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Stært is a Go library for loading and merging a program configuration structure from many sources.


Stært was born in order to merge two sources of configuration (Flæg, TOML). Now it also supports Key-Value Store.

We developed Flæg and Stært in order to simplify configuration maintenance on Træfik.


  • Load your configuration structure from many sources
  • Keep your configuration structure values unchanged if no overwriting (support defaults values)
  • Three native sources :
  • An interface to add your own sources
  • Handle pointers field :
    • You can give a structure of default values for pointers
    • Same comportment as Flæg
  • Stært is command oriented
    • It use flaeg.Command
    • Same comportment as Flæg commands
    • Stært supports only one command (the root-command)
    • Flæg allows you to use many commands
    • Only Flæg will be used if a sub-command is called. (because the configuration type could be different from one command to another)
    • You can add meta-data "parseAllSources" -> "true" to a sub-command if you want to parse all sources (it requires the same configuration type on the sub-command and the root-command)

Getting Started

The configuration

It works on your own configuration structure, like this one :

package example

import (

// Configuration is a struct which contains all different type to field
type Configuration struct {
	IntField     int                      `description:"An integer field"`
	StringField  string                   `description:"A string field"`
	PointerField *PointerSubConfiguration `description:"A pointer field"`

// PointerSubConfiguration is a SubStructure Configuration
type PointerSubConfiguration struct {
	BoolField  bool    `description:"A boolean field"`
	FloatField float64 `description:"A float field"`

Let's initialize it:

func main() {
	// Init with default value
	config := &Configuration{
		IntField:    1,
		StringField: "init",
		PointerField: &PointerSubConfiguration{
			FloatField: 1.1,
	// Set default pointers value
	defaultPointersConfig := &Configuration{
		PointerField: &PointerSubConfiguration{
			BoolField:  true,
			FloatField: 99.99,

The command

Stært uses flaeg.Command structure, like this:

// Create command
command := &flaeg.Command{
	Description:"This is an example of description",
	Run: func() error {
		fmt.Printf("Run example with the config :\n%+v\n", config)
		fmt.Printf("PointerField contains:%+v\n", config.PointerField)
		return nil

Use stært with sources

Initialize Stært:

s := staert.NewStaert(command)

Initialize TOML source:

toml := staert.NewTomlSource("example", []string{"./toml/", "/any/other/path"})

Initialize Flæg source:

f := flaeg.New(command, os.Args[1:])

Add sources

Add TOML and Flæg sources:


NB: You can change order, so that, Flæg configuration will overwrite TOML one.

Load your configuration

Just call LoadConfig function:

loadedConfig, err := s.LoadConfig();
if err != nil {
	// oops
// do what you want with `loadedConfig`
// or call run function

You can call Run

Run function will call run() from the command:

if err := s.Run(); err != nil {

NB: If you didn't call LoadConfig() before, your function run() will use your original configuration.

Let's run example

TOML file ./toml/example.toml:

IntField = 2

We can run the example program using following CLI arguments:

$ ./example --stringfield=owerwrittenFromFlag --pointerfield.floatfield=55.55
Run example with the config :
&{IntField:2 StringField:owerwrittenFromFlag PointerField:0xc82000ec80}
PointerField contains:&{BoolField:true FloatField:55.55}

Full example

Tagoæl is a trivial example which shows how Stært can be use. This funny GoLang program takes its configuration from both TOML and Flæg sources to display messages.

$ ./tagoael -h
tagoæl is an enhanced Hello World program to display messages with
an advanced configuration mechanism provided by Flæg & Stært.


Usage: tagoael [--flag=flag_argument] [-f[flag_argument]] ...     set flag_argument to flag(s)
   or: tagoael [--flag[=true|false| ]] [-f[true|false| ]] ...     set true/false to boolean flag(s)

        -c, --commandlineoverridesconfigfile               Whether configuration from command line overrides configuration from configuration file or not. (default "true")
        --configfile                                       Configuration file to use (TOML). (default "tagoael")
        -i, --displayindex                                 Whether to display index of each message (default "false")
        -m, --messagetodisplay                             Message to display (default "HELLO WOLRD")
        -n, --numbertodisplay                              Number of messages to display (default "1000")
        -h, --help                                         Print Help (this message) and exit

Thank you @debovema for this work :)


As with Flæg and TOML sources, the configuration structure can be loaded from a Key-Value Store. The package libkv provides connection to many KV Store like Consul, Etcd or Zookeeper.

The whole configuration structure is stored, using architecture like this pattern:

  • Key: <prefix1>/<prefix2>/.../<fieldNameLevel1>/<fieldNameLevel2>/.../<fieldName>
  • Value: <value>

It handles:

  • All mapstructure features(bool, int, ... , Squashed Embedded Sub struct, Pointer).
  • Maps with pattern : .../<MapFieldName>/<mapKey> -> <mapValue> (Struct as key not supported)
  • Slices (and Arrays) with pattern : .../<SliceFieldName>/<SliceIndex> -> <value>

Note: Hopefully, we provide the function StoreConfig to store your configuration structure ;)


KvSource implements Source:

type KvSource struct {
	Prefix string // like this "prefix" (without the /)


It can be initialized like this:

kv, err := staert.NewKvSource(backend store.Backend, addrs []string, options *store.Config, prefix string)


You can directly load data from the KV Store into the config structure (given by reference)

config := &ConfigStruct{} // Here your configuration structure by reference
err := kv.Parse(config)
// do what you want with `config`

Add to Stært sources

You can add this source to Stært, as with other sources:



You can also store your whole configuration structure into the KV Store:

// We assume that `config` is initialized
err := kv.StoreConfig(config)


  1. Fork it!
  2. Create your feature branch: git checkout -b my-new-feature
  3. Commit your changes: git commit -am 'Add some feature'
  4. Push to the branch: git push origin my-new-feature
  5. Submit a pull request :D

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