feat: add dynamic flagset reloading without restart

.gitignore

@@ -52,3 +52,4 @@ volume
 
 go.work
 go.work.sum
+main

cmd/relayproxy/config/config.go

@@ -218,6 +218,32 @@
 	return proxyConf, nil
 }
 
+// ReloadFromFile reloads the configuration from the config file.
+// It preserves command line flags and environment variables.
+func ReloadFromFile(flagSet *pflag.FlagSet, log *zap.Logger, version string) (*Config, error) {
+	// Reload config file (this will overwrite the file-based config)
+	loadConfigFile(log)
+
+	// Map environment variables (they take precedence over file config)
+	_ = k.Load(mapEnvVariablesProvider(k.String("envVariablePrefix"), log), nil)
+	_ = k.Set("version", version)
+
+	proxyConf := &Config{}
+	errUnmarshal := k.Unmarshal("", &proxyConf)
+	if errUnmarshal != nil {
+		return nil, errUnmarshal
+	}
+
+	processExporters(proxyConf)
+
+	return proxyConf, nil
+}
+
+// GetConfigFilePath returns the path to the configuration file being used.
+func GetConfigFilePath() (string, error) {
+	return locateConfigFile(k.String("config"))
+}
+
 // loadConfigFile handles the loading of configuration files
 func loadConfigFile(log *zap.Logger) {
 	configFileLocation, errFileLocation := locateConfigFile(k.String("config"))
@@ -232,6 +258,45 @@
 	}
 }
 
+// WatchConfigFile sets up a file watcher using koanf's built-in file watching
+// and calls the reloadCallback when the configuration file changes.
+// This function spawns a goroutine to watch for changes (Watch() is blocking).
+func WatchConfigFile(
+	configFilePath string,
+	reloadCallback func() error,
+	log *zap.Logger,
+) error {
+	parser := getParserForFile(configFilePath)
+	fileProvider := file.Provider(configFilePath)
+
+	// Watch for changes using koanf's built-in Watch() method
+	// Watch() is blocking and spawns a goroutine internally, so we call it in a goroutine
+	go func() {
+		if err := fileProvider.Watch(func(event interface{}, err error) {
+			if err != nil {
+				log.Error("error watching config file", zap.Error(err))
+				return
+			}
+
+			// Reload the configuration file
+			if err := k.Load(fileProvider, parser); err != nil {
+				log.Error("error reloading config file", zap.Error(err))
+				return
+			}
+
+			// Call the reload callback
+			if err := reloadCallback(); err != nil {
+				log.Error("error in reload callback", zap.Error(err))
+			}
+		}); err != nil {
+			log.Error("failed to start file watcher", zap.Error(err))
+		}
+	}()
+
+	log.Info("watching configuration file for changes", zap.String("file", configFilePath))
+	return nil
+}
+
 // getParserForFile returns the appropriate parser based on file extension
 func getParserForFile(configFileLocation string) koanf.Parser {
 	ext := filepath.Ext(configFileLocation)

cmd/relayproxy/helper/echo_helper_test.go

@@ -9,8 +9,11 @@
 	"github.com/labstack/echo/v4"
 	"github.com/stretchr/testify/assert"
 	ffclient "github.com/thomaspoignant/go-feature-flag"
+	"github.com/thomaspoignant/go-feature-flag/cmd/relayproxy/config"
 	"github.com/thomaspoignant/go-feature-flag/cmd/relayproxy/helper"
 	"github.com/thomaspoignant/go-feature-flag/cmd/relayproxy/service"
+	"github.com/thomaspoignant/go-feature-flag/notifier"
+	"go.uber.org/zap"
 )
 
 func TestGetAPIKey(t *testing.T) {
@@ -256,11 +259,11 @@
 	err     error
 }
 
-func (m *MockFlagsetManager) GetFlagSet(apiKey string) (*ffclient.GoFeatureFlag, error) {
+func (m *MockFlagsetManager) GetFlagSet(_ string) (*ffclient.GoFeatureFlag, error) {
 	return m.flagset, m.err
 }
 
-func (m *MockFlagsetManager) GetFlagSetName(apiKey string) (string, error) {
+func (m *MockFlagsetManager) GetFlagSetName(_ string) (string, error) {
 	return "", nil
 }
 
@@ -276,6 +279,11 @@
 	return false
 }
 
+func (m *MockFlagsetManager) ReloadFlagsets(newConfig *config.Config, logger *zap.Logger, notifiers []notifier.Notifier) error {
+	// nothing to do for mock
+	return nil
+}
+
 func (m *MockFlagsetManager) Close() {
 	// nothing to do
 }

cmd/relayproxy/main.go

@@ -120,5 +120,54 @@ func main() {
 		defer cancel()
 		apiServer.Stop(ctx)
 	}()
+
+	// Start config file watcher if flagsets are configured
+	if len(proxyConf.FlagSets) > 0 {
+		configFilePath, err := config.GetConfigFilePath()
+		if err == nil {
+			err = config.WatchConfigFile(configFilePath, func() error {
+				return reloadFlagsets(f, flagsetManager, logger.ZapLogger, version, []notifier.Notifier{
+					prometheusNotifier,
+					proxyNotifier,
+				})
+			}, logger.ZapLogger)
+			if err != nil {
+				logger.ZapLogger.Warn("could not start config file watcher", zap.Error(err))
+			}
+		} else {
+			logger.ZapLogger.Warn("could not start config file watcher", zap.Error(err))
+		}
+	}
+
 	apiServer.StartWithContext(context.Background())
 }
+
+// reloadFlagsets reloads the configuration file and updates flagsets
+func reloadFlagsets(
+	flagSet *pflag.FlagSet,
+	flagsetManager service.FlagsetManager,
+	logger *zap.Logger,
+	version string,
+	notifiers []notifier.Notifier,
+) error {
+	logger.Info("configuration file changed, reloading flagsets")
+
+	// Reload configuration from file
+	newConfig, err := config.ReloadFromFile(flagSet, logger, version)
+	if err != nil {
+		return fmt.Errorf("failed to reload configuration file: %w", err)
+	}
+
+	// Validate configuration
+	if err := newConfig.IsValid(); err != nil {
+		return fmt.Errorf("reloaded configuration is invalid: %w", err)
+	}
+
+	// Reload flagsets
+	if err := flagsetManager.ReloadFlagsets(newConfig, logger, notifiers); err != nil {
+		return fmt.Errorf("failed to reload flagsets: %w", err)
+	}
+
+	logger.Info("flagsets reloaded successfully")
+	return nil
+}