lalamove/konfig

Composable, observable and performant config handling for Go for the distributed processing era

Konfig

Composable, observable and performant config handling for Go. Written for larger distributed systems where you may have plenty of configuration sources - it allows you to compose configurations from multiple sources with reload hooks making it simple to build apps that live in a highly dynamic environment.

What's up with the name?

The name is Swedish for "config". We have a lot of nationalities here at Lalamove and to celebrate cultural diversity most of our open source packages will carry a name derived from a non-English language that is perhaps spoken by at least one of our employees(?).

Why another config package?

Most config packages for Golang are not very extensible and rarely expose interfaces. This makes it complex to build apps which can reload their state dynamically and difficult to mock. Fewer still come with sources such as Vault, Etcd and multiple encoding formats. In short, we didn't find a package that satisfied all of our requirements when we started out.

konfig is built around 4 small interfaces:

• Loader
• Watcher
• Parser
• Closer

Konfig features include:

• Dynamic configuration loading
• Composable load configs from multiple sources, such as vault, files and etcd
• Polyglot load configs from multiple format. Konfig supports JSON, YAML, TOML, Key=Value
• Fast, Lock-free, Thread safe Read Reads are up to 10x faster than Viper
• Observable config - Hot Reload mechanism and tooling to manage state
• Typed Read get typed values from config or bind a struct
• Metrics exposed prometheus metrics telling you how many times a config is reloaded, if it failed, and how long it takes to reload

Get started

go get github.com/lalamove/konfig

Load and watch a json formatted config file.

var configFiles = []klfile.File{
	{
		Path:   "./config.json",
		Parser: kpjson.Parser,
	},
}

func init() {
	konfig.Init(konfig.DefaultConfig())
}

func main() {
	// load from json file
	konfig.RegisterLoaderWatcher(
		klfile.New(&klfile.Config{
			Files: configFiles,
			Watch: true,
		}),
		// optionally you can pass config hooks to run when a file is changed
		func(c konfig.Store) error {
			return nil
		},
	)

	if err := konfig.LoadWatch(); err != nil {
		log.Fatal(err)
	}

	// retrieve value from config file
	konfig.Bool("debug")
}

Store

The Store is the base of the config package. It holds and gives access to values stored by keys.

Creating a Store

You can create a global Store by calling konfig.Init(*konfig.Config):

konfig.Init(konfig.DefaultConfig())

The global store is accessible directly from the package:

konfig.Get("foo") // calls store.Get("foo")

You can create a new store by calling konfig.New(*konfig.Config):

s := konfig.New(konfig.DefaultConfig())

Loading and Watching a Store

After registering Loaders and Watchers in the konfig.Store, you must load and watch the store.

You can do both by calling LoadWatch:

if err := konfig.LoadWatch(); err != nil {
	log.Fatal(err)
}

You can call Load only, it will load all loaders and return:

if err := konfig.Load(); err != nil {
	log.Fatal(err)
}

And finally you can call Watch only, it will start all watchers and return:

if err := konfig.Watch(); err != nil {
	log.Fatal(err)
}

Loaders

Loaders load config values into the store. A loader is an implementation of the loader interface.

type Loader interface {
	// Name return the name of the load, it is used to create labeled vectors metrics per loader
	Name() string
	// StopOnFailure indicates if a failure of the loader should trigger a stop
	StopOnFailure() bool
	// Loads the config and add it to the Store
	Load(Store) error
	// MaxRetry returns the maximum number of times to allow retrying on load failure
	MaxRetry() int
	// RetryDelay returns the delay to wait before retrying
	RetryDelay() time.Duration
}

You can register loaders in the config individually or with a watcher.

