Azure Funciton Service Principal

retrieveSecret.cs

using System.Linq;
using System.Net;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.Azure.WebJobs.Host;
using Newtonsoft.Json.Linq;
using Microsoft.Azure.Services.AppAuthentication;
using Microsoft.Azure.KeyVault;
using Microsoft.Azure.EventHubs;
using System.Text;
using System;

namespace ScaleTestV1_NoHost
{
    public static class Http
    {
        // I'm using a static HttpClient and EventHubClient so that 
        // multiple function executions within a single function app 
        // instance don't have to initialize new resources every execution.
        private static HttpClient client = new HttpClient();
        private static EventHubClient eventHubClient;

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

            // So I don't have to call key vault every execution, I only retrieve the secret
            // if I don't already have the client initialized. This way invoking my function 1000
            // times doesn't result in 1000 calls to key vault which would get throttled.
            if(eventHubClient == null)
            {
                log.Info("Retrieving secret from keyvault");
                
                // This is the part where I grab the secret.
                var azureServiceTokenProvider = new AzureServiceTokenProvider();
                var kvClient = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(azureServiceTokenProvider.KeyVaultTokenCallback), client);
                string eventHubConnectionString = (await kvClient.GetSecretAsync(Environment.GetEnvironmentVariable("EventHubSecretId"))).Value;
                
                // And now I use the secret to create an Event Hub Client
                eventHubClient = EventHubClient.CreateFromConnectionString(eventHubConnectionString);
            }

            // Do some work here
            
            // Send to Event Hub via my key-vaulted connection string client
            await eventHubClient.SendAsync(new EventData(Encoding.UTF8.GetBytes(message.ToString())));

            log.Info($"Sent message to Event Hub");
            return req.CreateResponse(HttpStatusCode.OK, message);
        }
    }
}