Send cloud-to-device messages with IoT Hub (Node.js)

Azure IoT Hub is a fully managed service that helps enable reliable and secure bi-directional communications between millions of devices and a solution back end. The Send telemetry from a device to an IoT hub quickstart shows how to create an IoT hub, provision a device identity in it, and code a simulated device app that sends device-to-cloud messages.

Note

The features described in this article are available only in the standard tier of IoT Hub. For more information about the basic and standard IoT Hub tiers, see Choose the right IoT Hub tier.

This tutorial builds on Send telemetry from a device to an IoT hub. It shows you how to:

  • From your solution back end, send cloud-to-device messages to a single device through IoT Hub.
  • Receive cloud-to-device messages on a device.
  • From your solution back end, request delivery acknowledgment (feedback) for messages sent to a device from IoT Hub.

You can find more information on cloud-to-device messages in the IoT Hub developer guide.

At the end of this tutorial, you run two Node.js console apps:

  • SimulatedDevice, a modified version of the app created in Send telemetry from a device to an IoT hub, which connects to your IoT hub and receives cloud-to-device messages.

  • SendCloudToDeviceMessage, which sends a cloud-to-device message to the simulated device app through IoT Hub, and then receives its delivery acknowledgment.

Note

IoT Hub has SDK support for many device platforms and languages (including C, Java, Python, and Javascript) through Azure IoT device SDKs. For step-by-step instructions on how to connect your device to this tutorial's code, and generally to Azure IoT Hub, see the Azure IoT Developer Center.

Prerequisites

  • Node.js version 10.0.x or later. Prepare your development environment describes how to install Node.js for this tutorial on either Windows or Linux.

  • An active Azure account. (If you don't have an account, you can create a free account in just a couple of minutes.)

Receive messages in the simulated device app

In this section, you modify the simulated device app you created in Send telemetry from a device to an IoT hub to receive cloud-to-device messages from the IoT hub.

  1. Using a text editor, open the SimulatedDevice.js file. This file is located in the iot-hub\Quickstarts\simulated-device folder off of the root folder of the Node.js sample code you downloaded in the Send telemetry from a device to an IoT hub quickstart.

  2. Register a handler with the device client to receive messages sent from IoT Hub. Add the call to client.on just after the line that creates the device client as in the following snippet:

    var client = DeviceClient.fromConnectionString(connectionString, Mqtt);
    
    client.on('message', function (msg) {
      console.log('Id: ' + msg.messageId + ' Body: ' + msg.data);
      client.complete(msg, function (err) {
        if (err) {
          console.error('complete error: ' + err.toString());
        } else {
          console.log('complete sent');
        }
      });
    });
    

    In this example, the device invokes the complete function to notify IoT Hub that it has processed the message. The call to complete is not required if you are using MQTT transport and can be omitted. It is required for HTTPS and AMQP.

    Note

    If you use HTTPS instead of MQTT or AMQP as the transport, the DeviceClient instance checks for messages from IoT Hub infrequently (less than every 25 minutes). For more information about the differences between MQTT, AMQP and HTTPS support, and IoT Hub throttling, see the IoT Hub developer guide.

Get the IoT hub connection string

In this article, you create a backend service to send cloud-to-device messages through the IoT hub you created in Send telemetry from a device to an IoT hub. To send cloud-to-device messages, your service needs the service connect permission. By default, every IoT Hub is created with a shared access policy named service that grants this permission.

To get the IoT Hub connection string for the service policy, follow these steps:

  1. In the Azure portal, select Resource groups. Select the resource group where your hub is located, and then select your hub from the list of resources.

  2. On the left-side pane of your IoT hub, select Shared access policies.

  3. From the list of policies, select the service policy.

  4. Under Shared access keys, select the copy icon for the Connection string -- primary key and save the value.

    Show how to retrieve the connection string

For more information about IoT Hub shared access policies and permissions, see Access control and permissions.

Send a cloud-to-device message

In this section, you create a Node.js console app that sends cloud-to-device messages to the simulated device app. You need the device ID of the device you added in the Send telemetry from a device to an IoT hub quickstart. You also need the IoT hub connection string you copied previously in Get the IoT hub connection string.

  1. Create an empty folder called sendcloudtodevicemessage. In the sendcloudtodevicemessage folder, create a package.json file using the following command at your command prompt. Accept all the defaults:

    npm init
    
  2. At your command prompt in the sendcloudtodevicemessage folder, run the following command to install the azure-iothub package:

    npm install azure-iothub --save
    
  3. Using a text editor, create a SendCloudToDeviceMessage.js file in the sendcloudtodevicemessage folder.

  4. Add the following require statements at the start of the SendCloudToDeviceMessage.js file:

    'use strict';
    
    var Client = require('azure-iothub').Client;
    var Message = require('azure-iot-common').Message;
    
  5. Add the following code to SendCloudToDeviceMessage.js file. Replace the "{iot hub connection string}" and "{device id}" placeholder values with the IoT hub connection string and device ID you noted previously:

    var connectionString = '{iot hub connection string}';
    var targetDevice = '{device id}';
    
    var serviceClient = Client.fromConnectionString(connectionString);
    
  6. Add the following function to print operation results to the console:

    function printResultFor(op) {
      return function printResult(err, res) {
        if (err) console.log(op + ' error: ' + err.toString());
        if (res) console.log(op + ' status: ' + res.constructor.name);
      };
    }
    
  7. Add the following function to print delivery feedback messages to the console:

    function receiveFeedback(err, receiver){
      receiver.on('message', function (msg) {
        console.log('Feedback message:')
        console.log(msg.getData().toString('utf-8'));
      });
    }
    
  8. Add the following code to send a message to your device and handle the feedback message when the device acknowledges the cloud-to-device message:

    serviceClient.open(function (err) {
      if (err) {
        console.error('Could not connect: ' + err.message);
      } else {
        console.log('Service client connected');
        serviceClient.getFeedbackReceiver(receiveFeedback);
        var message = new Message('Cloud to device message.');
        message.ack = 'full';
        message.messageId = "My Message ID";
        console.log('Sending message: ' + message.getData());
        serviceClient.send(targetDevice, message, printResultFor('send'));
      }
    });
    
  9. Save and close SendCloudToDeviceMessage.js file.

Run the applications

You are now ready to run the applications.

  1. At the command prompt in the simulated-device folder, run the following command to send telemetry to IoT Hub and to listen for cloud-to-device messages:

    node SimulatedDevice.js
    

    Run the simulated device app

  2. At a command prompt in the sendcloudtodevicemessage folder, run the following command to send a cloud-to-device message and wait for the acknowledgment feedback:

    node SendCloudToDeviceMessage.js
    

    Run the app to send the cloud-to-device command

    Note

    For simplicity, this tutorial does not implement any retry policy. In production code, you should implement retry policies (such as exponential backoff), as suggested in the article, Transient Fault Handling.

Next steps

In this tutorial, you learned how to send and receive cloud-to-device messages.

To see examples of complete end-to-end solutions that use IoT Hub, see Azure IoT Remote Monitoring solution accelerator.

To learn more about developing solutions with IoT Hub, see the IoT Hub developer guide.