How do you implement configuration hot-reloading in a Go service without restart?

Some configuration changes — feature flags, rate limits, log levels — should not require a pod restart. Hot-reloading reads new config from a file or config service and atomically swaps the configuration pointer.

// Atomic configuration pointer — reads and writes are goroutine-safe
type DynamicConfig struct {
    current atomic.Pointer[Config]
}

func NewDynamicConfig(initial *Config) *DynamicConfig {
    dc := &DynamicConfig{}
    dc.current.Store(initial)
    return dc
}

// Get returns the current config — zero allocation, lock-free
func (dc *DynamicConfig) Get() *Config {
    return dc.current.Load()
}

// Reload atomically swaps to a new config
func (dc *DynamicConfig) Reload(path string) error {
    data, err := os.ReadFile(path)
    if err != nil { return err }
    var cfg Config
    if err := yaml.Unmarshal(data, &cfg); err != nil { return err }
    if err := cfg.Validate(); err != nil { return err }
    dc.current.Store(&cfg) // atomic swap
    slog.Info("config reloaded", "path", path)
    return nil
}

// SIGHUP-triggered reload
func watchConfigSignal(dc *DynamicConfig, path string) {
    sighup := make(chan os.Signal, 1)
    signal.Notify(sighup, syscall.SIGHUP)
    for range sighup {
        if err := dc.Reload(path); err != nil {
            slog.Error("config reload failed", "err", err)
            // Keep running with old config
        }
    }
}

// File watcher approach
func watchConfigFile(dc *DynamicConfig, path string, ctx context.Context) {
    watcher, _ := fsnotify.NewWatcher()
    watcher.Add(path)
    defer watcher.Close()
    for {
        select {
        case <-ctx.Done(): return
        case event := <-watcher.Events:
            if event.Has(fsnotify.Write) {
                time.Sleep(100 * time.Millisecond) // wait for write to complete
                dc.Reload(path)
            }
        }
    }
}

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