Register a loader by itself:

configLoader := konfig.RegisterLoader(
	klfile.New(
		&klfile.Config{
			Files: []klfile.File{
				{
					Parser: kpjson.Parser,
					Path:   "./konfig.json",
				},
			},
		},
	),
)

Register a loader with a watcher:

To register a loader and a watcher together, you must register a LoaderWatcher which is an interface that implements both the Loader and the Watcher interface.

configLoader := konfig.RegisterLoaderWatcher(
	klfile.New(
		&klfile.Config{
			Files: []klfile.File{
				{
					Parser: kpjson.Parser,
					Path:   "./konfig.json",
				},
			},
			Watch: true,
		},
	),
)

You can also compose a loader and a watcher to create a LoaderWatcher:

configLoader := konfig.RegisterLoaderWatcher(
	// it creates a LoaderWatcher from a loader and a watcher
	konfig.NewLoaderWatcher(
		someLoader,
		someWatcher,
	),
)

Built in loaders

Konfig already has the following loaders, they all have a built in watcher:

• File Loader

Loads configs from files which can be watched. Files can have different parsers to load different formats. It has a built in file watcher which triggers a config reload (running hooks) when files are modified.

• Vault Loader

Loads configs from vault secrets. It has a built in Poll Watcher which triggers a config reload (running hooks) before the secret and the token from the auth provider expires.

• HTTP Loader

Loads configs from HTTP sources. Sources can have different parsers to load different formats. It has a built in Poll Diff Watcher which triggers a config reload (running hooks) if data is different.

• Etcd Loader

Loads configs from Etcd keys. Keys can have different parser to load different formats. It has a built in Poll Diff Watcher which triggers a config reload (running hooks) if data is different.

• Consul loader

Loads configs from Consul KV. Keys can have different parser to load different formats. It has built in Poll Diff Watcher which triggers a config reload (running hooks) if data is different.

• ENV Loader

Loads configs from environment variables.

• Flag Loader

Loads configs from command line flags.

• io.Reader Loader

Loads configs from an io.Reader.

Parsers

Parsers parse an io.Reader into a konfig.Store. These are used by some loaders to parse the data they fetch into the config store. The File Loader, Etcd Loader and HTTP Loader use Parsers.

Config already has the following parsers:

• JSON Parser
• TOML Parser
• YAML Parser
• KV Parser
• Map Parser

Watchers

Watchers trigger a call on a Loader on events. A watcher is an implementation of the Watcher interface.

type Watcher interface {
	// Start starts the watcher, it must not be blocking.
	Start() error
	// Done indicate whether the watcher is done or not
	Done() <-chan struct{}
	// Watch should block until an event unlocks it
	Watch() <-chan struct{}
	// Close closes the watcher, it returns a non nil error if it is already closed
	// or something prevents it from closing properly.
	Close() error
	// Err returns the error attached to the watcher
	Err() error
}

Built in watchers

Konfig already has the following watchers:

• File Watcher

Watches files for changes.

• Poll Watcher

Sends events at a given rate, or if diff is enabled. It takes a Getter and fetches the data at a given rate. If data is different, it sends an event.

Hooks

Hooks are functions ran after a successful loader Load() call. They are used to reload the state of the application on a config change.

Registering a loader with some hooks

You can register a loader or a loader watcher with hooks.

configLoader := konfig.RegisterLoaderWatcher(
	klfile.New(
		&klfile.Config{
			Files: []klfile.File{
				{
					Parser: kpyaml.Parser,
					Path:   "./konfig.yaml",
				},
			},
			Watch: true,
		},
	),
	func(s konfig.Store) error {
		// Here you should reload the state of your app
		return nil
	},
)

Adding hooks to an existing loader

You can register a Loader or a LoaderWatcher with hooks.

configLoader.AddHooks(
	func(s konfig.Store) error {
		// Here you should reload the state of your app
		return nil
	},
	func(s konfig.Store) error {
		// Here you should reload the state of your app
		return nil
	},
)

Adding hooks on keys

Alternatively, you can add hooks on keys. Hooks on keys will match for prefix in order to run a hook when any key with a given prefix is updated. A hook can only be run once per load event, even if multiple keys match that hook.

konfig.RegisterKeyHook(
	"db.",
	func(s konfig.Store) error {
		return nil
	},
)

Closers

Closers can be added to konfig so that if konfig fails to load, it will execute Close() on the registered Closers.

type Closer interface {
	Close() error
}

Register a Closer

konfig.RegisterCloser(closer)

Config Groups

You can namespace your configs using config Groups.

konfig.Group("db").RegisterLoaderWatcher(
	klfile.New(
		&klfile.Config{
			Files: []klfile.File{
				{
					Parser: kpyaml.Parser,
					Path:   "./db.yaml",
				},
			},
			Watch: true,
		},
	),
)

// accessing grouped config
dbHost := konfig.Group("db").MustString("credentials.host")

Binding a Type to a Store

You can bind a type to the konfig store if you want your config values to be unmarshaled to a struct or a map[string]interface{}. Then you can access an instance of that type in a thread safe manner (in order to be safe for dynamic config updates).

