Add feature flag system to support gradual rollouts (#151)

Introduce a lightweight feature flag implementation to enable safer and
more controlled feature releases. This system will allow us to:

- Reduce risk by gradually rolling out new features to subsets of users
- Quickly disable problematc features without requiring a new release
- Manage feature lifecycles more effectively across different environments

The implementation uses a simple map-based approach for for efficiently
introducing and graduating new features in ACK.

Usage example:
```go

func someLogic() {
    ...
    if cfg.FeatureGates.IsEnabled("FeatureName") {

    } else {

    }
}
```

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

pkg/config/config.go

@@ -34,6 +34,7 @@ import (
 	"sigs.k8s.io/controller-runtime/pkg/log/zap"
 
 	ackv1alpha1 "github.com/aws-controllers-k8s/runtime/apis/core/v1alpha1"
+	"github.com/aws-controllers-k8s/runtime/pkg/featuregate"
 	acktags "github.com/aws-controllers-k8s/runtime/pkg/tags"
 	ackutil "github.com/aws-controllers-k8s/runtime/pkg/util"
 )
@@ -59,6 +60,7 @@ const (
 	flagReconcileResourceResyncSeconds  = "reconcile-resource-resync-seconds"
 	flagReconcileDefaultMaxConcurrency  = "reconcile-default-max-concurrent-syncs"
 	flagReconcileResourceMaxConcurrency = "reconcile-resource-max-concurrent-syncs"
+	flagFeatureGates                    = "feature-gates"
 	envVarAWSRegion                     = "AWS_REGION"
 )
 
@@ -98,6 +100,9 @@ type Config struct {
 	ReconcileResourceResyncSeconds  []string
 	ReconcileDefaultMaxConcurrency  int
 	ReconcileResourceMaxConcurrency []string
+	// TODO(a-hilaly): migrate to k8s.io/component-base and implement a proper parser for feature gates.
+	FeatureGates    featuregate.FeatureGates
+	featureGatesRaw string
 }
 
 // BindFlags defines CLI/runtime configuration options
@@ -226,6 +231,13 @@ func (cfg *Config) BindFlags() {
 			" configuration maps resource kinds to maximum number of concurrent reconciles. If provided, "+
 			" resource-specific max concurrency takes precedence over the default max concurrency.",
 	)
+	flag.StringVar(
+		&cfg.featureGatesRaw, flagFeatureGates,
+		"",
+		"Feature gates to enable. The format is a comma-separated list of key=value pairs. "+
+			"Valid keys are feature names and valid values are 'true' or 'false'."+
+			"Available features: "+strings.Join(featuregate.GetDefaultFeatureGates().GetFeatureNames(), ", "),
+	)
 }
 
 // SetupLogger initializes the logger used in the service controller
@@ -323,6 +335,13 @@ func (cfg *Config) Validate(options ...Option) error {
 	if cfg.ReconcileDefaultMaxConcurrency < 1 {
 		return fmt.Errorf("invalid value for flag '%s': max concurrency default must be greater than 0", flagReconcileDefaultMaxConcurrency)
 	}
+
+	featureGatesMap, err := parseFeatureGates(cfg.featureGatesRaw)
+	if err != nil {
+		return fmt.Errorf("invalid value for flag '%s': %v", flagFeatureGates, err)
+	}
+	cfg.FeatureGates = featuregate.GetFeatureGatesWithOverrides(featureGatesMap)
+
 	return nil
 }
 
@@ -469,3 +488,43 @@ func parseWatchNamespaceString(namespace string) ([]string, error) {
 	}
 	return namespaces, nil
 }
+
+// parseFeatureGates converts a raw string of feature gate settings into a FeatureGates structure.
+//
+// The input string should be in the format "feature1=bool,feature2=bool,...".
+// For example: "MyFeature=true,AnotherFeature=false"
+//
+// This function:
+// - Parses the input string into individual feature gate settings
+// - Validates the format of each setting
+// - Converts the boolean values
+// - Applies these settings as overrides to the default feature gates
+func parseFeatureGates(featureGatesRaw string) (map[string]bool, error) {
+	featureGatesRaw = strings.TrimSpace(featureGatesRaw)
+	if featureGatesRaw == "" {
+		return nil, nil
+	}
+
+	featureGatesMap := map[string]bool{}
+	for _, featureGate := range strings.Split(featureGatesRaw, ",") {
+		featureGateKV := strings.SplitN(featureGate, "=", 2)
+		if len(featureGateKV) != 2 {
+			return nil, fmt.Errorf("invalid feature gate format: %s", featureGate)
+		}
+
+		featureName := strings.TrimSpace(featureGateKV[0])
+		if featureName == "" {
+			return nil, fmt.Errorf("invalid feature gate name: %s", featureGate)
+		}
+
+		featureValue := strings.TrimSpace(featureGateKV[1])
+		featureEnabled, err := strconv.ParseBool(featureValue)
+		if err != nil {
+			return nil, fmt.Errorf("invalid feature gate value for %s: %s", featureName, featureValue)
+		}
+
+		featureGatesMap[featureName] = featureEnabled
+	}
+
+	return featureGatesMap, nil
+}

