概述

k8s相关项目中，经常看到会引入库k8s.io/component-base/featuregate作为featureGate的开关，它只支持bool类型的gate，支持多个key。

细节

type Feature string

type FeatureSpec struct {
	// Default is the default enablement state for the feature
	Default bool
	// LockToDefault indicates that the feature is locked to its default and cannot be changed
	LockToDefault bool
	// PreRelease indicates the maturity level of the feature
	PreRelease prerelease
}

type prerelease string
const (
	// Values for PreRelease.
	Alpha = prerelease("ALPHA")
	Beta  = prerelease("BETA")
	GA    = prerelease("")

	// Deprecated
	Deprecated = prerelease("DEPRECATED")
)

type FeatureGate interface {
	// Enabled returns true if the key is enabled.
	Enabled(key Feature) bool
	// KnownFeatures returns a slice of strings describing the FeatureGate's known features.
	KnownFeatures() []string
	// DeepCopy returns a deep copy of the FeatureGate object, such that gates can be
	// set on the copy without mutating the original. This is useful for validating
	// config against potential feature gate changes before committing those changes.
	DeepCopy() MutableFeatureGate
}

type MutableFeatureGate interface {
	FeatureGate

	// AddFlag adds a flag for setting global feature gates to the specified FlagSet.
	AddFlag(fs *pflag.FlagSet)
	// Set parses and stores flag gates for known features
	// from a string like feature1=true,feature2=false,...
	Set(value string) error
	// SetFromMap stores flag gates for known features from a map[string]bool or returns an error
	SetFromMap(m map[string]bool) error
	// Add adds features to the featureGate.
	Add(features map[Feature]FeatureSpec) error
	// GetAll returns a copy of the map of known feature names to feature specs.
	GetAll() map[Feature]FeatureSpec
}

// featureGate implements FeatureGate as well as pflag.Value for flag parsing.
type featureGate struct {
	featureGateName string

	special map[Feature]func(map[Feature]FeatureSpec, map[Feature]bool, bool)

	// lock guards writes to known, enabled, and reads/writes of closed
	lock sync.Mutex
	// known holds a map[Feature]FeatureSpec
	known *atomic.Value
	// enabled holds a map[Feature]bool
	enabled *atomic.Value
	// closed is set to true when AddFlag is called, and prevents subsequent calls to Add
	closed bool
}

可见这个类非常简单，除了name和锁，只剩下：

• known: map[Feature]FeatureSpec
• enabled: map[Feature]bool
• special: 控制AllAlpha和AllBeta

special 看起来很复杂，其实就是当AllAlpha=true时，featureGate 中所有的Alpha特性都会被置为true