Viper: Configuration Management in Go

package config

import (
    "github.com/spf13/viper"
)

type Config struct {
    Server   ServerConfig
    Database DatabaseConfig
    Redis    RedisConfig
}

type ServerConfig struct {
    Port    int    `mapstructure:"port"`
    Host    string `mapstructure:"host"`
    Debug   bool   `mapstructure:"debug"`
}

type DatabaseConfig struct {
    Host     string `mapstructure:"host"`
    Port     int    `mapstructure:"port"`
    Name     string `mapstructure:"name"`
    User     string `mapstructure:"user"`
    Password string `mapstructure:"password"`
}

func Load() (*Config, error) {
    viper.SetConfigName("config")
    viper.SetConfigType("yaml")
    viper.AddConfigPath(".")
    viper.AddConfigPath("./config")
    
    // Environment variable support
    viper.AutomaticEnv()
    viper.SetEnvPrefix("APP")
    
    // Set defaults
    viper.SetDefault("server.port", 8080)
    viper.SetDefault("server.host", "0.0.0.0")
    
    if err := viper.ReadInConfig(); err != nil {
        // Config file not found is OK
        if _, ok := err.(viper.ConfigFileNotFoundError); !ok {
            return nil, err
        }
    }
    
    var cfg Config
    if err := viper.Unmarshal(&cfg); err != nil {
        return nil, err
    }
    
    return &cfg, nil
}

server:
  port: 8080
  host: "0.0.0.0"
  debug: false

database:
  host: "localhost"
  port: 5432
  name: "myapp"
  user: "postgres"
  password: "${DB_PASSWORD}"  # Reference env var

redis:
  host: "localhost"
  port: 6379

# Override with environment variables
export APP_SERVER_PORT=9090
export APP_DATABASE_HOST=db.prod.example.com

./myapp
# Uses port 9090 and production database

func WatchConfig(onChange func()) {
    viper.WatchConfig()
    viper.OnConfigChange(func(e fsnotify.Event) {
        log.Println("Config file changed:", e.Name)
        onChange()
    })
}

// Usage:
WatchConfig(func() {
    newCfg, _ := Load()
    updateRuntimeConfig(newCfg)
})

import (
    "github.com/spf13/pflag"
    "github.com/spf13/viper"
)

func init() {
    pflag.Int("port", 8080, "Server port")
    pflag.Bool("debug", false, "Enable debug mode")
    pflag.Parse()
    
    viper.BindPFlags(pflag.CommandLine)
}

// Precedence: flags > env > config file > defaults