What are the idiomatic Go patterns for managing application configuration?

Go applications typically load configuration from environment variables (twelve-factor app pattern), config files, or a combination. The idiomatic approach is to load all configuration at startup, validate it, and inject it into components as a struct — not read environment variables throughout the codebase.

// Config struct — single source of truth
type Config struct {
    HTTP struct {
        Port         int           `env:"HTTP_PORT"          default:"8080"`
        ReadTimeout  time.Duration `env:"HTTP_READ_TIMEOUT"  default:"5s"`
        WriteTimeout time.Duration `env:"HTTP_WRITE_TIMEOUT" default:"10s"`
    }
    Database struct {
        URL             string `env:"DATABASE_URL"      required:"true"`
        MaxConnections  int    `env:"DB_MAX_CONNECTIONS" default:"25"`
    }
    Auth struct {
        JWTSecret string        `env:"JWT_SECRET" required:"true"`
        TokenTTL  time.Duration `env:"TOKEN_TTL"  default:"24h"`
    }
}

// Load from environment — validate at startup
func loadConfig() (*Config, error) {
    var cfg Config
    // Use envconfig, viper, or manual os.Getenv
    cfg.HTTP.Port = mustEnvInt("HTTP_PORT", 8080)
    cfg.Database.URL = mustEnv("DATABASE_URL")
    cfg.Auth.JWTSecret = mustEnv("JWT_SECRET")
    if cfg.Auth.JWTSecret == "" {
        return nil, errors.New("JWT_SECRET must not be empty")
    }
    return &cfg, nil
}

func mustEnv(key string) string {
    v := os.Getenv(key)
    if v == "" {
        log.Fatalf("required environment variable %s is not set", key)
    }
    return v
}

func main() {
    cfg, err := loadConfig()
    if err != nil {
        log.Fatalf("invalid configuration: %v", err)
    }
    // Inject cfg into all components
    db := openDB(cfg.Database.URL, cfg.Database.MaxConnections)
    srv := newServer(cfg.HTTP, db)
    srv.Start()
}

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