kyverno/docs/dev/feature-flags

Feature Flags Guidelines

Introduction

Feature flags, also known as feature toggles, are a software development technique that allows developers to turn features on and off in a production environment without requiring a new deployment.

There are several ways to enable/disable features in Golang:

1. Feature Toggles: It provides a simple way to enable/disable features based on environment variables and/or command line arguments
2. Container arguments

Feature Flags

The toggle package exists in pkg/toggle can be used to define and manage a feature toggle for a new feature in Kyverno. Let's say we need to introduce a new feature toggle to enable/disable deferred loading. We can do it as follows:

1. Define a flag name, description, environment variable and a default value for this new feature in the toggle package:

   EnableDeferredLoadingFlagName    = "enableDeferredLoading"
   EnableDeferredLoadingDescription = "enable deferred loading of context variables"
   enableDeferredLoadingEnvVar      = "FLAG_ENABLE_DEFERRED_LOADING"
   defaultEnableDeferredLoading     = true
   

2. Create a new toggle for the new feature using the newToggle method which takes both default value and environment variable as arguments:

   EnableDeferredLoading    = newToggle(defaultEnableDeferredLoading, enableDeferredLoadingEnvVar)
   

   At this point, we have an instance of toggle which will be used later to call toggle.enabled() of the feature toggle to execute code conditionally.

3. Add a new method EnableDeferredLoading() bool in Toggles Interface at pkg/toggle/context.go to call the enabled method. It will be used later in Kyverno controllers:

   type Toggles interface {
       EnableDeferredLoading() bool
   }
   
   func (defaultToggles) EnableDeferredLoading() bool {
       return EnableDeferredLoading.enabled()
   }
   

4. In the controller, we can use it as follows:

   flag.Func(toggle.EnableDeferredLoadingFlagName, toggle.EnableDeferredLoadingDescription, toggle.EnableDeferredLoading.Parse)
   

Advantages

1. Feature toggles can be accessed globally. Its value can be checked anywhere in the code, no need to pass it as an argument among methods/functions.

2. Users can either enable/disable the feature by setting it as an argument to the container --enableDeferredLoading=false or setting its environment variable FLAG_ENABLE_DEFERRED_LOADING=0

Container Arguments

Container arguments can be used directly in the controller. Let's say we want to add a new container flag --enable-feature, we can do it as follows:

1. Create a variable for this new flag:

var(
    enableFeature bool
)

2. Define a bool flag with specified name, default value, and usage:

flagset.BoolVar(&enableFeature, "enable-feature", true, "Set this flag to 'false' to ....")