github.com/flei2000/viper@v0.0.0-20160111150723-a212099cbe6f/README.md

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Go configuration with fangs!

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## What is Viper?

Viper is a complete configuration solution for go applications including 12 factor apps. It is designed
to work within an application, and can handle all types of configuration needs
and formats. It supports:

* setting defaults
* reading from JSON, TOML, YAML and HCL config files
* live watching and re-reading of config files (optional)
* reading from environment variables
* reading from remote config systems (etcd or Consul), and watching changes
* reading from command line flags
* reading from buffer
* setting explicit values

Viper can be thought of as a registry for all of your applications
configuration needs.

## Why Viper?

When building a modern application, you don’t want to worry about
configuration file formats; you want to focus on building awesome software.
Viper is here to help with that.

Viper does the following for you:

1. Find, load, and unmarshal a configuration file in JSON, TOML, YAML or HCL.
2. Provide a mechanism to set default values for your different
   configuration options.
3. Provide a mechanism to set override values for options specified through
   command line flags.
4. Provide an alias system to easily rename parameters without breaking existing
   code.
5. Make it easy to tell the difference between when a user has provided a
   command line or config file which is the same as the default.

Viper uses the following precedence order. Each item takes precedence over the
item below it:

 * explicit call to Set
 * flag
 * env
 * config
 * key/value store
 * default

Viper configuration keys are case insensitive.

## Putting Values into Viper

### Establishing Defaults

A good configuration system will support default values. A default value is not
required for a key, but it's useful in the event that a key hasn’t been set via
config file, environment variable, remote configuration or flag.

Examples:

```go
viper.SetDefault("ContentDir", "content")
viper.SetDefault("LayoutDir", "layouts")
viper.SetDefault("Taxonomies", map[string]string{"tag": "tags", "category": "categories"})
```

### Reading Config Files

Viper requires minimal configuration so it knows where to look for config files.
Viper supports JSON, TOML, YAML and HCL files. Viper can search multiple paths, but
currently a single Viper instance only supports a single configuration file.
Viper does not default to any configuration search paths leaving defaults decision
to an application.

Here is an example of how to use Viper to search for and read a configuration file.
None of the specific paths are required, but at least one path should be provided
where a configuration file is expected.

```go
viper.SetConfigName("config") // name of config file (without extension)
viper.AddConfigPath("/etc/appname/")   // path to look for the config file in
viper.AddConfigPath("$HOME/.appname")  // call multiple times to add many search paths
viper.AddConfigPath(".")               // optionally look for config in the working directory
err := viper.ReadInConfig() // Find and read the config file
if err != nil { // Handle errors reading the config file
	panic(fmt.Errorf("Fatal error config file: %s \n", err))
}
```

### Watching and re-reading config files

Viper supports the ability to have your application live read a config file while running.

Gone are the days of needing to restart a server to have a config take effect,
viper powered applications can read an update to a config file while running and
not miss a beat.

Simply tell the viper instance to watchConfig. 
Optionally you can provide a function for Viper to run each time a change occurs.

**Make sure you add all of the configPaths prior to calling `WatchConfig()`**

```go
		viper.WatchConfig()
		viper.OnConfigChange(func(e fsnotify.Event) {
			fmt.Println("Config file changed:", e.Name)
		})
```

### Reading Config from io.Reader

Viper predefines many configuration sources such as files, environment
variables, flags, and remote K/V store, but you are not bound to them. You can
also implement your own required configuration source and feed it to viper.

```go
viper.SetConfigType("yaml") // or viper.SetConfigType("YAML")

// any approach to require this configuration into your program.
var yamlExample = []byte(`
Hacker: true
name: steve
hobbies:
- skateboarding
- snowboarding
- go
clothing:
  jacket: leather
  trousers: denim
age: 35
eyes : brown
beard: true
`)

viper.ReadConfig(bytes.NewBuffer(yamlExample))

viper.Get("name") // this would be "steve"
```

### Setting Overrides

These could be from a command line flag, or from your own application logic.

```go
viper.Set("Verbose", true)
viper.Set("LogFile", LogFile)
```