Let's see with an example of a json config file:

{
    "addr": ":8080",
    "debug": true,
    "db": {
        "username": "foo"
    },
    "redis": {
        "host": "127.0.0.1"
    }
}

type DBConfig struct {
	Username string
}
type Config struct {
	Addr      string
	Debug     string
	DB        DBConfig `konfig:"db"`
	RedisHost string   `konfig:"redis.host"`
}

// we init the root konfig store
konfig.Init(konfig.DefaultConfig())

// we bind the Config struct to the konfig.Store
konfig.Bind(Config{})

// we register our config file
konfig.RegisterLoaderWatcher(
	klfile.New(
		&klfile.Config{
			Files: []klfile.File{
				{
					Parser: kpjson.Parser,
					Path:   "./config.json",
				},
			},
			Watch: true,
		},
	),
)

// we load our config and start watching
if err := konfig.LoadWatch(); err != nil {
	log.Fatal(err)
}

// Get our config value
c := konfig.Value().(Config)

fmt.Println(c.Addr) // :8080

Note that you can compose your config sources. For example, have your credentials come from Vault and be renewed often and have the rest of your config loaded from a file and be updated on file change.

It is important to understand how Konfig unmarshals your config values into your struct. When a Loader calls konfig.Set(), if the konfig store has a value bound to it, it will try to unmarshal the key to the bound value.

• First, it will look for field tags in the struct, if a tag matches exactly the key, it will unmarshal the key to the struct field.
• Then, it will do a EqualFold on the field name and the key, if they match, it will unmarshal the key to the struct field.
• Then, if the key has a dot, it will check if the tag or the field name (to lowercase) is a prefix of the key, if yes, it will check if the type of the field is a struct of pointer, if yes, it will check the struct using what's after the prefix as the key.

Read from config

Apart from reading from the bound config value, konfig provides several methods to read values.

Every method to retrieve config values come in 2 flavours:

• Get reads a value at the given key. If key is not present it returns the zero value of the type.
• MustGet reads a value at the given key. If key is not present it panics.

All methods to read values from a Store:

// Exists checks whether the key k is set in the store.
Exists(k string) bool

// Get gets the value with the key k from the store. If the key is not set, Get returns nil. To check whether a value is really set, use Exists.
Get(k string) interface{}
// MustGet tries to get the value with the key k from the store. If the key k does not exist in the store, MustGet panics.
MustGet(k string) interface{}

// MustString tries to get the value with the key k from the store and casts it to a string. If the key k does not exist in the store, MustString panics.
MustString(k string) string
// String tries to get the value with the key k from the store and casts it to a string. If the key k does not exist it returns the Zero value.
String(k string) string

// MustInt tries to get the value with the key k from the store and casts it to a int. If the key k does not exist in the store, MustInt panics.
MustInt(k string) int
// Int tries to get the value with the key k from the store and casts it to a int. If the key k does not exist it returns the Zero value.
Int(k string) int

// MustFloat tries to get the value with the key k from the store and casts it to a float. If the key k does not exist in the store, MustFloat panics.
MustFloat(k string) float64
// Float tries to get the value with the key k from the store and casts it to a float. If the key k does not exist it returns the Zero value.
Float(k string) float64

// MustBool tries to get the value with the key k from the store and casts it to a bool. If the key k does not exist in the store, MustBool panics.
MustBool(k string) bool
// Bool tries to get the value with the key k from the store and casts it to a bool. If the key k does not exist it returns the Zero value.
Bool(k string) bool

