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Redesigning Configuration Refresh for Azure App Configuration



Since its inception, the .NET Core configuration provider for Azure App Configuration has provided the capability to monitor changes and sync them to the configuration within a running application. We recently redesigned this functionality to allow for on-demand refresh of the configuration. The new design paves the way for smarter applications that only refresh the configuration when necessary. As a result, inactive applications no longer have to monitor for configuration changes unnecessarily.

Initial design : Timer-based watch

In the initial design, configuration was kept in sync with Azure App Configuration using a watch mechanism which ran on a timer. At the time of initialization of the Azure App Configuration provider, users could specify the configuration settings to be updated and an optional polling interval. In case the polling interval was not specified, a default value of 30 seconds was used.

public static IWebHost BuildWebHost(string[] args)
{ WebHost.CreateDefaultBuilder(args) .ConfigureAppConfiguration((hostingContext, config) => { // Load settings from Azure App Configuration // Set up the provider to listen for changes triggered by a sentinel value var settings = config.Build(); string appConfigurationEndpoint = settings["AzureAppConfigurationEndpoint"]; config.AddAzureAppConfiguration(options => { options.ConnectWithManagedIdentity(appConfigurationEndpoint) .Use(keyFilter: "WebDemo:*") .WatchAndReloadAll(key: "WebDemo:Sentinel", label: LabelFilter.Null); }); settings = config.Build(); }) .UseStartup<Startup>() .Build();

For example, in the above code snippet, Azure App Configuration would be pinged every 30 seconds for changes. These calls would be made irrespective of whether the application was active or not. As a result, there would be unnecessary usage of network and CPU resources within inactive applications. Applications needed a way to trigger a refresh of the configuration on demand in order to be able to limit the refreshes to active applications. Then unnecessary checks for changes could be avoided.

This timer-based watch mechanism had the following fundamental design flaws.

  1. It could not be invoked on-demand.
  2. It continued to run in the background even in applications that could be considered inactive.
  3. It promoted constant polling of configuration rather than a more intelligent approach of updating configuration when applications are active or need to ensure freshness.

New design : Activity-based refresh

The new refresh mechanism allows users to keep their configuration updated using a middleware to determine activity. As long as the ASP.NET Core web application continues to receive requests, the configuration settings continue to get updated with the configuration store.

The application can be configured to trigger refresh for each request by adding the Azure App Configuration middleware from package Microsoft.Azure.AppConfiguration.AspNetCore in your application’s startup code.

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{ app.UseAzureAppConfiguration(); app.UseMvc();

At the time of initialization of the configuration provider, the user can use the ConfigureRefresh method to register the configuration settings to be updated with an optional cache expiration time. In case the cache expiration time is not specified, a default value of 30 seconds is used.

public static IWebHost BuildWebHost(string[] args)
{ WebHost.CreateDefaultBuilder(args) .ConfigureAppConfiguration((hostingContext, config) => { // Load settings from Azure App Configuration // Set up the provider to listen for changes triggered by a sentinel value var settings = config.Build(); string appConfigurationEndpoint = settings["AzureAppConfigurationEndpoint"]; config.AddAzureAppConfiguration(options => { options.ConnectWithManagedIdentity(appConfigurationEndpoint) .Use(keyFilter: "WebDemo:*") .ConfigureRefresh((refreshOptions) => { // Indicates that all settings should be refreshed when the given key has changed refreshOptions.Register(key: "WebDemo:Sentinel", label: LabelFilter.Null, refreshAll: true); }); }); settings = config.Build(); }) .UseStartup<Startup>() .Build();

In order to keep the settings updated and avoid unnecessary calls to the configuration store, an internal cache is used for each setting. Until the cached value of a setting has expired, the refresh operation does not update the value. This happens even when the value has changed in the configuration store.  

Try it now!

For more information about Azure App Configuration, check out the following resources. You can find step-by-step tutorials that would help you get started with dynamic configuration using the new refresh mechanism within minutes. Please let us know what you think by filing issues on GitHub.