### Registering and Using Aliases

Aliases permit a single value to be referenced by multiple keys

```go
viper.RegisterAlias("loud", "Verbose")

viper.Set("verbose", true) // same result as next line
viper.Set("loud", true)   // same result as prior line

viper.GetBool("loud") // true
viper.GetBool("verbose") // true
```

### Working with Environment Variables

Viper has full support for environment variables. This enables 12 factor
applications out of the box. There are four methods that exist to aid working
with ENV:

 * `AutomaticEnv()`
 * `BindEnv(string...) : error`
 * `SetEnvPrefix(string)`
 * `SetEnvReplacer(string...) *strings.Replacer`

_When working with ENV variables, it’s important to recognize that Viper
treats ENV variables as case sensitive._

Viper provides a mechanism to try to ensure that ENV variables are unique. By
using `SetEnvPrefix`, you can tell Viper to use add a prefix while reading from
the environment variables. Both `BindEnv` and `AutomaticEnv` will use this
prefix.

`BindEnv` takes one or two parameters. The first parameter is the key name, the
second is the name of the environment variable. The name of the environment
variable is case sensitive. If the ENV variable name is not provided, then
Viper will automatically assume that the key name matches the ENV variable name,
but the ENV variable is IN ALL CAPS. When you explicitly provide the ENV
variable name, it **does not** automatically add the prefix.

One important thing to recognize when working with ENV variables is that the
value will be read each time it is accessed. Viper does not fix the value when
the `BindEnv` is called.

`AutomaticEnv` is a powerful helper especially when combined with
`SetEnvPrefix`. When called, Viper will check for an environment variable any
time a `viper.Get` request is made. It will apply the following rules. It will
check for a environment variable with a name matching the key uppercased and
prefixed with the `EnvPrefix` if set.

`SetEnvReplacer` allows you to use a `strings.Replacer` object to rewrite Env
keys to an extent. This is useful if you want to use `-` or something in your
`Get()` calls, but want your environmental variables to use `_` delimiters. An
example of using it can be found in `viper_test.go`.

#### Env example

```go
SetEnvPrefix("spf") // will be uppercased automatically
BindEnv("id")

os.Setenv("SPF_ID", "13") // typically done outside of the app

id := Get("id") // 13
```

### Working with Flags

Viper has the ability to bind to flags. Specifically, Viper supports `Pflags`
as used in the [Cobra](https://github.com/spf13/cobra) library.

Like `BindEnv`, the value is not set when the binding method is called, but when
it is accessed. This means you can bind as early as you want, even in an
`init()` function.

The `BindPFlag()` method provides this functionality.

Example:

```go
serverCmd.Flags().Int("port", 1138, "Port to run Application server on")
viper.BindPFlag("port", serverCmd.Flags().Lookup("port"))
```

The use of [pflag](https://github.com/spf13/pflag/) in Viper does not preclude
the use of other packages that use the [flag](https://golang.org/pkg/flag/)
package from the standard library. The pflag package can handle the flags
defined for the flag package by importing these flags. This is accomplished
by a calling a convenience function provided by the pflag package called
AddGoFlagSet().

Example:

```go
package main

import (
	"flag"
	"github.com/spf13/pflag"
)

func main() {
	pflag.CommandLine.AddGoFlagSet(flag.CommandLine)
	pflag.Parse()
    ...
}
```

#### Flag interfaces

Viper provides two Go interfaces to bind other flag systems if you don't use `Pflags`.

`FlagValue` represents a single flag. This is a very simple example on how to implement this interface:

```go
type myFlag struct {}
func (f myFlag) IsChanged() { return false }
func (f myFlag) Name() { return "my-flag-name" }
func (f myFlag) ValueString() { return "my-flag-value" }
func (f myFlag) ValueType() { return "string" }
```

Once your flag implements this interface, you can simply tell Viper to bind it:

```go
viper.BindFlagValue("my-flag-name", myFlag{})
```

`FlagValueSet` represents a group of flags. This is a very simple example on how to implement this interface:

```go
type myFlagSet struct {
	flags []myFlag
}

func (f myFlagSet) VisitAll(fn func(FlagValue)) {
	for _, flag := range flags {
		fn(flag)	
	}
}
```