// MustDuration tries to get the value with the key k from the store and casts it to a time.Duration. If the key k does not exist in the store, MustDuration panics.
MustDuration(k string) time.Duration
// Duration tries to get the value with the key k from the store and casts it to a time.Duration. If the key k does not exist it returns the Zero value.
Duration(k string) time.Duration

// MustTime tries to get the value with the key k from the store and casts it to a time.Time. If the key k does not exist in the store, MustTime panics.
MustTime(k string) time.Time
// Time tries to get the value with the key k from the store and casts it to a time.Time. If the key k does not exist it returns the Zero value.
Time(k string) time.Time

// MustStringSlice tries to get the value with the key k from the store and casts it to a []string. If the key k does not exist in the store, MustStringSlice panics.
MustStringSlice(k string) []string
// StringSlice tries to get the value with the key k from the store and casts it to a []string. If the key k does not exist it returns the Zero value.
StringSlice(k string) []string

// MustIntSlice tries to get the value with the key k from the store and casts it to a []int. If the key k does not exist in the store, MustIntSlice panics.
MustIntSlice(k string) []int
// IntSlice tries to get the value with the key k from the store and casts it to a []int. If the key k does not exist it returns the Zero value.
IntSlice(k string) []int

// MustStringMap tries to get the value with the key k from the store and casts it to a map[string]interface{}. If the key k does not exist in the store, MustStringMap panics.
MustStringMap(k string) map[string]interface{}
// StringMap tries to get the value with the key k from the store and casts it to a map[string]interface{}. If the key k does not exist it returns the Zero value.
StringMap(k string) map[string]interface{}

// MustStringMapString tries to get the value with the key k from the store and casts it to a map[string]string. If the key k does not exist in the store, MustStringMapString panics.
MustStringMapString(k string) map[string]string
// StringMapString tries to get the value with the key k from the store and casts it to a map[string]string. If the key k does not exist it returns the Zero value.
StringMapString(k string) map[string]string

Strict Keys

You can define required keys on the konfig.Store by calling the Strict method. When calling strict method, konfig will set required keys on the store and during the first Load call on the store it will check if the keys are present, if not, Load will return a non nil error. Then, after every Load on a loader, konfig will check again if the keys are still present, if not, the loader Load will be considered a failure.

Usage:

// We init the root konfig store
konfig.Init(konfig.DefaultConfig()).Strict("debug", "username")

// Register our loaders
...

// We load our config and start watching.
// If strict keys are not found after the load operation, LoadWatch will return a non nil error.
if err := konfig.LoadWatch(); err != nil {
	log.Fatal(err)
}

Alternatively, BindStructStrict can be used to strictly bind config. Usage:

type DBConfig struct {
	Username string
}
type Config struct {
	Addr      string    `konfig:"-"` // this key will be non-strict
	DB        DBConfig  `konfig:"db"`
	RedisHost string    `konfig:"redis.host"`
}

// we init the root konfig store
konfig.Init(konfig.DefaultConfig())

// we bind the Config struct to the konfig.Store
konfig.BindStructStrict(Config{})

// Register our loaders
...

// We load our config and start watching.
// If any strict key is not found after the load operation, LoadWatch will return a non nil error.
if err := konfig.LoadWatch(); err != nil {
	log.Fatal(err)
}

Getter

To easily build services which can use dynamically loaded configs you can create getters for specific keys. A getter implements ngetter.GetterTyped from nui package. It is useful when building apps in larger distributed environments.

Example with a config value set for the debug key:

debug := konfig.Getter("debug")

debug.Bool() // true

Metrics

Konfig comes with prometheus metrics.

Two metrics are exposed:

• Config reloads counter vector with labels
• Config reload duration summary vector with labels

Example of metrics:

# HELP konfig_loader_reload Number of config loader reload
# TYPE konfig_loader_reload counter
konfig_loader_reload{loader="config-files",result="failure",store="root"} 0.0
konfig_loader_reload{loader="config-files",result="success",store="root"} 1.0

