koanf

koanf (pronounced conf; a play on the Japanese Koan) is a library for reading configuration from different sources in different formats in Go applications. It is a cleaner, lighter alternative to spf13/viper with better abstractions and extensibility and fewer dependencies.

koanf comes with built in support for reading configuration from files, command line flags, and environment variables, and can parse JSON, YAML, TOML, and Hashicorp HCL.

Installation

go get -u github.com/knadh/koanf

Contents

• Concepts
• Reading config from files
• Watching files for changes
• Reading from command line
• Reading environment variables
• Reading raw bytes
• Unmarshalling
• Unmarshalling with flat paths
• Setting default values
• Order of merge and key case senstivity
• Custom Providers and Parsers
• API

Concepts

• koanf.Provider is a generic interface that provides configuration, for example, from files, enviornment variables, HTTP sources, or anywhere. The configuration can either be raw bytes that a parser can parse, or it can be a nested map[string]interface{} that can be directly loaded.
• koanf.Parser is a generic interface that takes raw bytes, parses, and returns a nested map[string]interface{} representation. For example, JSON and YAML parsers.
• Once loaded into koanf, configuration are values queried by a delimited key path syntax. eg: app.server.port. Any delimiter can be chosen.
• Configuration from multiple sources can be loaded and merged into a koanf instance, for example, load from a file first and override certain values with flags from the command line.

With these two interface implementations, koanf can obtain configuration from multiple sources and parse any format and make it available to an application.

Reading config from files

package main

import (
	"fmt"
	"log"

	"github.com/knadh/koanf"
	"github.com/knadh/koanf/parsers/json"
	"github.com/knadh/koanf/parsers/yaml"
	"github.com/knadh/koanf/providers/file"
)

// Global koanf instance. Use "." as the key path delimiter. This can be "/" or any character.
var k = koanf.New(".")

func main() {
	// Load JSON config.
	if err := k.Load(file.Provider("mock/mock.json"), json.Parser()); err != nil {
		log.Fatalf("error loading config: %v", err)
	}

	// Load YAML config and merge into the previously loaded config (because we can).
	k.Load(file.Provider("mock/mock.yaml"), yaml.Parser())

	fmt.Println("parent's name is = ", k.String("parent1.name"))
	fmt.Println("parent's ID is = ", k.Int("parent1.id"))
}

Watching files for changes

The koanf.Provider interface has a Watch(cb) method that asks a provider to watch for changes and trigger the given callback that can live reload the configuration.

Currently, file.Provider supports this.

package main

import (
	"fmt"
	"log"

	"github.com/knadh/koanf"
	"github.com/knadh/koanf/parsers/json"
	"github.com/knadh/koanf/parsers/yaml"
	"github.com/knadh/koanf/providers/file"
)

// Global koanf instance. Use "." as the key path delimiter. This can be "/" or any character.
var k = koanf.New(".")

func main() {
	// Load JSON config.
	f := file.Provider("mock/mock.json")
	if err := k.Load(f, json.Parser()); err != nil {
		log.Fatalf("error loading config: %v", err)
	}

	// Load YAML config and merge into the previously loaded config (because we can).
	k.Load(file.Provider("mock/mock.yaml"), yaml.Parser())

	fmt.Println("parent's name is = ", k.String("parent1.name"))
	fmt.Println("parent's ID is = ", k.Int("parent1.id"))

	// Watch the file and get a callback on change. The callback can do whatever,
	// like re-load the configuration.
	// File provider always returns a nil `event`.
	f.Watch(func(event interface{}, err error) {
		if err != nil {
			log.Printf("watch error: %v", err)
			return
		}

		log.Println("config changed. Reloading ...")
		k.Load(f, json.Parser())
		k.Print()
	})

	// Block forever (and manually make a change to mock/mock.json) to
	// reload the config.
	log.Println("waiting forever. Try making a change to mock/mock.json to live reload")
	<-make(chan bool)
}

Reading from command line

The following example shows the use of posflag.Provider, a wrapper over spf13/pflag library, an advanced commandline lib. For Go's built in flag package, use basicflag.Provider.

package main

import (
	"fmt"
	"log"
	"os"

