In this tutorial we are going to learn how to get our Medical Device to Cloud data.

   Note:
This tutorial is from a chapter in the HL7 FHIR on Azure – Device Framework eBook.

The following are the Requirements.

Requirements

• Ability to capture our Medical Device output data.
• Ability to filter the data.
• Ability to update the Observation Resource document with the output data.
• Ability to modify the EndPoint Response after updating the Observation Resource document.
• Ability to parse the Observation Resource document and post it to any EndPoint.

Solution

We are going to create a Function App, with a EventHubTrigger to do the following:

1. Loop through the EventHub messages.
2. Create a EventGrid message.
3. Post the EventGrid message.

Next we will create a Function App, DeviceGrid that handles the updating of the Observation Resource document.

1. Subscribes to our EventGrid messages
2. Upserts the Observation Resource document with the data from the EventGrid messages

Leverage Azure API Management Policies.

1. Publish DeviceGrid Function App to Azure API Management.
2. Create a policy that subscribes to the EventGrid topic.
3. Extract specific device data and post it to any EndPoint.

Function Apps

EventHubTrigger – Function App

We are using an EventHubTrigger to loop through the EventHub Messages.
Then we create a EventGrid message and publish it.

Source Code

#region

using System;
using System.Configuration;
using System.Globalization;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;
using System.Web;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Host;
using Microsoft.Azure.WebJobs.ServiceBus;
using Newtonsoft.Json;

#endregion

namespace DeviceMetrics
{
    public static class DeviceToCloud
    {
        private static readonly string TopicEndpoint = ConfigurationManager.AppSettings["topicEndpoint"];
        private static readonly string TopicKey = ConfigurationManager.AppSettings["topicKey"];

        [FunctionName("DeviceToCloud")]
        public static void Run([EventHubTrigger("DeviceToCloud", Connection = " EventHubConnectionString")]
            string[] myEventHubMessages, TraceWriter log)
        {
            log.Info($"C# Event Hub trigger function processed a message: {myEventHubMessages.Length}");

            foreach (var msg in myEventHubMessages)
            {
                dynamic data = JsonConvert.DeserializeObject(msg);

                var eventGridMessage = new GridEvent
                {
                    Id = Guid.NewGuid().ToString(),
                    EventTime = DateTime.UtcNow,
                    EventType = data.Properties.EventType, 
                    Data = data.Properties.Data,
                    Subject = data.Properties.Name,
                    Topic = data.Properties.Topic,
                    DeviceId = data.Id
                };


                try
                {
                    SendEvent(TopicEndpoint, TopicKey, eventGridMessage).ConfigureAwait(false);
                }
                catch (HttpRequestException e)
                {
                    log.Error(e.Message, e);
                }
            }
        }


        public static async Task SendEvent(string topicEndpoint, string topicKey, object data)
        {
            // Create a SAS token for the call to the event grid. We can do this with 
            // the SAS key as well but wanted to show an alternative.
            var sas = CreateEventGridSasToken(topicEndpoint, DateTime.Now.AddDays(1), topicKey);

            // Instantiate an instance of the HTTP client with the 
            // event grid topic endpoint.
            var client = new HttpClient {BaseAddress = new Uri(topicEndpoint)};

            // Configure the request headers with the content type
            // and SAS token needed to make the request.
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            client.DefaultRequestHeaders.Add("aeg-sas-token", sas);

            // Serialize the data
            var json = JsonConvert.SerializeObject(data);
            var stringContent = new StringContent(json, Encoding.UTF8, "application/json");

            // Publish grid event
            await client.PostAsync(string.Empty, stringContent);
        }


        private static string CreateEventGridSasToken(string resourcePath, DateTime expirationUtc, string topicKey)
        {
            const char resource = 'r';
            const char expiration = 'e';
            const char signature = 's';

            // Encode the topic resource path and expiration parameters
            var encodedResource = HttpUtility.UrlEncode(resourcePath);
            var encodedExpirationUtc = HttpUtility.UrlEncode(expirationUtc.ToString(CultureInfo.InvariantCulture));

            // Format the unsigned SAS token
            var unsignedSas = $"{resource}={encodedResource}&{expiration}={encodedExpirationUtc}";

            // Create an HMCASHA256 policy with the topic key
            using (var hmac = new HMACSHA256(Convert.FromBase64String(topicKey)))
            {
                // Encode the signature and create the fully signed URL with the
                // appropriate parameters.
                var bytes = Convert.ToBase64String(hmac.ComputeHash(Encoding.UTF8.GetBytes(unsignedSas)));
                var encodedSignature = HttpUtility.UrlEncode(bytes);
                var signedSas = $"{unsignedSas}&{signature}={encodedSignature}";

                return signedSas;
            }
        }
    }
}

DeviceEventGrid – Function App

Source Code

#region

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.Azure.Documents;
using Microsoft.Azure.Documents.Client;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.Azure.WebJobs.Host;
using Newtonsoft.Json;

#endregion

namespace DeviceMetrics
{
    public static class DeviceEventGrid
    {
        private static readonly string Endpoint = ConfigurationManager.AppSettings["endpoint"];
        private static readonly string AuthKey = ConfigurationManager.AppSettings["authKey"];
        private static readonly string Database = ConfigurationManager.AppSettings["database"];
        private static readonly string Collection = ConfigurationManager.AppSettings["collection"];

        private static readonly DocumentClient Client = new DocumentClient(new Uri(Endpoint), AuthKey,
            new ConnectionPolicy
            {
                ConnectionMode = ConnectionMode.Direct,
                ConnectionProtocol = Protocol.Tcp
            }
        );