# HELP konfig_loader_reload_duration Histogram for the config reload duration
# TYPE konfig_loader_reload_duration summary
konfig_loader_reload_duration{loader="config-files",store="root",quantile="0.5"} 0.001227641
konfig_loader_reload_duration{loader="config-files",store="root",quantile="0.9"} 0.001227641
konfig_loader_reload_duration{loader="config-files",store="root",quantile="0.99"} 0.001227641
konfig_loader_reload_duration_sum{loader="config-files",store=""} 0.001227641
konfig_loader_reload_duration_count{loader="config-files",store=""} 1.0

To enable metrics, you must pass a custom config when creating a config store:

konfig.Init(&konfig.Config{
	Metrics: true,
	Name: "root",
})

Benchmark

Benchmarks are run on viper, go-config and konfig. Benchmark are done on reading ops and show that Konfig is 0 allocs on read and at leat 3x faster than Viper:

cd benchmarks && go test -bench . && cd ../
goos: linux
goarch: amd64
pkg: github.com/lalamove/konfig/benchmarks
BenchmarkGetKonfig-4            200000000                7.75 ns/op            0 B/op          0 allocs/op
BenchmarkStringKonfig-4         30000000                49.9 ns/op             0 B/op          0 allocs/op
BenchmarkGetViper-4             20000000               101 ns/op              32 B/op          2 allocs/op
BenchmarkStringViper-4          10000000               152 ns/op              32 B/op          2 allocs/op
BenchmarkGetGoConfig-4          10000000               118 ns/op              40 B/op          3 allocs/op
BenchmarkStringGoConfig-4       10000000               125 ns/op              40 B/op          3 allocs/op
PASS

Contributing

Contributions are welcome. To make contributions, fork the repository, create a branch and submit a Pull Request to the master branch.

konfig's Issues

Feature Request: Support Vault KV Version 2

Are there plans to support Vault's versioned KV store (Version 2) happy to do a proof of concept PR if this is something that konfig would like support in the future.

Issue with github.com/lucas-clemente/[email protected] (indirect via github.com/micro/[email protected])

The v0.12.0 tag for github.com/lucas-clemente/quic-go is broken (invalid signature, as discussed in quic-go/quic-go#2163).

Since github.com/lucas-clemente/quic-go is imported via go-micro (now Nitro), this is the package to bump.

A quick check shows that bumping go-micro to v1.12.0 should solve the issue.

Change the order of priority

Hello,

Currently having three sources to retrieve values (file, env, flag), I am looking for a way to get the following order of priority (from the lowest to the highest):
default (flag) - file - env - flag

Indeed, since the default values are defined in the FlagSet, they are in priority on file and env sources (so currently the priority order is as follows: file - env - default (flag) - flag)

Is it possible to achieve the expected result?

Thank you.

Support for unmarshalling of loaded config

In extensible/modular application, it's not possible to create statically defined struct with configuration. And, modular configuration would have for example following configuration:

# syntax: <module_key>.<options...>

module1.option1 = "..."
module1.option2.a = "..."
module1.option2.b = "..."
module1.option3.m = "..."
module1.option3.n = "..."
module1.option3.o = "..."

module2.option1.x = "..."
module2.option1.y = "..."
module2.option1.z = "..."
module2.option2 = "..."

# ...

So:

type Module interface {
    LoadConfig(s konfig.Store)
}

type ModuleX struct {}

func (m *ModuleX) LoadConfig(s konfig.Store) {
    var options ModuleXOptions
    s.Unmarshal(&options)
}

func main() {
    var modules map[string]Module = ...
    var config konfig.Store =  ...

    for key, module := range modules {
        // actually, don't know if this works in konfig,... 
        // get all options with prefix `<key>.`
        module.LoadConfig(config.Group(key))
    } 
}

Somewhat similar functionality is provided by https://github.com/knadh/koanf#unmarshalling. But, it would need a bit different Module interface:

type Module interface {
    LoadConfig(path string, k *koanf.Koanf)
}

type ModuleX struct {}

func (m *ModuleX) LoadConfig(path string, k *koanf.Koanf) {
    var options ModuleXOptions
    k.Unmarshal(path, &options)
}

func main() {
    ....
}

Are there any plans for such/similar support?

Loading string slices from env

I'm trying to load string slice from environment variable (format: item1,item2,item3) and bind it to struct field of type []string.