	"github.com/knadh/koanf"
	"github.com/knadh/koanf/parsers/toml"
	"github.com/knadh/koanf/providers/file"
	"github.com/knadh/koanf/providers/posflag"
	flag "github.com/spf13/pflag"
)

// Global koanf instance. Use "." as the key path delimiter. This can be "/" or any character.
var k = koanf.New(".")

func main() {
	// Use the POSIX compliant pflag lib instead of Go's flag lib.
	f := flag.NewFlagSet("config", flag.ContinueOnError)
	f.Usage = func() {
		fmt.Println(f.FlagUsages())
		os.Exit(0)
	}
	// Path to one or more config files to load into koanf along with some config params.
	f.StringSlice("conf", []string{"mock/mock.toml"}, "path to one or more .toml config files")
	f.String("time", "2020-01-01", "a time string")
	f.String("type", "xxx", "type of the app")
	f.Parse(os.Args[1:])

	// Load the config files provided in the commandline.
	cFiles, _ := f.GetStringSlice("conf")
	for _, c := range cFiles {
		if err := k.Load(file.Provider(c), toml.Parser()); err != nil {
			log.Fatalf("error loading file: %v", err)
		}
	}

	// "time" and "type" may have been loaded from the config file, but
	// they can still be overridden with the values from the command line.
	// The bundled posflag.Provider takes a flagset from the spf13/pflag lib.
	// Passing the Koanf instance to posflag helps it deal with default command
	// line flag values that are not present in conf maps from previously loaded
	// providers.
	if err := k.Load(posflag.Provider(f, ".", k), nil); err != nil {
		log.Fatalf("error loading config: %v", err)
	}

	fmt.Println("time is = ", k.String("time"))
}

Reading environment variables

package main

import (
	"fmt"
	"log"
	"strings"

	"github.com/knadh/koanf"
	"github.com/knadh/koanf/parsers/json"
	"github.com/knadh/koanf/providers/env"
	"github.com/knadh/koanf/providers/file"
)

// Global koanf instance. Use . as the key path delimiter. This can be / or anything.
var k = koanf.New(".")

func main() {
	// Load JSON config.
	if err := k.Load(file.Provider("mock/mock.json"), json.Parser()); err != nil {
		log.Fatalf("error loading config: %v", err)
	}

	// Load environment variables and merge into the loaded config.
	// "MYVAR" is the prefix to filter the env vars by.
	// "." is the delimiter used to represent the key hierarchy in env vars.
	// The (optional, or can be nil) function can be used to transform
	// the env var names, for instance, to lowercase them.
	//
	// For example, env vars: MYVAR_TYPE and MYVAR_PARENT1_CHILD1_NAME
	// will be merged into the "type" and the nested "parent1.child1.name"
	// keys in the config file here as we lowercase the key, 
	// replace `_` with `.` and strip the MYVAR_ prefix so that 
	// only "parent1.child1.name" remains.
	k.Load(env.Provider("MYVAR_", ".", func(s string) string {
		return strings.Replace(strings.ToLower(
			strings.TrimPrefix(s, "MYVAR_")), "_", ".", -1)
	}), nil)

	fmt.Println("name is = ", k.String("parent1.child1.name"))
}

Reading raw bytes

The bundled rawbytes Provider can be used to read arbitrary bytes from a source, like a database or an HTTP call.

package main

import (
	"fmt"

	"github.com/knadh/koanf"
	"github.com/knadh/koanf/parsers/json"
	"github.com/knadh/koanf/providers/rawbytes"
)

// Global koanf instance. Use . as the key path delimiter. This can be / or anything.
var k = koanf.New(".")

func main() {
	b := []byte(`{"type": "rawbytes", "parent1": {"child1": {"type": "rawbytes"}}}`)
	k.Load(rawbytes.Provider(b), json.Parser())
	fmt.Println("type is = ", k.String("parent1.child1.type"))
}

Unmarshalling

Unmarshalling is useful when you want to copy a nested config map into a struct (like unmarshalling JSON) instead of accessing individual config values using gettor methods.

package main

import (
"fmt"
"log"

