Golang Viper

Golang Viper Configuration

Viper is Go's most popular configuration library. It handles config files, environment variables, defaults, and remote config stores with a unified API.

Config Precedence (highest to lowest)

1. viper.Set() - explicit runtime overrides
2. Command-line flags (via pflag)
3. Environment variables
4. Config files
5. Remote key/value stores (etcd, Consul)
6. viper.SetDefault() - defaults

Quick Reference

// Basic setup
viper.SetConfigName("config")        // filename without extension
viper.SetConfigType("yaml")          // explicit format
viper.AddConfigPath(".")             // search current dir
viper.AddConfigPath("$HOME/.myapp")  // search home dir

// Set defaults before reading
viper.SetDefault("server.port", 8080)

// Read config
if err := viper.ReadInConfig(); err != nil {
    if _, ok := err.(viper.ConfigFileNotFoundError); ok {
        // Config file not found; use defaults
    } else {
        return fmt.Errorf("config error: %w", err)
    }
}

// Get values
host := viper.GetString("server.host")
port := viper.GetInt("server.port")
debug := viper.GetBool("debug")

// Unmarshal to struct
var config Config
viper.Unmarshal(&config)
viper.UnmarshalKey("server", &config.Server)