Ship code dark with feature flags

Without feature flags, deploying and releasing are the same event. Merging a half-finished feature means hiding it with a long-lived branch, and turning off a misbehaving feature means an emergency deployment.

A feature flag is a switch your code checks at runtime. Deploy the code with the flag off, turn it on when you're ready, and turn it off again in seconds if something goes wrong.

#Setting it up

Store flags in Azure App Configuration and read them with the feature management libraries:

builder.Configuration.AddAzureAppConfiguration(options =>
{
    options.Connect(new Uri("https://my-config.azconfig.io"), new DefaultAzureCredential())
           .UseFeatureFlags(flags => flags.SetRefreshInterval(TimeSpan.FromSeconds(30)));
});

builder.Services.AddAzureAppConfiguration();
builder.Services.AddFeatureManagement();

var app = builder.Build();
app.UseAzureAppConfiguration();   // refreshes flags as requests come in

The packages are Microsoft.Azure.AppConfiguration.AspNetCore and Microsoft.FeatureManagement.AspNetCore.

#Checking a flag

app.MapGet("/quotes/{id}", async (Guid id, IFeatureManager features, QuoteService quotes) =>
{
    var quote = await features.IsEnabledAsync("NewPricingEngine")
        ? await quotes.PriceWithNewEngineAsync(id)
        : await quotes.PriceAsync(id);

    return Results.Ok(quote);
});

For MVC controllers or actions, [FeatureGate("NewPricingEngine")] returns a 404 while the flag is off.

#Rolling out gradually

A flag doesn't have to be all or nothing. Built-in filters let you enable it for:

• A percentage of requests, for example 10%, then 50%, then 100%
• Specific users or groups, through the targeting filter, such as internal staff first
• A time window, such as a promotion that switches itself on and off

You change these in the App Configuration portal, and apps pick up the change at their next refresh, within the interval you set.

#The kill switch

Flags aren't only for new features. Wrapping a risky integration, such as a call to a new partner API, in a flag gives you an off switch that works faster than any deployment and doesn't need a developer.

#Clean up

Every flag is an if statement with two code paths to test and maintain. Once a feature is fully rolled out and stable, remove the flag and the old code path. Keep a short list of active flags with an owner and an expected removal date, or they accumulate for years.

#Takeaway

Use feature flags to separate deploying code from releasing it. Store them in Azure App Configuration, roll features out gradually to a percentage or a group of users, keep a kill switch around risky integrations, and remove flags once they've done their job.