Once your flag set implements this interface, you can simply tell Viper to bind it:

```go
fSet := myFlagSet{
	flags: []myFlag{myFlag{}, myFlag{}},
}
viper.BindFlagValues("my-flags", fSet)
```

### Remote Key/Value Store Support

To enable remote support in Viper, do a blank import of the `viper/remote`
package:

`import _ "github.com/spf13/viper/remote"`

Viper will read a config string (as JSON, TOML, YAML or HCL) retrieved from a path
in a Key/Value store such as etcd or Consul.  These values take precedence over
default values, but are overridden by configuration values retrieved from disk,
flags, or environment variables.

Viper uses [crypt](https://github.com/xordataexchange/crypt) to retrieve
configuration from the K/V store, which means that you can store your
configuration values encrypted and have them automatically decrypted if you have
the correct gpg keyring.  Encryption is optional.

You can use remote configuration in conjunction with local configuration, or
independently of it.

`crypt` has a command-line helper that you can use to put configurations in your
K/V store. `crypt` defaults to etcd on http://127.0.0.1:4001.

```bash
$ go get github.com/xordataexchange/crypt/bin/crypt
$ crypt set -plaintext /config/hugo.json /Users/hugo/settings/config.json
```

Confirm that your value was set:

```bash
$ crypt get -plaintext /config/hugo.json
```

See the `crypt` documentation for examples of how to set encrypted values, or
how to use Consul.

### Remote Key/Value Store Example - Unencrypted

```go
viper.AddRemoteProvider("etcd", "http://127.0.0.1:4001","/config/hugo.json")
viper.SetConfigType("json") // because there is no file extension in a stream of bytes
err := viper.ReadRemoteConfig()
```

### Remote Key/Value Store Example - Encrypted

```go
viper.AddSecureRemoteProvider("etcd","http://127.0.0.1:4001","/config/hugo.json","/etc/secrets/mykeyring.gpg")
viper.SetConfigType("json") // because there is no file extension in a stream of bytes
err := viper.ReadRemoteConfig()
```

### Watching Changes in etcd - Unencrypted

```go
// alternatively, you can create a new viper instance.
var runtime_viper = viper.New()

runtime_viper.AddRemoteProvider("etcd", "http://127.0.0.1:4001", "/config/hugo.yml")
runtime_viper.SetConfigType("yaml") // because there is no file extension in a stream of bytes

// read from remote config the first time.
err := runtime_viper.ReadRemoteConfig()

// unmarshal config
runtime_viper.Unmarshal(&runtime_conf)

// open a goroutine to watch remote changes forever
go func(){
	for {
	    time.Sleep(time.Second * 5) // delay after each request

	    // currently, only tested with etcd support
	    err := runtime_viper.WatchRemoteConfig()
	    if err != nil {
	        log.Errorf("unable to read remote config: %v", err)
	        continue
	    }

	    // unmarshal new config into our runtime config struct. you can also use channel
	    // to implement a signal to notify the system of the changes
	    runtime_viper.Unmarshal(&runtime_conf)
	}
}()
```

## Getting Values From Viper

In Viper, there are a few ways to get a value depending on the value's type.
The following functions and methods exist:

 * `Get(key string) : interface{}`
 * `GetBool(key string) : bool`
 * `GetFloat64(key string) : float64`
 * `GetInt(key string) : int`
 * `GetString(key string) : string`
 * `GetStringMap(key string) : map[string]interface{}`
 * `GetStringMapString(key string) : map[string]string`
 * `GetStringSlice(key string) : []string`
 * `GetTime(key string) : time.Time`
 * `GetDuration(key string) : time.Duration`
 * `IsSet(key string) : bool`

One important thing to recognize is that each Get function will return a zero
value if it’s not found. To check if a given key exists, the `IsSet()` method
has been provided.

