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Dynamic configuration management with hot reloading has become essential for modern Golang applications that require high availability and operational flexibility. I have spent considerable time working with systems that need to adapt to changing requirements without service interruptions, and implementing this pattern correctly makes the difference between a resilient application and one that struggles with configuration changes.
When I first encountered the need for dynamic configuration in a production environment, I realized that traditional static configuration files create significant operational overhead. Applications require restarts for every configuration change, leading to service downtime and potential data loss. The hot reloading approach addresses these challenges by monitoring configuration files and applying changes automatically.
The foundation of effective configuration management relies on a robust watching mechanism that can detect file system changes reliably. File system notifications provide the most efficient approach, as they eliminate the need for continuous polling while ensuring immediate response to configuration updates.
package main
import (
"context"
"encoding/json"
"fmt"
"log"
"os"
"path/filepath"
"sync"
"time"
"github.com/fsnotify/fsnotify"
)
type ConfigManager struct {
mu sync.RWMutex
config map[string]interface{}
filePath string
watcher *fsnotify.Watcher
callbacks []ConfigChangeCallback
errorHandler func(error)
validator ConfigValidator
ctx context.Context
cancel context.CancelFunc
}
type ConfigChangeCallback func(key string, oldValue, newValue interface{})
type ConfigValidator interface {
Validate(config map[string]interface{}) error
}
The core structure maintains configuration state while providing thread-safe access through read-write mutexes. This approach ensures that configuration reads remain fast and concurrent while serializing write operations to maintain consistency.
Configuration validation forms a critical component of the system. I have learned from experience that invalid configurations can cause cascading failures throughout an application. Implementing validation at the configuration layer prevents these issues before they propagate to business logic.
type DefaultValidator struct {
requiredKeys []string
constraints map[string]func(interface{}) error
}
func NewDefaultValidator(required []string) *DefaultValidator {
return &DefaultValidator{
requiredKeys: required,
constraints: make(map[string]func(interface{}) error),
}
}
func (v *DefaultValidator) AddConstraint(key string, constraint func(interface{}) error) {
v.constraints[key] = constraint
}
func (v *DefaultValidator) Validate(config map[string]interface{}) error {
for _, key := range v.requiredKeys {
if _, exists := config[key]; !exists {
return fmt.Errorf("required configuration key missing: %s", key)
}
}
for key, constraint := range v.constraints {
if value, exists := config[key]; exists {
if err := constraint(value); err != nil {
return fmt.Errorf("validation failed for key %s: %w", key, err)
}
}
}
return nil
}
The validation system supports both required key checks and custom constraint functions. This flexibility allows applications to enforce business rules at the configuration level, such as ensuring port numbers fall within valid ranges or database URLs follow expected formats.
Configuration initialization requires careful handling of initial loading and watcher setup. The process must be atomic to prevent race conditions between initial configuration loading and the start of file monitoring.
func NewConfigManager(filePath string) (*ConfigManager, error) {
watcher, err := fsnotify.NewWatcher()
if err != nil {
return nil, fmt.Errorf("failed to create file watcher: %w", err)
}
ctx, cancel := context.WithCancel(context.Background())
cm := &ConfigManager{
config: make(map[string]interface{}),
filePath: filePath,
watcher: watcher,
ctx: ctx,
cancel: cancel,
validator: NewDefaultValidator(nil),
}
if err := cm.loadConfig(); err != nil {
return nil, fmt.Errorf("failed to load initial config: %w", err)
}
if err := cm.startWatching(); err != nil {
return nil, fmt.Errorf("failed to start watching: %w", err)
}
return cm, nil
}
Type-safe accessor methods provide a clean interface for retrieving configuration values while handling JSON unmarshaling quirks. JSON parsing often converts numbers to float64, requiring careful type conversion for integer values.
func (cm *ConfigManager) GetString(key string) (string, bool) {
value, exists := cm.Get(key)
if !exists {
return "", false
}
str, ok := value.(string)
return str, ok
}
func (cm *ConfigManager) GetInt(key string) (int, bool) {
value, exists := cm.Get(key)
if !exists {
return 0, false
}
switch v := value.(type) {
case float64:
return int(v), true
case int:
return v, true
default:
return 0, false
}
}
func (cm *ConfigManager) GetDuration(key string) (time.Duration, bool) {
str, exists := cm.GetString(key)
if !exists {
return 0, false
}
duration, err := time.ParseDuration(str)
return duration, err == nil
}
The configuration loading process handles validation and change detection efficiently. When loading new configuration, the system compares values to detect actual changes and only triggers callbacks for modified keys.
