Configuration in .Net Core
  •  11 April 2020
  •  misc 

Configuration, it sounds very simple but many people do it the wrong way.

Configuration is an essential part of any application (The term Configuration here does not include internal application config, such as the route path definition). Configuration varies across deploys and environments, which provides confidential information to the application (such as database connection string) and defines the way the application should behave (for example, enable or disable specific features). It sounds quite simple but I found that many people did it the wrong way. That’s why we have this blog post.

Take a look at this

Recently, I worked on another project, which was written in ASP.Net Core 3.1. After a while digging into the code, I saw these patterns and felt really annoyed about it. They are related to the way the Configuration is implemented and passed around the application as well as the way it is consumed.

Here is the Setting interface and its implementations.

public interface ISetting
{
    Env Environment { get; }
    string MailgunApiKey { get; }
    string AppName { get; }
}

public class LocalSetting : ISetting
{
    Env Environment => Env.Local;
    public string MailgunApiKey => "key-xxxx";
    public string AppName => "Prod Name";
}

public class StagingSetting : ISetting
{
    Env Environment => Env.Staging;
    public string MailgunApiKey => "key-yyyy";
    public string AppName => "Prod Name";
}

public class ProdSetting : ISetting
{
    Env Environment => Env.Prod;
    public string MailgunApiKey => "key-zzzz";
    public string AppName => "Prod Name";
}

Here is an app component which uses the Setting object

public async Task Execute()
{
    var setting = Resolve<ISetting>();
    var emailToUse = "";

    switch (setting.Environment)
    {
        case Env.Local:
            emailToUse = "[email protected]";
            break;
        case Env.Staging:
            emailToUse = "[email protected]";
            break;
        case Env.Prod:
            emailToUse = request.ActualEmailAddress;
            break;
        default:
            break;
    }

    await SendEmail(emailToUse, setting.MailgunApiKey);
}

When Configuration gets wrong

A litmus test for whether an app has all config correctly factored out of the code is whether the codebase could be made open source at any moment, without compromising any credentials. The Twelve Factor App

You can spot a lot of problems with the above code. Some are quite obvious (like the hard-code values), some aren’t. Let’s take a look at each of them again and analyze

First, start with the code to define the ISetting interface

public interface ISetting
{
    Env Environment { get; }
    string MailgunApiKey { get; }
    string AppName { get; }
}

public class LocalSetting : ISetting
{
    Env Environment => Env.Local;
    public string MailgunApiKey => "key-xxxx";
    public string AppName => "Local Name";
}

public class StagingSetting : ISetting
{
    Env Environment => Env.Staging;
    public string MailgunApiKey => "key-yyyy";
    public string AppName => "Staging Name";
}

public class ProdSetting : ISetting
{
    Env Environment => Env.Prod;
    public string MailgunApiKey => "key-zzzz";
    public string AppName => "Prod Name";
}

What are the problems here?

  • Most obvious one, the hard-coded credentials. These things should not be stored in source control. What happen if a new guy join the team and he accidentally start the app with Prod as the default environment?
  • As a .Net (Core) developer, especially ASP.Net (Core) developer, this is confusing for me. It increases the complexity of the application in an unnecessary way. Why? Microsoft have already provided a standard for storing and loading hard-coded values via the appsettings.json file. Why do we have to make it more complex? Why do we have to invent another method for that purpose? The first time when I read the code, I saw all these classes along with other appsettings.json files and I 😳
  • It is not easy to update the configuration values. To change a single value, you will need to update your code, recompile it and trigger a redeployment of the app, which can take a lot of time. A better way should be to store them in environment variables, change the environment variables and then reload your app to run with the new values. Read more about Config in 12factor app here.

OK, coming to the next part

public async Task Execute()
{
    var config = Resolve<IConfiguration>();
    var emailToUse = "";

    switch (config.Environment)
    {
        case Env.Local:
            emailToUse = "[email protected]";
            break;
        case Env.Staging:
            emailToUse = "[email protected]";
            break;
        case Env.Prod:
            emailToUse = request.ActualEmailAddress;
            break;
        default:
            break;
    }

    await SendEmail(emailToUse, config.MailgunApiKey);
}

The above code piece tries to protect the developers from accidentally sending a test email to a real customer in Local or Staging environment by replacing the input email with an internal one.

However…

  • It is limited to just 3 environments: Local, Staging and Prod. What if you want to add one more environment, perhaps another Staging environment for a new team? You will have scan through your code base, find all the places using swtich/case like this and update them. No, it’s not a good idea at all. We had this problem before when the Vietnam team started working on an existing project of the US team and we had to set up a new environment for the Vietnam dev team, things were messed up as we couldn’t scan and update every single place in code.
  • The business logic is mixed with the Configuration while they should be separated.
    • The code should have no realization of the environment it is running on so it is easy to scale the app, to add more instances running in different architecture. This could be applied when you want to add a new temporary server to cope with the high workload or to add another server, which reflects the production server (with specific features disabled) for testing purpose.
    • This also exposes the problem of Cross-cutting concerns. Each component should be as simple as possible, it does only what it should. All the other cross-cutting concerns should be decomposed from the main component so it is easy to do unit testing.

Let’s fix it

So, what are the requirements?

