Configuration supplies values that can change without changing application code. Command-line options suit values chosen for a particular run; a file is a better fit for persistent, potentially complex settings that would otherwise need to be retyped. Environment variables are another way to provide configuration, particularly for 12-factor applications. section 2.2 section 2.2.1
Choose a format, then load its values at startup:
- JSON: define a Go struct matching the file’s names and nesting, open the file, and decode it with encoding/json. Export struct fields by capitalizing their names so the decoder can populate them; check file and decoding errors. section 2.2.1
- YAML: use github.com/kylelemons/go-gypsy/yaml to read a file and retrieve values such as a path and a Boolean enabled setting. section 2.2.1
- INI: use github.com/go-ini/ini to load a file and read keys from a named section, converting values to the types the application needs. section 2.2.1
Listing 2.9 Parsing a JSON configuration file: json_config.go
package main
import (
"encoding/json"
"fmt"
"os"
)
type configuration struct { #1
Enabled bool #1
Path string #1
} #1
func main() {
file, err := os.Open("config.json") #2
if err != nil {
panic(err)
}
defer file.Close()
decoder := json.NewDecoder(file) #3
conf := configuration{} #3
err = decoder.Decode(&conf) #3
if err != nil {
fmt.Println("Error:", err)
}
fmt.Println(conf.Path)
}
The JSON parser shows the complete path from config.json to a Go struct: open the file, defer its closure, decode into the struct, and use a loaded field. section 2.2.1
Configuration can also select resources for a deployment. When web assets are served separately—such as from a CDN—give each development, testing, and production environment its own asset location. Pass that location into the application and use it in the page template. A startup argument is one option; configuration files or a shared configuration service are others. section 10.1.4
Listing 10.7 Passing a URL location to a template
var t *template.Template
var l = flag.String("location",
"http://localhost:8080", "A location.") #1
var tpl = `<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>A Demo</title>
<link rel="stylesheet" href="{{.Location}}/styles.
css"> #2
</head>
<body>
<p>A demo.</p>
</body>
</html>`
func servePage(res http.ResponseWriter,
req *http.Request) { #3
data := struct{ Location *string }{ #3
Location: l, #3
} #3
t.Execute(res, data) #3
} #3
The template example takes an asset location from a flag and places it in the stylesheet URL, showing how an execution-time value reaches the rendered page. section 10.1.4
