mqtt_client

</a> </a>

The mqtt_client package provides a ROS 2 component node that enables connected ROS-based devices or robots to exchange ROS messages via an MQTT broker using the MQTT protocol. This works generically for arbitrary ROS message types. The mqtt_client can also exchange primitive messages with MQTT clients running on devices not based on ROS.

[!IMPORTANT]
This repository is open-sourced and maintained by the Institute for Automotive Engineering (ika) at RWTH Aachen University.
V2X Communication is one of many research topics within our Vehicle Intelligence & Automated Driving domain.
If you would like to learn more about how we can support your automated driving or robotics efforts, feel free to reach out to us!
:email: opensource@ika.rwth-aachen.de

• Installation
  • docker-ros
• Usage
  • Quick Start
  • Launch
  • Configuration
• Primitive Messages
• Latency Computation
• Package Summary
• Acknowledgements

Installation

The mqtt_client package is released as an official ROS 2 package and can easily be installed via a package manager.

sudo apt update
sudo apt install ros-$ROS_DISTRO-mqtt-client

If you would like to install mqtt_client from source, simply clone this repository into your ROS 2 workspace. All dependencies that are listed in the ROS package.xml can be installed using rosdep.

# mqtt_client$
rosdep install -r --ignore-src --from-paths .

# workspace$
colcon build --packages-up-to mqtt_client --cmake-args -DCMAKE_BUILD_TYPE=Release

docker-ros

mqtt_client is also available as a Docker image, containerized through docker-ros.

docker run --rm ghcr.io/ika-rwth-aachen/mqtt_client:latest # or distro-specific tags, e.g., :rolling

Usage

The mqtt_client can be easily integrated into an existing ROS-based system. Below, you first find a quick start guide to test the mqtt_client on a single machine. Then, more details are presented on how to launch and configure it in more complex applications.

Quick Start

Follow these steps to quickly launch a working mqtt_client that is sending ROS messages via an MQTT broker to itself.

Demo Broker

It is assumed that an MQTT broker (such as Mosquitto) is running on localhost:1883.

For this demo, you may easily launch Mosquitto with its default configuration using Docker.

docker run --rm --network host --name mosquitto eclipse-mosquitto

For a more advanced setup of your own broker, check out our instructions for running an MQTT broker in Docker with enabled authentication and encryption here.

Demo Configuration

The mqtt_client is best configured with a ROS parameter yaml file. The configuration shown below (also see params.yaml / params.yaml) allows an exchange of messages as follows:

• ROS messages received locally on ROS topic /ping/ros are sent to the broker on MQTT topic pingpong/ros;
• MQTT messages received from the broker on MQTT topic pingpong/ros are published locally on ROS topic /pong/ros;
• primitive ROS messages received locally on ROS topic /ping/primitive are sent as primitive (string) messages to the broker on MQTT topic pingpong/primitive;
• MQTT messages received from the broker on MQTT topic pingpong/primitive are published locally as primitive ROS messages on ROS topic /pong/primitive.

broker:
  host: localhost
  port: 1883
bridge:
  ros2mqtt:
    - ros_topic: /ping/ros
      mqtt_topic: pingpong/ros
    - ros_topic: /ping/primitive
      mqtt_topic: pingpong/primitive
      primitive: true
  mqtt2ros:
    - mqtt_topic: pingpong/ros
      ros_topic: /pong/ros
    - mqtt_topic: pingpong/primitive
      ros_topic: /pong/primitive
      primitive: true

Demo Client Launch

Launch the mqtt_client node with the pre-configured demo parameters:

ros2 launch mqtt_client standalone.launch.xml

[ WARN] [1665575657.358869079]: Parameter 'broker/tls/enabled' not set, defaulting to '0'
[ WARN] [1665575657.359798329]: Parameter 'client/id' not set, defaulting to ''
[ WARN] [1665575657.359810889]: Client buffer can not be enabled when client ID is empty
[ WARN] [1665575657.360300703]: Parameter 'client/clean_session' not set, defaulting to '1'
[ WARN] [1665575657.360576344]: Parameter 'client/keep_alive_interval' not set, defaulting to '60.000000'
[ WARN] [1665575657.360847295]: Parameter 'client/max_inflight' not set, defaulting to '65535'
[ INFO] [1665575657.361281461]: Bridging ROS topic '/ping/ros' to MQTT topic 'pingpong/ros'
[ INFO] [1665575657.361303380]: Bridging primitive ROS topic '/ping/primitive' to MQTT topic 'pingpong/primitive'
[ INFO] [1665575657.361352809]: Bridging MQTT topic 'pingpong/ros' to ROS topic '/pong/ros'
[ INFO] [1665575657.361370558]: Bridging MQTT topic 'pingpong/primitive' to primitive ROS topic '/pong/primitive'
[ INFO] [1665575657.362153083]: Connecting to broker at 'tcp://localhost:1883' ...
[ INFO] [1665575657.462622065]: Connected to broker at 'tcp://localhost:1883'

Note that the mqtt_client successfully connected to the broker and also echoed which ROS/MQTT topics are being bridged. For testing the communication between mqtt_client, itself, and other MQTT clients, open five new terminals.

