L14: Lifecycle Nodes & ROS 2 Bags#

Overview#

This lecture covers two ROS 2 capabilities essential for building, debugging, and analyzing real robotic systems. Lifecycle nodes follow a standardized state machine (Unconfigured, Inactive, Active, Finalized) that gives the system precise control over initialization order, resource allocation, and shutdown. Publishers, timers, and other resources are allocated in transition callbacks (on_configure, on_activate) rather than __init__, and external commands or programmatic service calls drive the node between states. ROS 2 bags record the messages published on a set of topics, with their timestamps, so a run can be replayed offline exactly as it happened. We compare the SQLite3 and MCAP storage backends, record a full Nav2 navigation run, inspect and replay it from the CLI, and finally visualize the recording in Foxglove Studio using a multi-panel layout (3D, Plot, Raw Messages). All hands-on examples use the lifecycle_demo and bag_demo packages.

Learning Objectives

By the end of this lecture, you will be able to:

  • Describe the four primary states of a lifecycle node and the transitions that connect them.

  • Explain why publishers and timers are allocated in on_configure and on_activate rather than __init__.

  • Distinguish SUCCESS, FAILURE, and ERROR return values from a transition callback and predict the resulting state.

  • Implement a Python lifecycle node by extending LifecycleNode and overriding the four transition callbacks.

  • Drive lifecycle transitions externally with ros2 lifecycle set and programmatically via the change_state service.

  • Record and replay topics with ros2 bag record and ros2 bag play, and choose between SQLite3 and MCAP storage.

  • Inspect a bag’s contents with ros2 bag info and identify the topics needed to reconstruct a navigation run offline.

  • Visualize an MCAP bag in Foxglove Studio using multi-panel layouts (3D, Plot, Raw Messages).

Next Steps#

  • This is the final regular lecture of the semester. The remaining class meetings are dedicated to final project status checks and office hours.

  • Complete the exercises from this lecture whenever you have some time.

  • Continue working on your final project deliverables.

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