Assembling the components

After a series of articles exploring how portions of solution could be built

• Simplicating and securing the HTTP handler
• Logging and the start of simplification
• Queuing uplink messages
• Provisioning Devices on demand
• First TTN payload to the cloud
• Out stupiding myself
• When Serialisation goes bad-payload_fields
• Basic JSON Deserialisation
• Infrastructure and payloads

I now had working code for receiving The Things Network(TTN) HTTP integration JSON messages with an Azure Function using an HTTPTrigger. (secured with an APIKey) and then putting them into an Azure Storage Queue for processing. This code was intentionally kept as small and as simple as possible so there was less to go wrong. The required configuration is also minimal.

In the last couple of posts I had been building an Azure Function with a QueueTrigger to process the uplink messages. The function used custom bindings so that the CloudQueueMessage could be accessed, and load the Azure Storage account plus queue name from configuration. I’m still using classes generated by JSON2CSharp (with minimal modifications) for deserialising the payloads with JSON.Net.

The message processor Azure Function uses a ConcurrentCollection to store AzureDeviceClient objects constructed using the information returned by the Azure Device Provisioning Service(DPS). This is so the DPS doesn’t have to be called for the connection details for every message.(When the Azure function is restarted the dictionary of DeviceClient objects has to be repopulated). If there is a backlog of messages the message processor can process more than a dozen messages concurrently so the telemetry events displayed in an application like Azure IoT Central can arrive out of order.

The solution uses DPS Group Enrollment with Symmetric Key Attestation so Azure IoT Hub devices can be “automagically” created when a message from a new device is processed. The processing code is multi-thread and relies on many error conditions being handled by the Azure Function retry mechanism. After a number of failed retries the messages are moved to a poison queue. Azure Storage Explorer is a good tool for viewing payloads and moving poison messages back to the processing queue.

public static class UplinkMessageProcessor
{
   static readonly ConcurrentDictionary<string, DeviceClient> DeviceClients = new ConcurrentDictionary<string, DeviceClient>();

   [FunctionName("UplinkMessageProcessor")]
   public static async Task Run(
      [QueueTrigger("%UplinkQueueName%", Connection = "AzureStorageConnectionString")]
      CloudQueueMessage cloudQueueMessage, // Used to get CloudQueueMessage.Id for logging
      Microsoft.Azure.WebJobs.ExecutionContext context,
      ILogger log)
   {
      PayloadV5 payloadObect;
      DeviceClient deviceClient = null;
      DeviceProvisioningServiceSettings deviceProvisioningServiceConfig;

      string environmentName = Environment.GetEnvironmentVariable("ENVIRONMENT");

      // Load configuration for DPS. Refactor approach and store securely...
      var configuration = new ConfigurationBuilder()
      .SetBasePath(context.FunctionAppDirectory)
      .AddJsonFile($"appsettings.json")
      .AddJsonFile($"appsettings.{environmentName}.json")
      .AddEnvironmentVariables()
      .Build();

      // Load configuration for DPS. Refactor approach and store securely...
      try
      {
         deviceProvisioningServiceConfig = (DeviceProvisioningServiceSettings)configuration.GetSection("DeviceProvisioningService").Get<DeviceProvisioningServiceSettings>(); ;
      }
      catch (Exception ex)
      {
         log.LogError(ex, $"Configuration loading failed");
         throw;
      }

      // Deserialise uplink message from Azure storage queue
      try
      {
         payloadObect = JsonConvert.DeserializeObject<PayloadV5>(cloudQueueMessage.AsString);
      }
      catch (Exception ex)
      {
         log.LogError(ex, $"MessageID:{cloudQueueMessage.Id} uplink message deserialisation failed");
         throw;
      }

      // Extract the device ID as it's used lots of places
      string registrationID = payloadObect.hardware_serial;

      // Construct the prefix used in all the logging
      string messagePrefix = $"MessageID: {cloudQueueMessage.Id} DeviceID:{registrationID} Counter:{payloadObect.counter} Application ID:{payloadObect.app_id}";
      log.LogInformation($"{messagePrefix} Uplink message device processing start");

      // See if the device has already been provisioned
      if (DeviceClients.TryAdd(registrationID, deviceClient))
      {
         log.LogInformation($"{messagePrefix} Device provisioning start");

         string enrollmentGroupSymmetricKey = deviceProvisioningServiceConfig.EnrollmentGroupSymmetricKeyDefault;

         // figure out if custom mapping for TTN applicationID
         if (deviceProvisioningServiceConfig.ApplicationEnrollmentGroupMapping != null)
        {
            deviceProvisioningServiceConfig.ApplicationEnrollmentGroupMapping.GetValueOrDefault(payloadObect.app_id, deviceProvisioningServiceConfig.EnrollmentGroupSymmetricKeyDefault);
         }