In order to test the communication among mqtt_clients, publish any ROS message on ROS topic /ping/ros and wait for a response on ROS topic /pong/ros.

# 1st terminal: publish ROS message to /ping
ros2 topic pub /ping/ros std_msgs/msg/String "{data: \"Hello MQTT\"}"

# 2nd terminal: listen for ROS messages on /pong
ros2 topic echo /pong/ros

In order to test the communication between mqtt_client and other MQTT clients, publish a primitive ROS message on ROS topic /ping/primitive, directly publish a primitive MQTT message on MQTT topic pingpong/primitive and wait for responses on ROS topic /pong/primitive. Note that you need to restart the ROS 2 mqtt_client with a different config file.

# mqtt_client$
ros2 launch mqtt_client standalone.launch.xml params_file:=$(ros2 pkg prefix mqtt_client)/share/mqtt_client/config/params.primitive.yaml

# 3rd terminal: publish primitive ROS message to /ping/primitive
ros2 topic pub /ping/primitive std_msgs/msg/Int32 "{data: 42}"

# 4th terminal: listen for primitive ROS messages on /pong/primitive
ros2 topic echo /pong/primitive

# 5th terminal: publish primitive MQTT message to pingpong/primitive directly using mosquitto_pub
docker run --rm --network host eclipse-mosquitto mosquitto_pub -h localhost -t "pingpong/primitive" --repeat 20 --repeat-delay 1 -m 69

If everything works as expected, the second terminal should print a message at 1Hz, while the fourth terminal should print two different messages at 1Hz.

Launch

You can start the mqtt_client node with:

ros2 launch mqtt_client standalone.launch.xml

This will automatically load the provided demo params.yaml / params.yaml. If you wish to load your custom configuration file, simply pass params_file.

ros2 launch mqtt_client standalone.launch.xml params_file:="</PATH/TO/PARAMS.YAML>"

In order to exploit the benefits of mqtt_client being a ROS 2 component, load the component into your own component container.

Configuration

All available ROS parameters supported by the mqtt_client and their default values (in []) are listed in the following.

Broker Parameters

broker:
  host:              # [localhost] IP address or hostname of the machine running the MQTT broker
  port:              # [1883] port the MQTT broker is listening on
  user:              # username used for authenticating to the broker (if empty, will try to connect anonymously)
  pass:              # password used for authenticating to the broker
  tls:
    enabled:           # [false] whether to connect via SSL/TLS
    ca_certificate:    # [/etc/ssl/certs/ca-certificates.crt] CA certificate file trusted by client (relative to ROS_HOME)

Client Parameters

client:
  id:                   # unique ID string used to identify the client (broker may allow empty ID and automatically generate one)
  buffer:
    size:                 # [0] maximum number of messages buffered by the bridge when not connected to broker (only available if client ID is not empty)
    directory:            # [buffer] directory used to buffer messages when not connected to broker (relative to ROS_HOME)
  last_will:
    topic:                # topic used for this client's last-will message (no last will, if not specified)
    message:              # [offline] last-will message
    qos:                  # [0] QoS value for last-will message
    retained:             # [false] whether to retain last-will message
  clean_session:        # [true] whether to use a clean session for this client
  keep_alive_interval:  # [60.0] keep-alive interval in seconds
  max_inflight:         # [65535] maximum number of inflight messages
  tls:
    certificate:          # client certificate file (only needed if broker requires client certificates; relative to ROS_HOME)
    key:                  # client private key file (relative to ROS_HOME)
    password:             # client private key password
    version:              # TLS version (https://github.com/eclipse/paho.mqtt.cpp/blob/master/src/mqtt/ssl_options.h#L305)
    verify:               # verify the client should conduct post-connect checks.
    alpn_protos:          # list of ALPN protocols (https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set_alpn_protos.html)