Using code like this:

store := konfig.New(&konfig.Config{
		Name:     "my-app-config",
		ExitCode: 1,
	})
store.RegisterLoader(klenv.New(&klenv.Config{
		Name:   "env-loader",
		Regexp: "^APP_(.*)",
		Replacer: nstrings.ReplacerFunc(func(s string) string {
			return strings.ToLower(strings.TrimPrefix(s, "APP_"))
		}),
	}))

type Config struct {
    StringSliceField []string `konfig:"string_slice_field"`
}
store.Load()
cfg := store.Value().(Config)
store.Bind(Config{})

and environment variable APP_STRING_SLICE_FIELD=item1,item2,item3 getting
[]string{"item1,item2,item3"} in cfg.StringSliceField insteand of expected []string{"item1","item2","item3"}.
But with klfile loader with kpjson parser I`m getting correct result.

I try to make pull request with suggested improvement to klenv package.

General functionality of konfig

When I ran into konfig, I really liked your approach and extensibility. I started writing an extension for support of consul, fixed couple of bugs and was almost ready to submit a pull request, however I ran into more philosophical issue and wanted to check with you to see what's your thinking.

You build konfig to behave like first class citizen within other people code. That means you terminate code if significant issues are found with the integrity of your package, which is fine. However you also completely stop processing if you run into any kind of integrity errors within a config itself.

This in general is fine for local configs and debugging, but poses major issues for all remote configs. You see, people are sloppy. Just a simple typo in a yaml config will take down the whole parser which in turns stops konfig from functioning.

Yes, you added some helpers like NoExitOnError but this will only let the program to continue working without letting it to retry to pull a remote config while wrecking a havoc in the logs.

My general strategy why using remote configs is:

• add local config that is used up until new remote config is not provided
• overwrite the default config with remote config if found

By stoping the whole process means that no new configs will be pulled (without informing the user). Imagine running 10,000 servers in k8 and because of a typo, none of them can pull a config from etcd. Your only solution is to restart/rebuild the whole cluster.

My suggestion would be to rebuild the logic in a way that it will not exhibit this behavior. I'm happy to chip in. Otherwise I can submit what I have, but I think that will be for me.

Go modules dependency fetching doesn't work

1:

>> go version
go version go1.14.1 linux/amd64

2:

>> git clone https://github.com/lalamove/konfig && cd konfig

3:

>> go mod download
go: contrib.go.opencensus.io/exporter/[email protected] requires
	github.com/census-instrumentation/[email protected]: invalid pseudo-version: version before v0.1.0 would have negative patch number

Value hooks feature

Add hooks on values so that when specific values are changed, no matter what's the loader, some hooks can be ran.

Smaller integers cannot be marshalled into

Here's an example

type Config struct {
	Port    uint16
}

config.yml:

port: 8080

FILEWATCHER | 2022/06/11 19:46:20 adding file to watch: ./config.yml
panic: reflect.Set: value of type int is not assignable to type uint16

goroutine 1 [running]:
reflect.Value.assignTo({0x111ce80?, 0x13a84e0?, 0xc0001a4300?}, {0x1177edc, 0xb}, 0x111d7c0, 0x0)
        C:/Program Files/Go/src/reflect/value.go:3062 +0x2ac
reflect.Value.Set({0x111d7c0?, 0xc0001a4300?, 0x111ce80?}, {0x111ce80?, 0x13a84e0?, 0x111d7c0?})
        C:/Program Files/Go/src/reflect/value.go:2088 +0xeb
github.com/lalamove/konfig.(*value).setStruct(0xc00019a3f0, {0xc0001a2600, 0x4}, {0x111ce80, 0x13a84e0}, {0x1113e20?, 0xc0001a4300?, 0x30000?})
        C:/Users/steve/go/pkg/mod/github.com/lalamove/[email protected]/value.go:221 +0x34f
github.com/lalamove/konfig.(*value).setValues(0xc00019a3f0, 0xc00019a8a0?)
        C:/Users/steve/go/pkg/mod/github.com/lalamove/[email protected]/value.go:196 +0x22f
github.com/lalamove/konfig.Values.load(0xc00019a510, 0x111b440?, 0xc00019c0b0)
        C:/Users/steve/go/pkg/mod/github.com/lalamove/[email protected]/values.go:61 +0x738
github.com/lalamove/konfig.(*S).loaderLoadRetry(0xc00019c0b0, 0xc000180060, 0x0)
        C:/Users/steve/go/pkg/mod/github.com/lalamove/[email protected]/loader.go:149 +0x276
github.com/lalamove/konfig.(*S).Load(0xc00019c0b0)
        C:/Users/steve/go/pkg/mod/github.com/lalamove/[email protected]/loader.go:69 +0x74
github.com/lalamove/konfig.(*S).LoadWatch(0x1?)
        C:/Users/steve/go/pkg/mod/github.com/lalamove/[email protected]/loader.go:49 +0x1e
github.com/lalamove/konfig.LoadWatch()
        C:/Users/steve/go/pkg/mod/github.com/lalamove/[email protected]/loader.go:44 +0x1e

