ROS 2 QoS Profiles: Reliable vs Best-Effort Robot Communication

ROS 2 QoS Profiles: Reliable vs Best-Effort Robot Communication Robot systems continuously exchange data with very different requirements. A dropped camera frame is usually acceptable. A dropped emergency command may not be. ROS 2 Quality of Service (QoS) lets you express these requirements. The Two Common Reliability Modes Reliable Reliable communication attempts to ensure that samples reach compatible subscribers. Useful for: Commands Configuration Important state transitions Critical application data Best Effort Best effort prioritizes timely delivery and may tolerate lost samples. Useful for: Cameras LiDAR High-frequency IMU streams Other continuously refreshed sensor data Example Imagine a camera producing 30 frames per second. If frame 100 is lost, the system can often process frame 101 immediately. For a command: MOVE_FORWARD losing the message may be unacceptable. Therefore: Camera -> Best Effort Command -> Reliable is often a sensible starting point. QoS Dimensions Reliability is only one QoS policy. Important policies include: Reliability Durability History Depth Deadline Lifespan Liveliness C++ Example auto sensor_qos = rclcpp :: SensorDataQoS (); auto publisher = create_publisher < sensor_msgs :: msg :: Image > ( "/camera/image" , sensor_qos ); For important application data, you might explicitly configure reliable communication: auto qos = rclcpp :: QoS ( rclcpp :: KeepLast ( 10 )) . reliable (); auto publisher = create_publisher < std_msgs :: msg :: String > ( "/robot/status" , qos ); QoS Compatibility A publisher and subscriber need compatible QoS settings. A common mistake is: Publisher: Best Effort Subscriber: Reliable and then wondering why messages are not received as expected. Always inspect the effective QoS of both endpoints. A Practical Decision Table Topic Suggested Starting Point Camera image Best Effort Point cloud Best Effort IMU Best Effort Navigation command Reliable Configuration Reliable Robot state Reliable Diagnostics Reliable These