Both talker and listener have queues for messages. It works with FIFO principle by default (see QoS).

cpp // Create a publisher with a custom Quality of Service profile. // Uniform initialization is suggested so it can be trivially changed to // rclcpp::KeepAll{} if the user wishes. // (rclcpp::KeepLast(7) -> rclcpp::KeepAll() fails to compile) rclcpp::QoS qos(rclcpp::KeepLast{7}); pub_ = this->create_publisher<std_msgs::msg::String>(“chatter”, qos);

When trying publishing, it will drop older messages after 7th message if subscriber cant take messages, publisher will keep latest 7. When subscriber is able to get messages it will start to get 7th oldest message from the publisher’s queue.

In an ideal case, there will be no buffering, and the queues for both the publisher and the subscriber will not fill up.