         // Do DPS magic first time device seen
         await DeviceRegistration(log, messagePrefix, deviceProvisioningServiceConfig.GlobalDeviceEndpoint, deviceProvisioningServiceConfig.ScopeID, enrollmentGroupSymmetricKey, registrationID);
      }

      // Wait for the Device Provisioning Service to complete on this or other thread
      log.LogInformation($"{messagePrefix} Device provisioning polling start");
      if (!DeviceClients.TryGetValue(registrationID, out deviceClient))
      {
         log.LogError($"{messagePrefix} Device provisioning polling TryGet before while failed");

         throw new ApplicationException($"{messagePrefix} Device provisioning polling TryGet before while failed");
      }

      while (deviceClient == null)
      {
         log.LogInformation($"{messagePrefix} provisioning polling delay");
         await Task.Delay(deviceProvisioningServiceConfig.DeviceProvisioningPollingDelay);

         if (!DeviceClients.TryGetValue(registrationID, out deviceClient))
         {
            log.LogError($"{messagePrefix} Device provisioning polling TryGet while loop failed");

            throw new ApplicationException($"{messagePrefix} Device provisioning polling TryGet while loopfailed");
         }
      }

      // Assemble the JSON payload to send to Azure IoT Hub/Central.
      log.LogInformation($"{messagePrefix} Payload assembly start");
      JObject telemetryEvent = new JObject();
      try
      {
         JObject payloadFields = (JObject)payloadObect.payload_fields;
         telemetryEvent.Add("HardwareSerial", payloadObect.hardware_serial);
         telemetryEvent.Add("Retry", payloadObect.is_retry);
         telemetryEvent.Add("Counter", payloadObect.counter);
         telemetryEvent.Add("DeviceID", payloadObect.dev_id);
         telemetryEvent.Add("ApplicationID", payloadObect.app_id);
         telemetryEvent.Add("Port", payloadObect.port);
         telemetryEvent.Add("PayloadRaw", payloadObect.payload_raw);
         telemetryEvent.Add("ReceivedAt", payloadObect.metadata.time);

         // If the payload has been unpacked in TTN backend add fields to telemetry event payload
         if (payloadFields != null)
         {
            foreach (JProperty child in payloadFields.Children())
            {
               EnumerateChildren(telemetryEvent, child);
            }
         }
      }
      catch (Exception ex)
      {
         if (DeviceClients.TryRemove(registrationID, out deviceClient))
         {
            log.LogWarning($"{messagePrefix} TryRemove payload assembly failed");
         }

         log.LogError(ex, $"{messagePrefix} Payload assembly failed");
         throw;
      }

      // Send the message to Azure IoT Hub/Azure IoT Central
      log.LogInformation($"{messagePrefix} Payload SendEventAsync start");
      try
      {
         using (Message ioTHubmessage = new Message(Encoding.ASCII.GetBytes(JsonConvert.SerializeObject(telemetryEvent))))
         {
            // Ensure the displayed time is the acquired time rather than the uploaded time. esp. importan for messages that end up in poison queue
            ioTHubmessage.Properties.Add("iothub-creation-time-utc", payloadObect.metadata.time.ToString("s", CultureInfo.InvariantCulture));
            await deviceClient.SendEventAsync(ioTHubmessage);
         }
      }
      catch (Exception ex)
      {
         if (DeviceClients.TryRemove(registrationID, out deviceClient))
         {
            log.LogWarning($"{messagePrefix} TryRemove SendEventAsync failed");
         }

         log.LogError(ex, $"{messagePrefix} SendEventAsync failed");
         throw;
      }

   log.LogInformation($"{messagePrefix} Uplink message device processing completed");
   }
}

There is also support for using a specific GroupEnrollment based on the application_id in the uplink message payload.

"DeviceProvisioningService": {
      "GlobalDeviceEndpoint": "global.azure-devices-provisioning.net",
      "ScopeID": "",
      "EnrollmentGroupSymmetricKeyDefault": "TopSecretKey",
      "DeviceProvisioningPollingDelay": 500,
      "ApplicationEnrollmentGroupMapping": {
         "Application1": "TopSecretKey1",
         "Application2": "TopSecretKey2"
      }
   }

In addition to the appsettings.json there is configuration for application insights, uplink message queue name and Azure Storage connection strings. The “Environment” setting is important as it specifies what appsettings.json file should be used if code is being debugged etc..

The deployed solution application consists of Azure IoTHub and DPS instances. There are two Azure functions, one for putting the messages from the TTN into a queue the other is for processing them. The Azure Functions are hosted in an Azure AppService plan.

An Azure Storage account is used for the queue and Azure Function synchronisation information and Azure Application Insights is used to monitor the solution.