Bridge Parameters

bridge:
  ros2mqtt:                # Object specifying which ROS topics to map to which MQTT topics
    ros_topics:            # Array specifying which ROS topics to bridge
      - {{ ros_topic_name }} # The ROS topic that should be bridged, corresponds to the sub-object in the YAML
    {{ ros_topic_name }}:
      mqtt_topic:          # MQTT topic on which the corresponding ROS messages are sent to the broker
      primitive:           # [false] whether to publish as primitive message
      ros_type:            # [*empty*] If set, the ROS msg type provided will be used. If empty, the type is automatically deduced via the publisher
      inject_timestamp:    # [false] whether to attach a timestamp to a ROS2MQTT payload (for latency computation on receiver side)
      advanced:
        ros:
          queue_size:      # [1] ROS subscriber queue size
          qos:
            reliability:   # [auto] One of "auto", "system_default", "reliable", "best_effort". If auto, the QoS is automatically determined via the publisher
            durability:    # [auto] One of "auto", "system_default", "volatile", "transient_local". If auto, the QoS is automatically determined via the publisher
        mqtt:
          qos:             # [0] MQTT QoS value
          retained:        # [false] whether to retain MQTT message
  mqtt2ros:                # Object specifying which MQTT topics to map to which ROS topics
    mqtt_topics:           # Array specifying which ROS topics to bridge
      - {{ mqtt_topic_name }} # The MQTT topic that should be bridged, corresponds to the sub-object in the YAML
    {{ mqtt_topic_name }}:
      ros_topic:           # ROS topic on which corresponding MQTT messages are published
      ros_type:            # [*empty*] If set, the ROS msg type provided will be used. If empty, the type is automatically deduced via the MQTT message
      primitive:           # [false] whether to publish as primitive message (if coming from non-ROS MQTT client)
      advanced:
        mqtt:
          qos:             # [0] MQTT QoS value
        ros:
          queue_size:      # [1] ROS publisher queue size
          latched:         # [false] whether to latch ROS message
          qos:
            reliability:   # [system_default] One of "system_default", "reliable", "best_effort". 
            durability:    # [system_default] One of "system_default", "volatile", "transient_local". 

Primitive Messages

As seen in the Quick Start, the mqtt_client can not only exchange arbitrary ROS messages with other mqtt_clients, but it can also exchange primitive message data with other non-mqtt_client MQTT clients. This allows ROS-based devices to exchange primitive messages with devices not based on ROS. The primitive parameter can be set for both ROS-to-MQTT (bridge/ros2mqtt) and for MQTT-to-ROS (bridge/mqtt2ros) transmissions.

If a ROS-to-MQTT transmission is configured as primitive and the ROS message type is one of the supported primitive ROS message types, the raw data is published as a string. The supported primitive ROS message types are std_msgs/String, std_msgs/Bool, std_msgs/Char, std_msgs/UInt8, std_msgs/UInt16, std_msgs/UInt32, std_msgs/UInt64, std_msgs/Int8, std_msgs/Int16, std_msgs/Int32, std_msgs/Int64, std_msgs/Float32, std_msgs/Float64.

If an MQTT-to-ROS transmission is configured as primitive, the MQTT message is interpreted and published as a primitive data type, if possible. The message is probed in the following order: bool (std_msgs/Bool), int (std_msgs/Int32), float (std_msgs/Float32), string (std_msgs/String).

Latency Computation

The mqtt_client provides built-in functionality to measure the latency of transferring a ROS message via an MQTT broker back to ROS. Note that this functionality is only available for non-primitive messages (see Primitive Messages). To this end, the sending client injects the current timestamp into the MQTT message. The receiving client can then compute the latency between message reception time and the injected timestamp. Naturally, this is only accurate to the level of synchronization between clocks on sending and receiving machine.

In order to inject the current timestamp into outgoing MQTT messages, the parameter inject_timestamp has to be set for the corresponding bridge/ros2mqtt entry. The receiving mqtt_client will then automatically publish the measured latency in seconds as a ROS std_msgs/Float64 message on topic /<mqtt_client_name>/latencies/<mqtt2ros/ros_topic>.

These latencies can be printed easily with:

ros2 topic echo /<mqtt_client_name>/latencies/<mqtt2ros/ros_topic>/data

or plotted with rqt_plot:

ros2 run rqt_plot rqt_plot /<mqtt_client_name>/latencies/<mqtt2ros/ros_topic>/data

Package Summary

This short package summary documents the package in line with the ROS Wiki Style Guide.

Components

mqtt_client/MqttClient

Enables connected ROS-based devices or robots to exchange ROS messages via an MQTT broker using the MQTT protocol.

Subscribed Topics

• <bridge/ros2mqtt/ros_topic> (rclcpp::SerializedMessage) ROS topic whose messages are transformed to MQTT messages and sent to the MQTT broker. May have arbitrary ROS message type.

Published Topics

• <bridge/mqtt2ros/ros_topic> (rclcpp::SerializedMessage) ROS topic on which MQTT messages received from the MQTT broker are published. May have arbitrary ROS message type.
• ~/latencies/<bridge/mqtt2ros/ros_topic> (std_msgs/Float64) Latencies measured on the message transfer to <bridge/mqtt2ros/ros_topic> are published here, if the received messages have a timestamp injected (see Latency Computation).

Services

• ~/is_connected (mqtt_client/srv/IsConnected) Returns whether the client is connected to the MQTT broker.

• ~/new_ros2mqtt_bridge (mqtt_client/srv/NewRos2MqttBridge) Returns whether a new ROS -> MQTT bridge was created.

• ~/new_mqtt2ros_bridge (mqtt_client/srv/NewMqtt2RosBridge) Returns whether a new MQTT -> ROS bridge was created.

Parameters

See Configuration.

Acknowledgements

This research is accomplished within the projects 6GEM (FKZ 16KISK036K) and UNICARagil (FKZ 16EMO0284K). We acknowledge the financial support for the projects by the Federal Ministry of Education and Research of Germany (BMBF).