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Use MQTTnet to develop distributed application workloads that connect to MQTT broker

  • Artikkeli

MQTTnet is an open-source, high performance .NET library for MQTT based communication. This article uses a Kubernetes service account token and MQTTnet to connect to MQTT broker. You should use service account tokens to connect in-cluster applications.

Sample code

The sample code performs the following steps:

  1. Creates an MQTT client using the MqttFactory class:

    var mqttFactory = new MqttFactory();
var mqttClient = mqttFactory.CreateMqttClient();

  2. The Kubernetes pod specification mounts the service account on the container file system. The contents of the file are read: ##3. The mounted token is used as the password with well-known username K8S-SAT:

    static string sat_auth_file = "/var/run/secrets/tokens/mqtt-client-token";
...
var satToken = File.ReadAllBytes(sat_auth_file);

  3. The MQTT client options are configured using the MqttClientOptions class. Using the MqttClientOptionsBuilder as advised in the client documentation is the advised way of setting the options:

    var mqttClientOptions = new MqttClientOptionsBuilder()
    .WithTcpServer(hostname, tcp_port)
    .WithProtocolVersion(MqttProtocolVersion.V500)
    .WithClientId("mqtt-client-dotnet")
    .WithAuthentication("K8S-SAT", satToken);

  4. After setting up the MQTT client options, a connection can be established. The following code shows how to connect with a server. You can replace the CancellationToken.None with a valid CancellationToken if needed.

    var response = await mqttClient.ConnectAsync(mqttClientOptions.Build(), CancellationToken.None);

  5. MQTT messages can be created using the properties directly or using MqttApplicationMessageBuilder. This class has overloads that allow dealing with different payload formats. The API of the builder is a fluent API. The following code shows how to compose an application message and publish them to an article called sampletopic:

    var applicationMessage = new MqttApplicationMessageBuilder()
    .WithTopic("sampletopic")
    .WithPayload("samplepayload" + counter++)
    .Build();

await mqttClient.PublishAsync(applicationMessage, CancellationToken.None);

Pod specification

The serviceAccountName field in the pod configuration must match the service account associated with the token being used. Also, note the serviceAccountToken.expirationSeconds is set to 86400 seconds, and once it expires, you need to reload the token from disk. This logic isn't implemented in this sample.

apiVersion: v1
kind: ServiceAccount
metadata:
  name: mqtt-client
  namespace: azure-iot-operations

---
apiVersion: v1
kind: Pod
metadata:
  name: mqtt-client-dotnet
  namespace: azure-iot-operations
spec:
  serviceAccountName: mqtt-client

  volumes:

  # SAT token used to authenticate between the application and the MQTT broker  
  - name: mqtt-client-token
    projected:
      sources:
      - serviceAccountToken:
          path: mqtt-client-token
          audience: aio-internal
          expirationSeconds: 86400

  # Certificate chain for the application to validate the MQTT broker              
  - name: aio-ca-trust-bundle
    configMap:
      name: azure-iot-operations-aio-ca-trust-bundle

  containers:
  - name: mqtt-client-dotnet
    image: ghcr.io/azure-samples/explore-iot-operations/mqtt-client-dotnet:latest
    volumeMounts:
    - name: mqtt-client-token
      mountPath: /var/run/secrets/tokens/
    - name: aio-ca-trust-bundle
      mountPath: /var/run/certs/aio-internal-ca-cert/
    env:
    - name: hostname
      value: "aio-broker"
    - name: tcpPort
      value: "18883"
    - name: useTls
      value: "true"
    - name: caFile
      value: "/var/run/certs/aio-internal-ca-cert/ca.crt"
    - name: satAuthFile
      value: "/var/run/secrets/tokens/mqtt-client-token"

To run the sample, follow the instructions in its README.

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