func (cm *ConfigManager) loadConfig() error {
data, err := os.ReadFile(cm.filePath)
if err != nil {
return fmt.Errorf("failed to read config file: %w", err)
}
var newConfig map[string]interface{}
if err := json.Unmarshal(data, &newConfig); err != nil {
return fmt.Errorf("failed to parse config JSON: %w", err)
}
if cm.validator != nil {
if err := cm.validator.Validate(newConfig); err != nil {
return fmt.Errorf("configuration validation failed: %w", err)
}
}
cm.mu.Lock()
defer cm.mu.Unlock()
for key, newValue := range newConfig {
oldValue := cm.config[key]
if !equal(oldValue, newValue) {
cm.config[key] = newValue
go cm.notifyCallbacks(key, oldValue, newValue)
}
}
for key := range cm.config {
if _, exists := newConfig[key]; !exists {
oldValue := cm.config[key]
delete(cm.config, key)
go cm.notifyCallbacks(key, oldValue, nil)
}
}
return nil
}
File system monitoring requires careful handling of different event types and editor behaviors. Many text editors create temporary files during save operations, which can trigger multiple events for a single logical change.
func (cm *ConfigManager) watchLoop() {
for {
select {
case <-cm.ctx.Done():
return
case event, ok := <-cm.watcher.Events:
if !ok {
return
}
if event.Name != cm.filePath {
continue
}
if event.Op&fsnotify.Write == fsnotify.Write || event.Op&fsnotify.Create == fsnotify.Create {
time.Sleep(100 * time.Millisecond)
if err := cm.loadConfig(); err != nil {
if cm.errorHandler != nil {
cm.errorHandler(fmt.Errorf("failed to reload config: %w", err))
} else {
log.Printf("Config reload error: %v", err)
}
} else {
log.Printf("Configuration reloaded from %s", cm.filePath)
}
}
case err, ok := <-cm.watcher.Errors:
if !ok {
return
}
if cm.errorHandler != nil {
cm.errorHandler(fmt.Errorf("file watcher error: %w", err))
}
}
}
}
The callback mechanism enables different application components to respond selectively to configuration changes. This pattern allows for graceful reconfiguration without full service restarts.
func (cm *ConfigManager) AddCallback(callback ConfigChangeCallback) {
cm.mu.Lock()
defer cm.mu.Unlock()
cm.callbacks = append(cm.callbacks, callback)
}
func (cm *ConfigManager) notifyCallbacks(key string, oldValue, newValue interface{}) {
for _, callback := range cm.callbacks {
callback(key, oldValue, newValue)
}
}
Error handling throughout the system provides multiple layers of resilience. The configuration manager continues operating even when individual reload attempts fail, preventing configuration errors from causing service outages.
Programmatic configuration updates support testing scenarios and administrative interfaces. The Set method includes validation and rollback capabilities to maintain configuration consistency.
func (cm *ConfigManager) Set(key string, value interface{}) error {
cm.mu.Lock()
defer cm.mu.Unlock()
oldValue := cm.config[key]
cm.config[key] = value
if cm.validator != nil {
if err := cm.validator.Validate(cm.config); err != nil {
cm.config[key] = oldValue
return fmt.Errorf("configuration validation failed: %w", err)
}
}
cm.notifyCallbacks(key, oldValue, value)
return nil
}
Practical implementation requires careful consideration of deployment scenarios. Configuration files should be managed through proper deployment pipelines that ensure atomicity and rollback capabilities. I recommend using symbolic links or atomic file replacement techniques to prevent partial configuration updates.
Performance considerations become important in high-throughput applications. The read-write mutex design prioritizes read performance while ensuring write consistency. Configuration reads typically far outnumber writes, making this optimization valuable for most applications.
Security aspects of configuration management require attention to file permissions and sensitive data handling. Configuration files containing secrets should use appropriate file system permissions and potentially encryption at rest.
The system integrates well with container orchestration platforms like Kubernetes through ConfigMaps and mounted volumes. The file-based approach enables seamless integration with existing deployment automation while providing the flexibility of runtime configuration changes.
Testing dynamic configuration systems requires careful simulation of file system events and validation of callback behaviors. Mock filesystems and controlled event generation help verify system behavior under various scenarios.
func main() {
configPath := "config.json"
initialConfig := map[string]interface{}{
"server_port": 8080,
"debug_mode": false,
"timeout": "30s",
"max_connections": 100,
"database_url": "postgres://localhost/myapp",
}
configData, _ := json.MarshalIndent(initialConfig, "", " ")
os.WriteFile(configPath, configData, 0644)
cm, err := NewConfigManager(configPath)
if err != nil {
log.Fatalf("Failed to create config manager: %v", err)
}
defer cm.Close()
validator := NewDefaultValidator([]string{"server_port", "database_url"})
validator.AddConstraint("server_port", func(value interface{}) error {
port, ok := value.(float64)
if !ok || port < 1 || port > 65535 {
return fmt.Errorf("invalid port number")
}
return nil
})
cm.SetValidator(validator)
cm.AddCallback(func(key string, oldValue, newValue interface{}) {
log.Printf("Config changed: %s = %v (was %v)", key, newValue, oldValue)
})
if port, ok := cm.GetInt("server_port"); ok {
fmt.Printf("Server will start on port: %d\n", port)
}
fmt.Println("Application started. Modify config.json to see hot reloading...")
time.Sleep(30 * time.Second)
}
This implementation provides a production-ready foundation for dynamic configuration management in Golang applications. The pattern enables operational flexibility while maintaining system stability through validation and error handling mechanisms. Applications can adapt to changing requirements without service interruptions, improving both user experience and operational efficiency.
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