    "github.com/knadh/koanf"
    "github.com/knadh/koanf/parsers/json"
    "github.com/knadh/koanf/providers/file"

)

// Global koanf instance. Use . as the key path delimiter. This can be / or anything.
var k = koanf.New(".")

func main() {
// Load JSON config.
if err := k.Load(file.Provider("mock/mock.json"), json.Parser()); err != nil {
log.Fatalf("error loading config: %v", err)
}

    // Structure to unmarshal nested conf to.
    type childStruct struct {
    	Name       string            `koanf:"name"`
    	Type       string            `koanf:"type"`
    	Empty      map[string]string `koanf:"empty"`
    	GrandChild struct {
    		Ids []int `koanf:"ids"`
    		On  bool  `koanf:"on"`
    	} `koanf:"grandchild1"`
    }

    var out childStruct

    // Quick unmarshal.
    k.Unmarshal("parent1.child1", &out)
    fmt.Println(out)

    // Unmarshal with advanced config.
    out = childStruct{}
    k.UnmarshalWithConf("parent1.child1", &out, koanf.UnmarshalConf{Tag: "koanf"})
    fmt.Println(out)
}

Unmarshalling with flat paths

Sometimes it is necessary to unmarshal an assortment of keys from various nested structures into a flat target structure. This is possible with the UnmarshalConf.FlatPaths flag.

package main

import (
	"fmt"
	"log"

	"github.com/knadh/koanf"
	"github.com/knadh/koanf/parsers/json"
	"github.com/knadh/koanf/providers/file"
)

// Global koanf instance. Use . as the key path delimiter. This can be / or anything.
var k = koanf.New(".")

func main() {
	// Load JSON config.
	if err := k.Load(file.Provider("mock/mock.json"), json.Parser()); err != nil {
		log.Fatalf("error loading config: %v", err)
	}

	type rootFlat struct {
		Type                        string            `koanf:"type"`
		Empty                       map[string]string `koanf:"empty"`
		Parent1Name                 string            `koanf:"parent1.name"`
		Parent1ID                   int               `koanf:"parent1.id"`
		Parent1Child1Name           string            `koanf:"parent1.child1.name"`
		Parent1Child1Type           string            `koanf:"parent1.child1.type"`
		Parent1Child1Empty          map[string]string `koanf:"parent1.child1.empty"`
		Parent1Child1Grandchild1IDs []int             `koanf:"parent1.child1.grandchild1.ids"`
		Parent1Child1Grandchild1On  bool              `koanf:"parent1.child1.grandchild1.on"`
	}

	// Unmarshal the whole root with FlatPaths: True.
	var o1 rootFlat
	k.UnmarshalWithConf("", &o1, koanf.UnmarshalConf{Tag: "koanf", FlatPaths: true})
	fmt.Println(o1)

	// Unmarshal a child structure of "parent1".
	type subFlat struct {
		Name                 string            `koanf:"name"`
		ID                   int               `koanf:"id"`
		Child1Name           string            `koanf:"child1.name"`
		Child1Type           string            `koanf:"child1.type"`
		Child1Empty          map[string]string `koanf:"child1.empty"`
		Child1Grandchild1IDs []int             `koanf:"child1.grandchild1.ids"`
		Child1Grandchild1On  bool              `koanf:"child1.grandchild1.on"`
	}

	var o2 subFlat
	k.UnmarshalWithConf("parent1", &o2, koanf.UnmarshalConf{Tag: "koanf", FlatPaths: true})
	fmt.Println(o2)
}

Setting default values.

koanf does not provide any special functions to set default values but uses the Provider interface to enable it.