  • Configuration should be separated from code logic. The code should just focus on business logic and should not contain any of the logic to check for specific environment (like all the switch/case we saw above).
  • We should be able to override any specific value seamlessly. It should allow us to enable or disable any feature easily by turning on or off some environment variables rather than scanning through all places in the code to change manually.
  • It should be easy to add new or switch to another environment.
  • Of course, environment types and credentials should not be hard-coded in code.

Here is the sample flow of building the Configuration instance

Flow

  1. All the setting values are loaded from different locations, through several layers of override that we defined. This is the normal way that you would do with ASP.Net Core. The final result would be an instance of IConfiguration, which is a key-value dictionary of all the raw setting values. This allows us to switch the settings very easily by changing the input files or just some specific environment variables.
  2. For simplicity in usage, the IConfiguration instance will be injected into another layer of custom AppConfig object, where you can build, validate and transform all the raw string values to the correct format that you want.
  3. The App components then consume the configuration simply by resolving the AppConfig instance from the DI Container.

1. Load Configuration values

Microsoft has already documented and explained all the steps to build and register the IConfiguration implementation with DI Container here Configuration in ASP.NET Core. Basically, what you need to do is just specify the load order when creating IHostBuilder instance.

Host.CreateDefaultBuilder(args)
    .ConfigureAppConfiguration((hostingContext, config) =>
    {
        var environmentName = hostingContext.HostingEnvironment.EnvironmentName;
        // Use this for hard-coded values that are not confidential
        config.AddJsonFile($"appsettings.{environmentName}.json", optional: true);
        config.AddJsonFile($"appsettings.json", optional: true);
        // Use this for credentials running on Prod
        config.AddEnvironmentVariables();
    })

If you are writing your own Console application (or other types of application which are not ASP.Net Core), simply install these extensions on Nuget

  • Microsoft.Extensions.Configuration
  • Microsoft.Extensions.Configuration.Json
  • Microsoft.Extensions.Configuration.FileExtensions
  • Microsoft.Extensions.Configuration.EnvironmentVariables

Here is a simple example showing how to register it with Autofac Module

protected override void Load(ContainerBuilder builder)
{
    builder.Register(f =>
    {
        var environmentName =
            Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT");

        var config = new ConfigurationBuilder();
        config
            .AddJsonFile($"appsettings.{environmentName}.json", optional: true)
            .AddJsonFile($"appsettings.json", optional: true)
            .AddEnvironmentVariables();

        return config.Build();
    }).As<IConfiguration>();
}

2. Transform raw string values

The preferred way is to load Config values from environment variables. That means, all the raw values should be of type string. For simplicity in later usage, it is recommended to add another layer of AppConfig, where you can validate and transform the raw string values to the correct types that you expect. This can be easily achieved using extension methods.

public static string GetRequiredString(this IConfiguration configuration, string key)
{
  var val = configuration[key];
  if (!string.IsNullOrWhiteSpace(val))
    return val;

  throw new RequiredValueMissingException(key);
}

public static int GetInt(this IConfiguration configuration, string key, int defaultValue)
{
  var val = configuration[key];
  if (string.IsNullOrWhiteSpace(val))
    return defaultValue;

  return Convert.ToInt32(val);
}
public class AppConfig
{
    public string LogLevel { get; set; }
    public string MailgunApiKey { get; set; }
    public bool SendRealEmail { get; set; }
    public string DefaultToEmail { get; set; }

    public AppConfig(IConfiguration config)
    {
        LogLevel = config.GetString("LOG_LEVEL", defaultValue: "Info");

        // validate this is required
        MailgunApiKey = config.GetRequiredString("MAILGUN_API_KEY");

        // to be safe, default value for this should be false
        SendRealEmail = config.GetBool("SEND_REAL_EMAIL", defaultValue: false);
        DefaultToEmail = config.GetString(
            "DEFAULT_TO_EMAIL", defaultValue: "[email protected]"
        );
    }
}

And here is how to register it with Autofac

protected override void Load(ContainerBuilder builder)
{
    // register directly
    builder.RegisterType<AppConfig>();

    // or if you want an interface
    builder.RegisterType<AppConfig>().As<IAppConfig>();
}

3. Consume the AppConfig object

Finally, to consume AppConfig, simply resolve it from your DI Container.

public class SendEmail
{
    public async Task Execute(string email)
    {
        var config = scope.Resolve<AppConfig>();

        // whether to send real email?
        var sendRealEmail = config.SendRealEmail;
        if (!sendRealEmail) email = config.DefaultToEmail;

        //
        await mailgunClient.Send(email);
    }
}

The main component logic is now extremely simple and very easy to write unit test. It doesn’t care about the current environment that it is running at all, no matter if you are running a Console application, a Docker container in your local computer or a Docker container on your Production Kubernetes cluster. To add a new environment or a new server with some features disabled, simple override the necessary environment variables, there is no need to update the code and rebuild any component. In case of error and you want to disable the email feature on your Prod, what you need to is also just to update an environment variable and reload your application.

Hmmmm…

This is easy, this is simple, there is a standard solution from Microsoft and 12factor App, so

  • Do it the right way!
  • Don’t reinvent the wheel! (unless you have a really good reason)