12 factor configuration with Go's `flag` package

III. Config
Store config in the environment
An app’s config is everything that is likely to vary between deploys (staging, production, developer environments, etc). This includes:
- Resource handles to the database, Memcached, and other backing services
- Credentials to external services such as Amazon S3 or Twitter
- Per-deploy values such as the canonical hostname for the deploy
Apps sometimes store config as constants in the code. This is a violation of twelve-factor, which requires strict separation of config from code. Config varies substantially across deploys, code does not.

package main

import (
	"flag"
	"fmt"
	"os"
	"strconv"
	"log"
)

var (
	// set by build process
	Git_Revision string
	Consul_URL string = "http://consul.local:8500"
	Statsd_URL string
	HTTP_ListenAddr string = ":8080"
	HTTP_Timeout    int    = 16
)

func main() {
	flag.StringVar(&Consul_URL, "consul-url", LookupEnvOrString("CONSUL_URL", Consul_URL), "service discovery url")
	flag.StringVar(&Statsd_URL, "statsd-url", LookupEnvOrString("STATSD_URL", Statsd_URL), "statsd's host:port")
	flag.StringVar(&HTTP_ListenAddr, "http-listen-addr", LookupEnvOrString("HTTP_LISTEN_ADDR", HTTP_ListenAddr), "http service listen address")
	flag.IntVar(&HTTP_Timeout, "http-timeout", LookupEnvOrInt("HTTP_TIMEOUT", HTTP_Timeout), "http timeout requesting http services")

	flag.Parse()
	log.Printf("app.config %v\n", getConfig(flag.CommandLine))

	log.Println("app.status=starting")
	defer log.Println("app.status=shutdown")

	log.Println("hello world")
}

func LookupEnvOrString(key string, defaultVal string) string {
	if val, ok := os.LookupEnv(key); ok {
		return val
	}
	return defaultVal
}

func LookupEnvOrInt(key string, defaultVal int) int {
	if val, ok := os.LookupEnv(key); ok {
		v, err := strconv.Atoi(val)
		if err != nil {
			log.Fatalf("LookupEnvOrInt[%s]: %v", key, err)
		}
		return v
	}
	return defaultVal
}

func getConfig(fs *flag.FlagSet) []string {
	cfg := make([]string, 0, 10)
	fs.VisitAll(func(f *flag.Flag) {
		cfg = append(cfg, fmt.Sprintf("%s:%q", f.Name, f.Value.String()))
	})

	return cfg
}