From a map

The bundled confmap provider takes a map[string]interface{} that can be loaded into a koanf instance.

package main

import (
	"fmt"
	"log"

	"github.com/knadh/koanf"
	"github.com/knadh/koanf/providers/confmap"
	"github.com/knadh/koanf/providers/file"
	"github.com/knadh/koanf/parsers/json"
	"github.com/knadh/koanf/parsers/yaml"
)

// Global koanf instance. Use "." as the key path delimiter. This can be "/" or any character.
var k = koanf.New(".")

func main() {
	// Load default values using the confmap provider.
	// We provide a flat map with the "." delimiter.
	// A nested map can be loaded by setting the delimiter to an empty string "".
	k.Load(confmap.Provider(map[string]interface{}{
		"parent1.name": "Default Name",
		"parent3.name": "New name here",
	}, "."), nil)

	// Load JSON config on top of the default values.
	if err := k.Load(file.Provider("mock/mock.json"), json.Parser()); err != nil {
		log.Fatalf("error loading config: %v", err)
	}

	// Load YAML config and merge into the previously loaded config (because we can).
	k.Load(file.Provider("mock/mock.yaml"), yaml.Parser())

	fmt.Println("parent's name is = ", k.String("parent1.name"))
	fmt.Println("parent's ID is = ", k.Int("parent1.id"))
}

From a struct

The bundled structs provider can be used to read data from a struct to load into a koanf instance.

package main

import (
	"fmt"

	"github.com/knadh/koanf"
	"github.com/knadh/koanf/providers/structs"
)

// Global koanf instance. Use "." as the key path delimiter. This can be "/" or any character.
var k = koanf.New(".")

type parentStruct struct {
	Name   string      `koanf:"name"`
	ID     int         `koanf:"id"`
	Child1 childStruct `koanf:"child1"`
}
type childStruct struct {
	Name        string            `koanf:"name"`
	Type        string            `koanf:"type"`
	Empty       map[string]string `koanf:"empty"`
	Grandchild1 grandchildStruct  `koanf:"grandchild1"`
}
type grandchildStruct struct {
	Ids []int `koanf:"ids"`
	On  bool  `koanf:"on"`
}
type sampleStruct struct {
	Type    string            `koanf:"type"`
	Empty   map[string]string `koanf:"empty"`
	Parent1 parentStruct      `koanf:"parent1"`
}

func main() {
	// Load default values using the structs provider.
	// We provide a struct along with the struct tag `koanf` to the
	// provider.
	k.Load(structs.Provider(sampleStruct{
		Type:  "json",
		Empty: make(map[string]string),
		Parent1: parentStruct{
			Name: "parent1",
			ID:   1234,
			Child1: childStruct{
				Name:  "child1",
				Type:  "json",
				Empty: make(map[string]string),
				Grandchild1: grandchildStruct{
					Ids: []int{1, 2, 3},
					On:  true,
				},
			},
		},
	}, "koanf"), nil)

	fmt.Printf("name is = `%s`\n", k.String("parent1.child1.name"))
}

Order of merge and key case senstivity

• Config keys are case sensitive in koanf. For example, app.server.port and APP.SERVER.port are not the same.
• koanf does not impose any ordering on loading config from various providers. Every successive Load() or Load() merges new config into existing config. That means it is possible to load environment variables first, then files on top of it, and then command line variables on top of it, or any such order.

Custom Providers and Parsers

A Provider can provide a nested map[string]interface{} config that can be loaded into koanf with koanf.Load() or raw bytes that can be parsed with a Parser (loaded using koanf.Load().

Writing Providers and Parsers are easy. See the bundled implementations in the providers and parses directory.

API

Bundled providers

Bundled parsers

Instance functions

Getter functions

Alternative to viper

koanf is a light weight alternative to the popular spf13/viper. It does not aim to do everything viper does (such as mutating config maps and writing them back to files), but provides simpler primitives for reading and accessing configuration. It was written as a result of multiple stumbling blocks encountered with some of viper's fundamental flaws.

• viper breaks JSON, YAML, TOML, HCL language specs by forcibly lowercasing keys.
• Tightly couples config parsing with file extensions.
• Has poor semantics and abstractions. Commandline, env, file etc. and various parses are hardcoded in the core. There are no primitives that can be extended.
• Pulls a large number of third party dependencies into the core package. For instance, even if you do not use YAML or flags, the dependencies are still pulled as a result of the coupling.
• Imposes arbitrary ordering conventions (eg: flag -> env -> config etc.)
• Get() returns references to slices and maps. Mutations made outside change the underlying values inside the conf map.
• Does non-idiomatic things such as throwing away O(1) on flat maps.
• There are a large number of open issues.