it to assert that all expected topics are present before running a scenario. ROS2's component container and lifecycle manager use it to coordinate node startup order.

⚠️ Common Pitfalls

• get_node_names() vs get_node_names_and_namespaces(): get_node_names() returns just names. get_node_names_and_namespaces() returns tuples — don't confuse them.
• The node sees itself: Your own node name will appear in the graph. This is normal.