Overview: Azure App configuration
Tutorial: Use dynamic configuration in an ASP.NET Core app
Tutorial: Use dynamic configuration in a .NET Core app
Related Blog: Configuring a Server-side Blazor app with Azure App Configuration


Software Engineer, Azure App Configuration


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A Penny Saved is a Ton of Serverless Compute Earned

Scott Guthrie recently shared one of my favorite anecdotes on his Azure Red Shirt Tour. A Microsoft customer regularly invokes 1 billion (yes, that’s with a “B”) Azure Functions per day. The customer reached out to support after the first month thinking there was a bug in the billing system, only to find out that the $72 was in fact correct. How is that possible? Azure Functions is a serverless compute platform that allows you to focus on code that only executes when triggered by events, and you only pay for CPU time and memory used during execution (versus a traditional web server where you are paying a fee even if your app is idle). This is called micro-billing, and is one key reason serverless computing is so powerful.

Curious about Azure Functions? Follow the link to get up and running with your first function in minutes.

Scott Guthrie Red Shirt

Scott Guthrie on the Azure Red Shirt Tour

In fact, micro-billing is so important, it’s one of three rules I use to verify if a service is serverless. There is not an official set of rules and there is no standard for serverless. The closest thing to a standard is the whitepaper published by the Cloud Native Computing Foundation titled CNCF WG-Serverless Whitepaper v1.0 (PDF). The paper describes serverless computing as “building and running applications that do not require server management.” The paper continues to state they are “executed, scaled, and billed in response to the exact demand needed at the moment.”

It’s easy to label almost everything serverless, but there is a difference between managed and serverless. A managed service takes care of responsibilities for you, such as standing up a website or hosting a Docker container. Serverless is a managed service but requires a bit more. Here is Jeremy’s Serverless Rules.

  1. The service should be capable of running entirely in the cloud. Running locally is fine and often preferred for developing, testing, and debugging, but ultimately it should end up in the cloud.
  2. You don’t have to configure a virtual machine or cluster. Docker is great, but containers require a Docker host to run. That host typically means setting up a VM and, for resiliency and scale, using an orchestrator like Kubernetes to scale the solution. There are also services like Azure Web Apps that provide a fully managed experience for running web apps and containers, but I don’t consider them serverless because they break the next rule.
  3. You only pay for active invocations and never for idle time. This rule is important, and the essence of micro-billing. ACI is a great way to run a container, but I pay for it even when it’s not being used. A function, on the other hand, only bills when it’s called.

These rules are why I stopped calling managed databases “serverless.” So, what, then, does qualify as serverless?

The Azure serverless platform includes Azure Functions, Logic Apps, and Event Grid. In this post, we’ll take a closer look at Azure Functions.

Azure Functions

Azure Functions allows you to write code that is executed based on an event, or trigger. Triggers may include an HTTP request, a timer, a message in a queue, or any other number of important events. The code is passed details of the trigger but can also access bindings that make it easier to connect to resources like databases and storage. The serverless Azure Functions model is based on two parameters: invocations and gigabyte seconds.

Invocations are the number of times the function is invoked based on its trigger. Gigabyte seconds is a function of memory usage. Image a graph that shows time on the x-axis and memory consumption on the y-axis. Plot the memory usage of your function over time. Gigabyte seconds represent the area under the curve.

Let’s assume you have a microservice that is called every minute and takes one second to scan and aggregate data. It uses a steady 128 megabytes of memory during the run. Using the Azure Pricing Calculator, you’ll find that the cost is free. That’s because the first 400,000 Gigabyte seconds and 1 million invocations are free every month. Running every second (there are 2,628,000 seconds in a month) with double memory (256 megabytes), the entire monthly cost is estimated at $4.51.

Azure Functions pricing

Pricing calculator for Azure Functions

Recently I tweeted about my own experience with serverless cost (or lack thereof). I wrote a link-shortening tool. It uses a function to take long URLs and turn them into a shorter code I can easily share. I also have a function that takes the short code and performs the redirect, then stores the data in a queue. Another microservice processes items in the queue and stores metadata that I can analyze for later. I have tens of thousands of invocations per month and my total cost is less than a dollar.

Link shortener stats

A tweet about cost of running serverless code in Azure

Do I have your attention?

In future posts I will explore the cost model for Logic Apps and Event Grid. In the meantime…

Learn about and get started with your first Azure Function by following this link: