configurationsource

ASP.NET Core has a lightweight configuration system that is designed to be highly extensible. It lets you aggregate many configuration values from multiple different sources, and then access those in a strongly typed fashion using the Options pattern.

Using packages in the Microsoft.Extensions.Configuration namespace, you can read configuration from:

• Azure Key Vault
• Azure App Configuration
• Command-line arguments
• Custom providers (installed or created)
• Directory files
• Environment variables
• In-memory .NET objects
• Settings files

For PineBlog I wanted some values that can be set in the appsettings.json to be set from the admin UI as well. These values are stored in the database and exposed through a Entity Framework Core (EF Core) DbContext.

In this post I'm going to describe creating a custom configuration provider that uses EF Core. For the sake of simplicity it is a more general description than the implementation in PineBlog.

Default configuration

Web apps based on the ASP.NET Core dotnet new templates call CreateDefaultBuilder when building a host. This provides default configuration for the app, for instance the appsettings.json using the File Configuration Provider. For more information see the official documentation on this topic.

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>();;

The ConfigOptions

In this example we will have a ConfigOptions class that maps to the configuration in appsettings.json. And a ConfigEntity class that overrides some of those configuration values from the database.

public class ConfigOptions
{
    public string BackgroundColor { get; set; }
    public int ItemsPerPage { get; set; }
    public bool ShowHeader { get; set; }

    // this property will not be overridden by ConfigEntity
    public string ApiKey { get; set; } 
}

public class ConfigEntity
{
    public string BackgroundColor { get; set; }
    public int ItemsPerPage { get; set; }
    public bool ShowHeader { get; set; }
}

The appsettings.json look like this:

{
  "ConfigOptions": {
    "BackgroundColor": "#ff0000",
    "ItemsPerPage": 2,
    "ShowHeader": true,
    "ApiKey": "h&Ww1vbFP6y2RW7Nx$$Q&&KW"
  }
}

Creating a custom configuration provider

With the basics out of the way, we can now start creating our custom configuration provider.

In order to create a custom provider, you need to implement the IConfigurationProvider and IConfigurationSource interfaces from the Microsoft.Extensions.Configuration.Abstractions package. Or you can use any of the provided base classes to get started.

Configuration source

The IConfigurationSource interface only has one method that needs implementing.

And because we will be using EF Core as a configuration source we need a DbContextOptionsBuilder action to use the DbContext from the configuration provider. We also have a ReloadDelay to avoid triggering a reload before a change is completely saved.

public class ConfigEntityConfigurationSource : IConfigurationSource
{
    public Action<DbContextOptionsBuilder> OptionsAction { get; set; }

    public bool ReloadOnChange { get; set; }

    // Number of milliseconds that reload will wait before calling Load. This helps avoid triggering a reload before a change is completely saved. Default is 500.
    public int ReloadDelay { get; set; } = 500;

    public IConfigurationProvider Build(IConfigurationBuilder builder)
    {
        return new ConfigEntityConfigurationProvider(this);
    }
}

Configuration provider

For our custom configuration provider we use the ConfigurationProvider base class, this is the most basic implementation of IConfigurationProvider. The configuration provider initializes the database when it's empty.

We override the Load method with our custom implementation, this method loads (or reloads) the data for the provider. Here we instantiate the DbContext using the OptionsAction from the configuration source. And we then get a ConfigEntity from the database and set its values to the configuration dictionary of the configuration provider.
And by using the same keys for the configuration values (e.g. ConfigOptions.BackgroundColor) as we used in the appsettings.json we will effectively override those values with the values from the database.

public class ConfigEntityConfigurationProvider : ConfigurationProvider
{
    private readonly ConfigEntityConfigurationSource _source;

    public ConfigEntityConfigurationProvider(ConfigEntityConfigurationSource source)
    {
        _source = source;
    }

    public override void Load()
    {
        var builder = new DbContextOptionsBuilder<EntityDbContext>();
        _source.OptionsAction(builder);

        using (var context = new CustomDbContext(builder.Options))
        {
            context.Database.EnsureCreated();

            var config = context.ConfigEntity.SingleOrDefault();
            if (config == null) return;

            Data = new Dictionary<string, string>();
            Data.Add($"{nameof(ConfigOptions)}.{nameof(ConfigOptions.BackgroundColor)}", config.BackgroundColor);
            Data.Add($"{nameof(ConfigOptions)}.{nameof(ConfigOptions.ItemsPerPage)}", config.ItemsPerPage);
            Data.Add($"{nameof(ConfigOptions)}.{nameof(ConfigOptions.ShowHeader)}", config.ShowHeader);
        }
    }
}

We now have a configuration provider that loads its values from the database. But it will only do this once, when the application is started, and we want it to reload as well when the user updates the ConfigEntity in the database.

Reloading configuration on entity changes

To trigger a reload of the configuration when the ConfigEntity in the database changes, we need to let the configuration provider know that the entity has changed. We'll solve this by triggering an event on a entity change observer class and listening for this event in our configuration provider.

Entity change observer

We create a singleton class that has an EventHandler for the entity changes. Our configuration provider can listen for this event to update the configuration.

I've made this class an old school singleton, since injecting it into the DbContext was a bit complicated. And we use ThreadPool.QueueUserWorkItem offload the invoking of the event to a background thread, so it doesn't block the DbContext.SaveChanges.

public class EntityChangeObserver
{
    public event EventHandler<EntityChangeEventArgs> Changed;

    public void OnChanged(EntityChangeEventArgs e)
    {
        ThreadPool.QueueUserWorkItem((_) => Changed?.Invoke(this, e));
    }

    #region singleton

    private static readonly Lazy<EntityChangeObserver> lazy = new Lazy<EntityChangeObserver>(() => new EntityChangeObserver());

    private EntityChangeObserver() { }

    public static EntityChangeObserver Instance => lazy.Value;

    #endregion singleton
}

Notify the observer

Create a custom (base) class that extends DbContext, and override the SaveChanges methods.

public abstract class EntityDbContext : DbContext
{
    public override int SaveChanges()
    {
        OnEntityChange();
        base.SaveChanges()
    }

    public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken)
    {
        OnEntityChange();
        return await base.SaveChangesAsync(cancellationToken);
    }

    private void OnEntityChange()
    {
        foreach(var entry in ChangeTracker.Entries()
            .Where(i => i.State == EntityState.Modified || i.State == EntityState.Added))
        {
            EntityChangeObserver.Instance.OnChanged(new EntityChangeEventArgs(entry));
        }
    }
}

Trigger a reload

To let the configuration provider know the entity has changed and to trigger a reload, we need to listen for the EntityChangeObserver.Changed event. And because we only want to reload when the ConfigEntity changes, we add a check to see if that is the type of the changed entity.

public ConfigEntityConfigurationProvider(ConfigEntityConfigurationSource source)
{
    _source = source;

    if (_source.ReloadOnChange)
        EntityChangeObserver.Instance.Changed += EntityChangeObserver_Changed;
}

private void EntityChangeObserver_Changed(object sender, EntityChangeEventArgs e)
{
    if (e.Entry.Entity.GetType() != typeof(ConfigEntity))
        return;

    Thread.Sleep(_source.ReloadDelay);
    Load();
}

Add the configuration in the application

And finally in the Program.cs we can now configure the IConfigurationSource.

WebHost.CreateDefaultBuilder(args)
    .UseStartup<Startup>()
    .ConfigureAppConfiguration((hostingContext, config) => {
        config.Add(new ConfigEntityConfigurationSource {
            OptionsAction = o => o.UseInMemoryDatabase("db", new InMemoryDatabaseRoot()),
            ReloadOnChange = true
        });
    });

Our application will now load the ConfigOptions from the appsettings.json, then use the ConfigEntityConfigurationSource to override some of the values (when present in the database). And when the user updates the ConfigEntity in the database the values will be reloaded.

Note: Use IOptionsSnapshot to support reloading options with minimal processing overhead. Options are computed once per request when accessed and cached for the lifetime of the request. See the official documentation on reload configuration data with IOptionsSnapshot.