L10: Parameters, Custom Interfaces, Services & Actions#

Overview#

This lecture covers four interconnected pillars of ROS 2 communication beyond publish/subscribe. You will learn how to configure nodes at runtime using parameters, define domain-specific data types with custom interfaces (.msg, .srv, .action), implement request/response communication with services, and build goal-oriented long-running tasks with actions. Together these mechanisms form the complete ROS 2 communication toolkit required for building sophisticated robotic systems. All hands-on examples use dedicated demo packages: parameters_demo, custom_interfaces, message_demo, service_demo, and action_demo, built via the lecture10_demo metapackage.

Learning Objectives

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

  • Declare, retrieve, and set ROS 2 parameters, and react to runtime changes with a parameter callback.

  • Define custom message, service, and action interfaces in a CMake package and use them in Python nodes.

  • Write a service server and client using both asynchronous (call_async) and synchronous (call) patterns.

  • Explain when to use a MultiThreadedExecutor and a separate callback group to avoid deadlocks in a synchronous service client.

  • Write an action server that publishes feedback, handles cancellation cooperatively, and returns a terminal result.

  • Write an action client that manages the full asynchronous callback chain: goal response, feedback, cancel response, and result.

  • Choose the appropriate communication mechanism (topic, service, action, parameter) for a given robotics task.

Next Steps#

  • In the next lecture, we will cover L11: Simulation and Mobile Robot Control:

    • Gazebo Harmonic architecture and SDF world files

    • Spawning robots and configuring sensors (LiDAR, camera, IMU)

    • ros_gz_bridge for Gazebo–ROS 2 communication

    • TF2 coordinate frames, broadcasters, and listeners

    • Mobile robot control with cmd_vel and differential drive

    • Reading sensor data in Python nodes

  • Complete the exercises from this lecture before the next class.

  • Set up the Docker simulation environment (see Simulation page).

  • Read Getting Started with Gazebo Harmonic.

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