Example:
```go
viper.GetString("logfile") // case-insensitive Setting & Getting
if viper.GetBool("verbose") {
    fmt.Println("verbose enabled")
}
```
### Accessing nested keys

The accessor methods also accept formatted paths to deeply nested keys. For
example, if the following JSON file is loaded:

```json
{
    "host": {
        "address": "localhost",
        "port": 5799
    },
    "datastore": {
        "metric": {
            "host": "127.0.0.1",
            "port": 3099
        },
        "warehouse": {
            "host": "198.0.0.1",
            "port": 2112
        }
    }
}

```

Viper can access a nested field by passing a `.` delimited path of keys:

```go
GetString("datastore.metric.host") // (returns "127.0.0.1")
```

This obeys the precedence rules established above; the search for the root key
(in this example, `datastore`) will cascade through the remaining configuration
registries until found. The search for the sub-keys (`metric` and `host`),
however, will not.

For example, if the `metric` key was not defined in the configuration loaded
from file, but was defined in the defaults, Viper would return the zero value.

On the other hand, if the primary key was not defined, Viper would go through
the remaining registries looking for it.

Lastly, if there exists a key that matches the delimited key path, its value
will be returned instead. E.g.

```json
{
    "datastore.metric.host": "0.0.0.0",
    "host": {
        "address": "localhost",
        "port": 5799
    },
    "datastore": {
        "metric": {
            "host": "127.0.0.1",
            "port": 3099
        },
        "warehouse": {
            "host": "198.0.0.1",
            "port": 2112
        }
    }
}

GetString("datastore.metric.host") //returns "0.0.0.0"
```

### Extract sub-tree

Extract sub-tree from Viper.

For example, `viper` represents:

```json
app:
  cache1:
    max-items: 100
    item-size: 64
  cache2:
    max-items: 200
    item-size: 80
```

After executing:

```go
subv := viper.Sub("app.cache1")
```

`subv` represents:

```json
max-items: 100
item-size: 64
```

Suppose we have:

```go
func NewCache(cfg *Viper) *Cache {...}
```

which creates a cache based on config information formatted as `subv`.
Now it's easy to create these 2 caches separately as:

```go
cfg1 := viper.Sub("app.cache1")
cache1 := NewCache(cfg1)

cfg2 := viper.Sub("app.cache2")
cache2 := NewCache(cfg2)
```

### Unmarshaling

You also have the option of Unmarshaling all or a specific value to a struct, map,
etc.

There are two methods to do this:

 * `Unmarshal(rawVal interface{}) : error`
 * `UnmarshalKey(key string, rawVal interface{}) : error`

Example:

```go
type config struct {
	Port int
	Name string
	PathMap string `mapstructure:"path_map"`
}

var C config

err := Unmarshal(&C)
if err != nil {
	t.Fatalf("unable to decode into struct, %v", err)
}
```

## Viper or Vipers?

Viper comes ready to use out of the box. There is no configuration or
initialization needed to begin using Viper. Since most applications will want
to use a single central repository for their configuration, the viper package
provides this. It is similar to a singleton.

In all of the examples above, they demonstrate using viper in it's singleton
style approach.

### Working with multiple vipers

You can also create many different vipers for use in your application. Each will
have it’s own unique set of configurations and values. Each can read from a
different config file, key value store, etc. All of the functions that viper
package supports are mirrored as methods on a viper.

Example:

```go
x := viper.New()
y := viper.New()

x.SetDefault("ContentDir", "content")
y.SetDefault("ContentDir", "foobar")

//...
```

When working with multiple vipers, it is up to the user to keep track of the
different vipers.

## Q & A

Q: Why not INI files?

A: Ini files are pretty awful. There’s no standard format, and they are hard to
validate. Viper is designed to work with JSON, TOML or YAML files. If someone
really wants to add this feature, I’d be happy to merge it. It’s easy to specify
which formats your application will permit.

Q: Why is it called “Viper”?

A: Viper is designed to be a [companion](http://en.wikipedia.org/wiki/Viper_(G.I._Joe))
to [Cobra](https://github.com/spf13/cobra). While both can operate completely
independently, together they make a powerful pair to handle much of your
application foundation needs.

Q: Why is it called “Cobra”?

A: Is there a better name for a [commander](http://en.wikipedia.org/wiki/Cobra_Commander)?