pkg/config/config_test.go

@@ -14,6 +14,7 @@
 package config
 
 import (
+	"reflect"
 	"strings"
 	"testing"
 )
@@ -107,3 +108,78 @@ func TestParseNamespace(t *testing.T) {
 		}
 	}
 }
+
+func TestParseFeatureGates(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		want    map[string]bool
+		wantErr bool
+	}{
+		{
+			name:  "Empty input",
+			input: "",
+			want:  nil,
+		},
+		{
+			name:  "Single feature enabled",
+			input: "Feature1=true",
+			want:  map[string]bool{"Feature1": true},
+		},
+		{
+			name:  "Single feature disabled",
+			input: "Feature1=false",
+			want:  map[string]bool{"Feature1": false},
+		},
+		{
+			name:  "Multiple features",
+			input: "Feature1=true,Feature2=false,Feature3=true",
+			want: map[string]bool{
+				"Feature1": true,
+				"Feature2": false,
+				"Feature3": true,
+			},
+		},
+		{
+			name:  "Whitespace in input",
+			input: " Feature1 = true , Feature2 = false ",
+			want: map[string]bool{
+				"Feature1": true,
+				"Feature2": false,
+			},
+		},
+		{
+			name:    "Invalid format",
+			input:   "Feature1:true",
+			wantErr: true,
+		},
+		{
+			name:    "Invalid boolean value",
+			input:   "Feature1=yes",
+			wantErr: true,
+		},
+		{
+			name:    "Missing value",
+			input:   "Feature1=",
+			wantErr: true,
+		},
+		{
+			name:    "Missing key",
+			input:   "=true",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := parseFeatureGates(tt.input)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("parseFeatureGates() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("parseFeatureGates() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

pkg/featuregate/features.go

@@ -0,0 +1,102 @@
+// Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"). You may
+// not use this file except in compliance with the License. A copy of the
+// License is located at
+//
+//     http://aws.amazon.com/apache2.0/
+//
+// or in the "license" file accompanying this file. This file is distributed
+// on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+// express or implied. See the License for the specific language governing
+// permissions and limitations under the License.
+
+// Package featuregate provides a simple mechanism for managing feature gates
+// in ACK controllers. It allows for default gates to be defined and
+// optionally overridden.
+package featuregate
+
+// defaultACKFeatureGates is a map of feature names to Feature structs
+// representing the default feature gates for ACK controllers.
+var defaultACKFeatureGates = FeatureGates{
+	// Set feature gates here
+	// "feature1": {Stage: Alpha, Enabled: false},
+}
+
+// FeatureStage represents the development stage of a feature.
+type FeatureStage string
+
+const (
+	// Alpha represents a feature in early testing, potentially unstable.
+	// Alpha features may be removed or changed at any time and are disabled
+	// by default.
+	Alpha FeatureStage = "alpha"
+
+	// Beta represents a feature in advanced testing, more stable than alpha.
+	// Beta features are enabled by default.
+	Beta FeatureStage = "beta"
+
+	// GA represents a feature that is generally available and stable.
+	GA FeatureStage = "ga"
+)
+
+// Feature represents a single feature gate with its properties.
+type Feature struct {
+	// Stage indicates the current development stage of the feature.
+	Stage FeatureStage
+
+	// Enabled determines if the feature is enabled.
+	Enabled bool
+}
+
+// FeatureGates is a map representing a set of feature gates.
+type FeatureGates map[string]Feature
+
+// IsEnabled checks if a feature with the given name is enabled.
+// It returns true if the feature exists and is enabled, false
+// otherwise.
+func (fg FeatureGates) IsEnabled(name string) bool {
+	feature, ok := fg[name]
+	return ok && feature.Enabled
+}
+
+// GetFeature retrieves a feature by its name.
+// It returns the Feature struct and a boolean indicating whether the
+// feature was found.
+func (fg FeatureGates) GetFeature(name string) (Feature, bool) {
+	feature, ok := fg[name]
+	return feature, ok
+}
+
+// GetFeatureNames returns a slice of feature names in the FeatureGates
+// instance.
+func (fg FeatureGates) GetFeatureNames() []string {
+	names := make([]string, 0, len(fg))
+	for name := range fg {
+		names = append(names, name)
+	}
+	return names
+}
+
+// GetDefaultFeatureGates returns a new FeatureGates instance initialized with the default feature set.
+// This function should be used when no overrides are needed.
+func GetDefaultFeatureGates() FeatureGates {
+	gates := make(FeatureGates)
+	for name, feature := range defaultACKFeatureGates {
+		gates[name] = feature
+	}
+	return gates
+}
+
+// GetFeatureGatesWithOverrides returns a new FeatureGates instance with the default features,
+// but with the provided overrides applied. This allows for runtime configuration of feature gates.
+func GetFeatureGatesWithOverrides(featureGateOverrides map[string]bool) FeatureGates {
+	gates := GetDefaultFeatureGates()
+	for name, enabled := range featureGateOverrides {
+		if feature, ok := gates[name]; ok {
+			feature.Enabled = enabled
+			gates[name] = feature
+		}
+	}
+	return gates
+}