Wrong version of nlogger in go.mod

Using version v0.4.0 of konfig in a go module enabled project fails with undefined: nlogger.Provider

Checking version 0.4.0's go.mod I can see that version v.0.0.2 of nlogger is stated there but the Provider type was not introduced till v0.0.4.

I manually fixed it by specifying v0.0.4 in my go.mod file but Konfig's go.mod will also need to be updated, I will trying submitting a PR for that.

Thanks for the awesome package.

Strict keys feature

Add the possibility to specifify config keys which should be mandatory. Once a method such as store.Strict(...string) is called, the store will look if the given keys are present, if not it will panic. Then for every following Load, the store will check if the keys are still present, if no, will consider the load to be a failure.

Inverse NoStopOnFailure and move to Loader level, rather than global only

Currently, we force configurations to specify when they think failure is acceptable. However - most configuration sources most likely think that failure is acceptable for certain content of the configuration.
#19 has a good example where if you put the wrong YAML into a dynamically loaded configuration, it will exit the application after hitting MaxRetry unless you specify NoStopOnFailure. This may be unintuitive and cause major outages unless people read the documentation closely.

We have the requirement however to exit on a specific loader (that is if it fails to load credentials from Vault, the application may as well exit and let Kubernetes handle retries/backoff etc), so we need to support specifying StopOnFailure for each individual Loader.

Support for multiline env vars

If you update the test in kpkeyval.TestKVParser to use a multi line env var we error out on multiline env vars with InvalidConfigFileFormat.

However this does not support multiline vars, e.g.

e.g.

SSH_KEY=-----BEGIN OPENSSH PRIVATE KEY-----
asdfsadfsdfasdf
asdfasdfasdfasdf
-----END OPENSSH PRIVATE KEY-----

		{
			name:   `no error, default separator`,
			err:    false,
			reader: strings.NewReader("BAR=FOO\nFOO=BAR\nBAZ=FOO\nBAR"),
			asserts: func(t *testing.T, cfg konfig.Values) {
				require.Equal(t, "BAR", cfg["FOO"])
				require.Equal(t, "FOO", cfg["BAR"])
				require.Equal(t, "FOO\nBAR", cfg["BAZ"])
			},
		},

Feature Request: Allow marking all struct field as strict

Hi, thanks for the wonderful library.

Currently I bind my Store to a configuration struct and I have a yaml file that should always have defaults for all flags which can then be overriden with env parameters.

I think if we could add something like a BindStrict that takes in a struct and adds all the exposed fields as strict keys would allow not needing to keep a separate list of canonicalised keys to be passed manually to the StrictKey function.

NopWatcher doesn't implement Err method of Watcher interface

version: 0.4.0

Registering any loader that does not watch i.e internal uses the NopWatcher e.g FlagWatcher causes a panic.
runtime error: invalid memory address or nil pointer dereference

Traced the panic to loader.go

		        case <-wl.Done():
			if err := wl.Err(); err != nil {
				c.cfg.Logger.Get().Error(err.Error())
			}
			// the watcher is closed
			return

After the watchLoader which would have a NopWatcher as it's watcher returns a closed channel in wl.Done() the block tries to get any errors in the wl which causes a panic.