Viper Examples

Viper is a configuration library fo Go. It has been my go to library for configs. Some of the best features of Viper are:

  • Ability to unmarshal subsets of a configuration file into a Go struct
  • Ability to override contents of a configuration file using OS environment variables

Here’s a brief example that demonstrates both features:

Assume you have a configuration file at $HOME/server.toml

resolve_dns = true
http_keep_alive = 5

[example_com]
doc_root = "/var/www/example_com"
allow_files = "*.html"
login_required = true
login_lockout_count = 3

[example_org]
doc_root = "/var/www/example_org"
allow_files = "*.html, *.jpg"
login_required = false
login_lockout_count = 5

Then you can utilize the configuration file using this snippet:

package main

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

func main() {
	userHomeDir, err := os.UserHomeDir()
	if err != nil {
		panic(err)
	}
	baseDir := fmt.Sprintf("%s", userHomeDir)

	// This section demonstrates the ability to
	// 1. Unmarshal a subtree using Sub()
	// 2. Ability to override subtree configurations using SetEnvPrefix
	//
	// Run this via:
	//
	//     COM_DOC_ROOT=123 ORG_DOC_ROOT=456 go run main.go
	//
	viper.SetConfigName("server")
	viper.AddConfigPath(fmt.Sprintf("%s", baseDir))
	if err := viper.ReadInConfig(); err != nil {
		panic(err)
	}

	var sc ServerConf
	if err := viper.Unmarshal(&sc); err != nil {
		panic(err)
	}
	fmt.Printf("ServerConf:\n%+v\n", sc)

	com_section := viper.Sub("example_com")
	com_section.SetEnvPrefix("com")
	com_section.AutomaticEnv() // Or viper.BindEnv("doc_root")

	var vc_com VHostConf
	if err := com_section.Unmarshal(&vc_com); err != nil {
		panic(err)
	}
	fmt.Printf("example.com:\n%+v\n", vc_com)

	org_section := viper.Sub("example_org")
	org_section.SetEnvPrefix("org")
	org_section.AutomaticEnv()

	var vc_www VHostConf
	if err := org_section.Unmarshal(&vc_www); err != nil {
		panic(err)
	}
	fmt.Printf("example.org\n%+v\n", vc_www)

}

type ServerConf struct {
	ResolveDNS    bool `mapstructure:"resolve_dns"`
	HttpKeepAlive int  `mapstructure:"http_keep_alive"`
}

type VHostConf struct {
	DocRoot           string `mapstructure:"doc_root"`
	AllowFiles        string `mapstructure:"allow_files"`
	LoginRequired     bool   `mapstructure:"login_required"`
	LoginLockoutCount int    `mapstructure:"login_lockout_count"`
}

Running the command COM_DOC_ROOT=123 ORG_DOC_ROOT=456 go run main.go gets you

ServerConf:
{ResolveDNS:true HttpKeepAlive:5}
example.com:
{DocRoot:123 AllowFiles:*.html LoginRequired:true LoginLockoutCount:3}
example.org
{DocRoot:456 AllowFiles:*.html, *.jpg LoginRequired:false LoginLockoutCount:5}

The DocRoot from each vhost has been overriden with environment variables, allowing you to make changes quickly without touhing your configuration file, which is frequently useful when prototyping new software.