quickstart-feature-flag-azure-functions-csharp.md

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Quickstart for adding feature flags to Azure Functions | Microsoft Docs	In this quickstart, use Azure Functions with feature flags from Azure App Configuration and test the function locally.	azure-app-configuration	AlexandraKemperMS	azure-app-configuration	csharp	devx-track-csharp, mode-other	quickstart	8/26/2020	alkemper

Quickstart: Add feature flags to an Azure Functions app

In this quickstart, you create an Azure Functions app and use feature flags in it. You use the feature management from Azure App Configuration to centrally store all your feature flags and control their states.

The .NET Feature Management libraries extend the framework with feature flag support. These libraries are built on top of the .NET configuration system. They integrate with App Configuration through its .NET configuration provider.

Prerequisites

• Azure subscription - create one for free
• Visual Studio 2019 with the Azure development workload.
• Azure Functions tools

Create an App Configuration store

[!INCLUDE azure-app-configuration-create]

7. Select Feature Manager > +Add to add a feature flag called Beta.

   [!div class="mx-imgBorder"]

   Leave label and Description undefined for now.

8. Select Apply to save the new feature flag.

Create a Functions app

[!INCLUDE Create a project using the Azure Functions template]

Connect to an App Configuration store

This project will use dependency injection in .NET Azure Functions. It adds Azure App Configuration as an extra configuration source where your feature flags are stored.

1. Right-click your project, and select Manage NuGet Packages. On the Browse tab, search for and add following NuGet packages to your project.

   • Microsoft.Extensions.Configuration.AzureAppConfiguration version 4.1.0 or later
   • Microsoft.FeatureManagement version 2.2.0 or later
   • Microsoft.Azure.Functions.Extensions version 1.1.0 or later

2. Add a new file, Startup.cs, with the following code. It defines a class named Startup that implements the FunctionsStartup abstract class. An assembly attribute is used to specify the type name used during Azure Functions startup.

   using System;
   using Microsoft.Azure.Functions.Extensions.DependencyInjection;
   using Microsoft.Extensions.Configuration;
   using Microsoft.FeatureManagement;
   
   [assembly: FunctionsStartup(typeof(FunctionApp.Startup))]
   
   namespace FunctionApp
   {
       class Startup : FunctionsStartup
       {
           public override void ConfigureAppConfiguration(IFunctionsConfigurationBuilder builder)
           {
           }
   
           public override void Configure(IFunctionsHostBuilder builder)
           {
           }
       }
   }

3. Update the ConfigureAppConfiguration method, and add Azure App Configuration provider as an extra configuration source by calling AddAzureAppConfiguration().

   The UseFeatureFlags() method tells the provider to load feature flags. All feature flags have a default cache expiration of 30 seconds before rechecking for changes. The expiration interval can be updated by setting the FeatureFlagsOptions.CacheExpirationInterval property passed to the UseFeatureFlags method.

   public override void ConfigureAppConfiguration(IFunctionsConfigurationBuilder builder)
   {
       builder.ConfigurationBuilder.AddAzureAppConfiguration(options =>
       {
           options.Connect(Environment.GetEnvironmentVariable("ConnectionString"))
                  .Select("_")
                  .UseFeatureFlags();
       });
   }

   [!TIP] If you don't want any configuration other than feature flags to be loaded to your application, you can call Select("_") to only load a nonexisting dummy key "_". By default, all configuration key-values in your App Configuration store will be loaded if no Select method is called.

4. Update the Configure method to make Azure App Configuration services and feature manager available through dependency injection.

   public override void Configure(IFunctionsHostBuilder builder)
   {
       builder.Services.AddAzureAppConfiguration();
       builder.Services.AddFeatureManagement();
   }

5. Open Function1.cs, and add the following namespaces.

   using System.Linq;
   using Microsoft.FeatureManagement;
   using Microsoft.Extensions.Configuration.AzureAppConfiguration;

   Add a constructor used to obtain instances of _featureManagerSnapshot and IConfigurationRefresherProvider through dependency injection. From the IConfigurationRefresherProvider, you can obtain the instance of IConfigurationRefresher.

   private readonly IFeatureManagerSnapshot _featureManagerSnapshot;
   private readonly IConfigurationRefresher _configurationRefresher;
   
   public Function1(IFeatureManagerSnapshot featureManagerSnapshot, IConfigurationRefresherProvider refresherProvider)
   {
       _featureManagerSnapshot = featureManagerSnapshot;
       _configurationRefresher = refresherProvider.Refreshers.First();
   }

6. Update the Run method to change the value of the displayed message depending on the state of the feature flag.

   The TryRefreshAsync method is called at the beginning of the Functions call to refresh feature flags. It will be a no-op if the cache expiration time window isn't reached. Remove the await operator if you prefer the feature flags to be refreshed without blocking the current Functions call. In that case, later Functions calls will get updated value.

   [FunctionName("Function1")]
   public async Task<IActionResult> Run(
       [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequest req,
       ILogger log)
   {
       log.LogInformation("C# HTTP trigger function processed a request.");
   
       await _configurationRefresher.TryRefreshAsync();
   
       string message = await _featureManagerSnapshot.IsEnabledAsync("Beta")
               ? "The Feature Flag 'Beta' is turned ON"
               : "The Feature Flag 'Beta' is turned OFF";
   
       return (ActionResult)new OkObjectResult(message);
   }

Test the function locally

1. Set an environment variable named ConnectionString, where the value is the connection string you retrieved earlier in your App Configuration store under Access Keys. If you use the Windows command prompt, run the following command and restart the command prompt to allow the change to take effect:

       setx ConnectionString "connection-string-of-your-app-configuration-store"

   If you use Windows PowerShell, run the following command:

       $Env:ConnectionString = "connection-string-of-your-app-configuration-store"
   

   If you use macOS or Linux, run the following command:

       export ConnectionString='connection-string-of-your-app-configuration-store'

2. Press F5 to test your function. If prompted, accept the request from Visual Studio to download and install Azure Functions Core (CLI) tools. You might also need to enable a firewall exception so that the tools can handle HTTP requests.

3. Copy the URL of your function from the Azure Functions runtime output.

   

4. Paste the URL for the HTTP request into your browser's address bar. The following image shows the response indicating that the feature flag Beta is disabled.

   

5. Sign in to the Azure portal. Select All resources, and select the App Configuration store that you created.

6. Select Feature manager, and change the state of the Beta key to On.

7. Refresh the browser a few times. When the cached feature flag expires after 30 seconds, the page should have changed to indicate the feature flag Beta is turned on, as shown in the image below.

   

Note

The example code used in this tutorial can be downloaded from the Azure App Configuration GitHub repo.

Clean up resources

[!INCLUDE azure-app-configuration-cleanup]

Next steps

In this quickstart, you created a feature flag and used it with an Azure Functions app via the Microsoft.FeatureManagement library.

• Learn more about feature management
• Manage feature flags
• Use conditional feature flags
• Enable staged rollout of features for targeted audiences
• Use dynamic configuration in an Azure Functions app