Demonstrate publishing messages with the AWS IoT Device Client - AWS IoT Core
Publish default MQTT messagePublish custom MQTT message

Demonstrate publishing messages with the AWS IoT Device Client

The procedures in this section demonstrate how the AWS IoT Device Client can send default and custom MQTT messages.

These policy statements in the policy that you created in the previous step for these exercises give the Raspberry Pi permission to perform these actions:

  • iot:Connect

    Gives the client named PubSubTestThing, your Raspberry Pi running the AWS IoT Device Client, to connect.

        {
      "Effect": "Allow",
      "Action": [
        "iot:Connect"
      ],
      "Resource": [
        "arn:aws:iot:us-west-2:57EXAMPLE833:client/PubSubTestThing"
      ]
    }
  • iot:Publish

    Gives the Raspberry Pi permission to publish messages with an MQTT topic of test/dc/pubtopic.

        {
      "Effect": "Allow",
      "Action": [
        "iot:Publish"
      ],
      "Resource": [
        "arn:aws:iot:us-west-2:57EXAMPLE833:topic/test/dc/pubtopic"
      ]
    }

    The iot:Publish action gives permission to publish to the MQTT topics listed in the Resource array. The content of those messages is not controlled by the policy statement.

Publish the default message using the AWS IoT Device Client

This procedure runs the AWS IoT Device Client so that it publishes a single default MQTT message that the MQTT test client receives and displays.

To send the default MQTT message from the AWS IoT Device Client

  1. Make sure that both the terminal window on your local host computer that's connected to your Raspberry Pi and the window with the MQTT test client are visible while you perform this procedure.

  2. In the terminal window, enter these commands to run the AWS IoT Device Client using the config file created in Create the config file.

    cd ~/aws-iot-device-client/build
./aws-iot-device-client --config-file ~/dc-configs/dc-pubsub-config.json

    In the terminal window, the AWS IoT Device Client displays information messages and any errors that occur when it runs.

    If no errors are displayed in the terminal window, review the MQTT test client.

  3. In the MQTT test client, in the Subscriptions window, see the Hello World! message sent to the test/dc/pubtopic message topic.

  4. If the AWS IoT Device Client displays no errors and you see Hello World! sent to the test/dc/pubtopic message in the MQTT test client, you've demonstrated a successful connection.

  5. In the terminal window, enter ^C (Ctrl-C) to stop the AWS IoT Device Client.

After you've demonstrated that the AWS IoT Device Client published the default MQTT message, you can continue to the Publish a custom message using the AWS IoT Device Client.

Publish a custom message using the AWS IoT Device Client

The procedures in this section create a custom MQTT message and then runs the AWS IoT Device Client so that it publishes the custom MQTT message one time for the MQTT test client to receive and display.

Create a custom MQTT message for the AWS IoT Device Client

Perform these steps in the terminal window on the local host computer that's connected to your Raspberry Pi.

To create a custom message for the AWS IoT Device Client to publish

  1. In the terminal window, open a text editor, such as nano.

  2. Into the text editor, copy and paste the following JSON document. This will be the MQTT message payload that the AWS IoT Device Client publishes.

    {
  "temperature": 28,
  "humidity": 80,
  "barometer": 1013,
  "wind": {
    "velocity": 22,
    "bearing": 255
  }
}

  3. Save the contents of the text editor as ~/messages/sample-ws-message.json.

  4. Enter the following command to set the permissions of the message file that you just created.

    chmod 600 ~/messages/*
To create a config file for the AWS IoT Device Client to use to send the custom message

  1. In the terminal window, in a text editor such as nano, open the existing AWS IoT Device Client config file: ~/dc-configs/dc-pubsub-config.json.

  2. Edit the samples object to look like this. No other part of this file needs to be changed.

      "samples": {
    "pub-sub": {
      "enabled": true,
      "publish-topic": "test/dc/pubtopic",
      "publish-file": "~/messages/sample-ws-message.json",
      "subscribe-topic": "test/dc/subtopic",
      "subscribe-file": "~/.aws-iot-device-client/log/pubsub_rx_msgs.log"

  3. Save the contents of the text editor as ~/dc-configs/dc-pubsub-custom-config.json.

  4. Run this command to set the permissions on the new config file.

    chmod 644 ~/dc-configs/dc-pubsub-custom-config.json

Publish the custom MQTT message by using the AWS IoT Device Client

This change affects only the contents of the MQTT message payload, so the current policy will continue to work. However, if the MQTT topic (as defined by the publish-topic value in ~/dc-configs/dc-pubsub-custom-config.json) was changed, the iot::Publish policy statement would also need to be modified to allow the Raspberry Pi to publish to the new MQTT topic.

To send the MQTT message from the AWS IoT Device Client

  1. Make sure that both the terminal window and the window with the MQTT test client are visible while you perform this procedure. Also, make sure that your MQTT test client is still subscribed to the # topic filter. If it isn't, subscribe to the # topic filter again.

  2. In the terminal window, enter these commands to run the AWS IoT Device Client using the config file created in Create the config file.

    cd ~/aws-iot-device-client/build
./aws-iot-device-client --config-file ~/dc-configs/dc-pubsub-custom-config.json

    In the terminal window, the AWS IoT Device Client displays information messages and any errors that occur when it runs.

    If no errors are displayed in the terminal window, review the MQTT test client.

  3. In the MQTT test client, in the Subscriptions window, see the custom message payload sent to the test/dc/pubtopic message topic.

  4. If the AWS IoT Device Client displays no errors and you see the custom message payload that you published to the test/dc/pubtopic message in the MQTT test client, you've published a custom message successfully.

  5. In the terminal window, enter ^C (Ctrl-C) to stop the AWS IoT Device Client.

After you've demonstrated that the AWS IoT Device Client published a custom message payload, you can continue to Demonstrate subscribing to messages with the AWS IoT Device Client.