Sample startup code showing how we can configure a VMSS instance based on its hostname and role.

Global.asax.cs

namespace MyDemo.Website.WebApi
{
    public class WebApiApplication : System.Web.HttpApplication
    {
        // This value is used if for some reason the configuration doesn't have a defined
        // application insights instrumentation key
        private const string DefaultAppInsightsKey = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx";

        // Yes, these are hardcoded - need to change? Recompile and build VM Image and redeploy.
        private const string ConfigurationDbId = "apiconfigs";
        private const string ConfigurationDbCollection = "apiconfigentries";
        private const string ServiceIdentifier = "webapi";
	
	private static Dictionary<string, string> instanceTelemetry;

        internal static string LoggingDatabaseCollection;
        internal static string LoggingDatabaseKey;
        internal static string LoggingDatabase;
        internal static string LoggingDatabaseCollection;
        internal static string ConfigDatabaseAccount;
        internal static string ConfigDatabaseKey;

	protected void Application_Start()
        {
            ApplyConfiguration();
	    // removed other initialisation code
        }

        private void ApplyConfiguration()
        {
	     // Assumes you correctly version your assemblies... you do, don't you? :)
	     instanceTelemetry = new Dictionary<string, string>
                                {
                                    { "Version", Assembly.GetExecutingAssembly().GetName().Version.ToString() },
                                    { "Hostname", Environment.MachineName }
                                };
	
            // Assumption that VM is an instance in an Azure VM Scale Set where
            // instance names end with a 6 character hexatrigesimal value to uniquely identify them.
            var instanceKey = Environment.MachineName.Substring(0, Environment.MachineName.Length - 6); // strip last 6 characters

            // Secret URI in keyvault that will contain config DB information.
            // Yes, the KeyVault instance is hardcoded which requires the solution to be re-compiled and re-deployed if
            // changed. This is on purpose to stop people editing configuration on running instances. They
            // should be editing the source code and pushing through the CD pipeline to re-bake the VM image.
            var configDbSecretUri = $"https://yourkeyvaultinstance.vault.azure.net/secrets/{instanceKey}-{ServiceIdentifier}-configdb-account/";
            var configDbKeySecretUri = $"https://yourkeyvaultinstance.vault.azure.net/secrets/{instanceKey}-{ServiceIdentifier}-configdb-key/";

            // Call Key Vault and retrieve secret.
            // Uses an Azure AD Service Principal to access 
            var keyVaultClient = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(KevVaultUtils.GetToken));

            // Configuration DB account and key read from KeyVault.
            ConfigDatabaseAccount = keyVaultClient.GetSecretAsync(configDbSecretUri).Result.Value;
            ConfigDatabaseKey = keyVaultClient.GetSecretAsync(configDbKeySecretUri).Result.Value;

            var configResult = new ApiConfiguration();

            try
            {
		            // DocumentDbClient is just a helper class that provides a singleton client class I can use.
                configResult = DocumentDbClient.ConfigInstance.CreateDocumentQuery<ApiConfiguration>(
                                UriFactory.CreateDocumentCollectionUri(ConfigurationDbId, ConfigurationDbCollection), new FeedOptions { MaxItemCount = 1 })
                                .Where(ac => ac.Id == $"{instanceKey}-{ServiceIdentifier}" && ac.AccessKey == ConfigDatabaseKey).AsEnumerable().FirstOrDefault();
            
                if (!string.IsNullOrWhiteSpace(configResult.Id))
                {
                    // Default fallback App Insights instance list above.
                    // This means even if we have a misconfiguration we should get telemetry *somewhere*
                    TelemetryConfiguration.Active.InstrumentationKey = string.IsNullOrWhiteSpace(configResult.AppInsightsKey) ? DefaultAppInsightsKey : configResult.AppInsightsKey;

                    // Our runtime values that can be used by our code
                    LoggingDatabaseAccount = configResult.LoggingDatabaseAccount;
                    LoggingDatabaseKey = configResult.LoggingDatabaseKey;
                    LoggingDatabase = configResult.LoggingDatabase;
                    LoggingDatabaseCollection = configResult.LoggingDatabaseCollection;                
                }
                else
                {
                    throw new ConfigurationErrorsException("Service is not configured correctly.");
                }
            }
            catch (Exception ex)
            {
                AppInsightsClient.ClientInstance.TrackException(ex, instanceTelemetry);
                // Bubble Exception
                throw;
            }
        }
    }
}