spf13--viper/README.md

viper

Go configuration with fangs

What is Viper?

Viper is a complete configuration solution for go applications. It has been designed to work within an application to handle all types of configuration. It supports

• setting defaults
• reading from json, toml and yaml config files
• reading from environment variables
• reading from remote config systems (Etcd or Consul)
• reading from command line flags
• setting explicit values

It 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 have 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 marshal a configuration file in JSON, TOML or YAML.
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 can establish a default to be used in the event that the key hasn’t be set via config file, environment variable, remote configuration or flag.

Examples:

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

Reading Config Files

If you want to support a config file, Viper requires a minimal configuration so it knows where to look for the config file. Viper supports json, toml and yaml files. Viper can search multiple paths, but currently a single viper only supports a single config file.

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
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))
}

Setting Overrides

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

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

Registering and Using Aliases

Aliases permit a single value to be referenced by multiple keys

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 with 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. It 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

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 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:

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

Remote Key/Value Store Support

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

Viper uses 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.

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:

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

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

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()

Getting Values From Viper

In Viper, there are a few ways to get a value depending on what type of value you want to retrieved. 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 its zero value if it’s not found. To check if a given key exists, the IsSet() method has been provided.

Example:

viper.GetString("logfile") // case-insensitive Setting & Getting
if viper.GetBool("verbose") {
    fmt.Println("verbose enabled")
}

Marshaling

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

There are two methods to do this:

• Marshal(rawVal interface{}) : error
• MarshalKey(key string, rawVal interface{}) : error

Example:

type config struct {
	Port int
	Name string
}

var C config

err := Marshal(&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 its 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:

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 to 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?