Step 2: Demonstrate publishing messages with the Amazon IoT Device Client - Amazon IoT Core
Publish the default message using the Amazon IoT Device ClientPublish a custom message using the Amazon IoT Device Client.
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Step 2: Demonstrate publishing messages with the Amazon IoT Device Client

The procedures in this section demonstrate how the Amazon 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 Amazon 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 Amazon IoT Device Client

This procedure runs the Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon IoT Device Client.

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

Publish a custom message using the Amazon IoT Device Client.

The procedures in this section create a custom MQTT message and then runs the Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon 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 Amazon IoT Device Client.

After you've demonstrated that the Amazon IoT Device Client published a custom message payload, you can continue to Step 3: Demonstrate subscribing to messages with the Amazon IoT Device Client.