        [FunctionName("DeviceEventGrid")]
        public static async Task<HttpResponseMessage> Run(
            [HttpTrigger(AuthorizationLevel.Function, "post")]
            HttpRequestMessage req,
            TraceWriter log)
        {
            log.Info("C# HTTP trigger function processed a request.");


            var jsonContent = await req.Content.ReadAsStringAsync();
            var gridEvent = JsonConvert.DeserializeObject<List<GridEvent<Dictionary<string, string>>>>(jsonContent)
                ?.SingleOrDefault();

            if (gridEvent == null) return req.CreateErrorResponse(HttpStatusCode.BadRequest, $@"Missing event details");

            var gridEventType = req.Headers.GetValues("Aeg-Event-Type").FirstOrDefault();

            if (gridEventType == "SubscriptionValidation")
            {
                // Retrieve the validation code and echo back.
                var validationCode = gridEvent.Data["validationCode"];
                var validationResponse = JsonConvert.SerializeObject(new
                {
                    validationResponse = validationCode
                });

                return new HttpResponseMessage
                {
                    StatusCode = HttpStatusCode.OK,
                    Content = new StringContent(validationResponse)
                };
            }

            if (gridEventType == "Notification")
            {
                // Get the Observation Resource Document
                dynamic doc = await Client.ReadDocumentAsync(UriFactory.CreateDocumentUri(Database, Collection,
                    gridEvent.Data["deviceId"]));


                var valueQuanity = new ValueQuantity
                {
                    Code = doc.Component.ValueQuantity.Code,
                    System = doc.Component.ValueQuantity.System,
                    Unit = gridEvent.Data["unit"],
                    Value = gridEvent.Data["value"]
                };

                var component = new Component
                {
                    Code = doc.Component.Code,
                    ValueQuantity = valueQuanity
                };


                // partial values 
                var observation = new CompoundNumericObservation
                {
                    Id = doc.Id,
                    Component = {[0] = component},
                    EffectiveDateTime = new DateTimeOffset(DateTime.Parse(gridEvent.Data["timestamp"]))
                };


                try
                {
                    var result =
                        await Client.UpsertDocumentAsync(UriFactory.CreateDocumentUri(Database, Collection, gridEvent.Data["deviceId"]),
                            observation);

                    return req.CreateResponse(result.StatusCode);
                }
                catch (DocumentClientException e)
                {
                    var resp = new HttpResponseMessage
                    {
                        StatusCode = (HttpStatusCode) e.StatusCode,
                        Content = new StringContent(e.Message)
                    };
                    return resp;
                }
            }

            return req.CreateErrorResponse(HttpStatusCode.BadRequest,
                $@"Unknown request type");
        }

        public class GridEvent<T> where T : class
        {
            public string Id { get; set; }
            public string EventType { get; set; }
            public string Subject { get; set; }
            public DateTime EventTime { get; set; }
            public T Data { get; set; }
            public string Topic { get; set; }
        }
    }
}

Subscribing to EventGrid Events Using Azure API Management

After we pubish our Function App to Api Management, we will create a new Policy for our Function App. This allows us to extract specific device data post it to any EndPoint.

The following is an example policy.

<policies>
  <inbound>
    <base />
    <set-variable value="@(context.Request.Headers["Aeg-Event-Type"].Contains("SubscriptionValidation"))" name="isEventGridSubscriptionValidation" />
    <set-variable value="@(context.Request.Headers["Aeg-Event-Type"].Contains("Notification"))" name="isEventGridNotification" />
    <choose>
      <when condition="@(context.Variables.GetValueOrDefault<bool>("isEventGridSubscriptionValidation"))">
        <return-response>
          <set-status code="200" reason="OK" />
          <set-body>@{
            var events = context.Request.Body.As<string>();
            JArray a = JArray.Parse(events);
            var eventGridData = a.First["data"];
            var validationCode = eventGridData["validationCode"];
            var jOutput =
              new JObject(
                new JProperty("validationResponse", validationCode)
                );
            return jOutput.ToString();
          }</set-body>
        </return-response>
      </when>
      <when condition="@(context.Variables.GetValueOrDefault<bool>("isEventGridNotification"))">
        <send-one-way-request mode="new">
          <set-url>https://XXXXXXXXXXXXXXXXXXXXXX}</set-url>
          <set-method>POST</set-method>
          <set-body>@{
            var events = context.Request.Body.As<string>();
            JArray a = JArray.Parse(events);
            var eventGridData = a.First["data"];
            var deviceId = eventGridData["deviceId"];
            var deviceData = eventGridData["deviceData"];
            var patientd = eventGridData["patientd"];
            return new JObject(
                new JProperty("deviceId", deviceId),
                new JProperty("data", deviceData),
                new JProperty("patientd", patientd)                ;
          }</set-body>
        </send-one-way-request>
      </when>
    </choose>
  </inbound>
  <backend>
    <base />
  </backend>
  <outbound>
    <base />
  </outbound>
  <on-error>
    <base />
  </on-error>
</policies>

Summary

• We are able to leverage the use an Azure EventGrid to create an EventGrid message.
• We are using a Function App to loop through the EventHub messages.
• We are able to create and publish EventGrid messages within the Function App.
• We created another Function App that subscribes to our EventGrid Topic.
• This Function App upserts the H7 FHIR Observation Resource with the ValueQuantity data.
• We are using a Azure API management Policy to parse the ValueQuantity data and post it to any EndPoint.

Next Steps

• We are going to create a Durable Function to orchestrate the message flow