MQTTnet Azure Function Binding

It Looks promising

I’m using MQTTnet to build my The Things Industries client and it looked like the amount of code for my Message Queue Telemetry Transport (MQTT) Data API Integration could be reduced by using the AzureFunction MQTT Binding by Kees Schollaart.

I used The Things Industries simulate uplink functionality for my initial testing

TTI uplink message simulator

The first version of the Azure function code proof of concept(PoC) was very compact

namespace MQTTnetAzureFunction
{
   using System;
   using System.Text;
   using Microsoft.Azure.WebJobs;
   using Microsoft.Extensions.Logging;

   using CaseOnline.Azure.WebJobs.Extensions.Mqtt;
   using CaseOnline.Azure.WebJobs.Extensions.Mqtt.Messaging;
   using CaseOnline.Azure.WebJobs.Extensions.Mqtt.Config;
   using CaseOnline.Azure.WebJobs.Extensions.Mqtt.Bindings;

   using MQTTnet.Client.Options;
   using MQTTnet.Extensions.ManagedClient;

   public static class Subscribe
   {
      [FunctionName("UplinkMessageProcessor")]
      public static void UplinkMessageProcessor(
            [MqttTrigger("v3/application123456789012345/devices/+/up", ConnectionString = "TTNMQTTConnectionString")] IMqttMessage message,
IMqttMessage message,
            ILogger log)
      {
         var body = Encoding.UTF8.GetString(message.GetMessage());

         log.LogInformation($"Advanced: message from topic {message.Topic} \nbody: {body}");
      }
   }
}

I configured the TTNMQTTConnectionString in the application’s local.settings.json file

{
    "IsEncrypted": false,
   "Values": {
      "AzureWebJobsStorage": "DefaultEndpointsProtocol=https;AccountName=...",
      "AzureWebJobsDashboard": "DefaultEndpointsProtocol=https;AccountName=...",
      "FUNCTIONS_WORKER_RUNTIME": "dotnet",
      "TTNMQTTConnectionString": "Server=...;Username=application1@...;Password=...",
   }
}

This was a good start but I need to be able to configure the MQTT topic for deployments.

After looking at the binding source code plus some trial and error based on the AdvancedConfiguration sample I have a nasty PoC

public static class Subscribe
{
   [FunctionName("UplinkMessageProcessor")]
   public static void UplinkMessageProcessor(
         [MqttTrigger(typeof(ExampleMqttConfigProvider), "v3/%TopicName%/devices/+/up")] IMqttMessage message,
         ILogger log)
   {
      var body = Encoding.UTF8.GetString(message.GetMessage());

      log.LogInformation($"Advanced: message from topic {message.Topic} \nbody: {body}");
   }
}

public class MqttConfigExample : CustomMqttConfig
{
  public override IManagedMqttClientOptions Options { get; }

  public override string Name { get; }

  public MqttConfigExample(string name, IManagedMqttClientOptions options)
  {
     Options = options;
     Name = name;
   }
}

public class ExampleMqttConfigProvider : ICreateMqttConfig
{
   public CustomMqttConfig Create(INameResolver nameResolver, ILogger logger)
   {
      var connectionString = new MqttConnectionString(nameResolver.Resolve("TTNMQTTConnectionString"), "CustomConfiguration");

      var options = new ManagedMqttClientOptionsBuilder()
             .WithAutoReconnectDelay(TimeSpan.FromSeconds(5))
             .WithClientOptions(new MqttClientOptionsBuilder()
                  .WithClientId(connectionString.ClientId.ToString())
                  .WithTcpServer(connectionString.Server, connectionString.Port)
                  .WithCredentials(connectionString.Username, connectionString.Password)
                  .Build())
             .Build();

      return new MqttConfigExample("CustomConnection", options);
   }
}

The TTNMQTTConnectionString and TopicName can be configured in the application’s local.settings.json file

{
    "IsEncrypted": false,
   "Values": {
      "AzureWebJobsStorage": "DefaultEndpointsProtocol=https;AccountName=...",
      "AzureWebJobsDashboard": "DefaultEndpointsProtocol=https;AccountName=...",
      "FUNCTIONS_WORKER_RUNTIME": "dotnet",
      "TTNMQTTConnectionString": "Server=...;Username=application1@...;Password=...",
      "TopicName": "application1@..."
   }
}

When run in the Azure Functions Core Tools the simulated message properties and payload are displayed

The message “The ‘UplinkMessageProcessor’ function is in error: Unable to configure binding ‘message’ of type ‘mqttTrigger’. This may indicate invalid function.json properties. Can’t figure out which ctor to call.” needs further investigation.

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