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spf13 /viper

Go configuration with fangs

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Go configuration with fangs!

Many Go projects are built using Viper including:


go get

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, HCL, envfile and Java properties 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, HCL, INI, envfile or Java properties formats.
  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
  • flag
  • env
  • config
  • key/value store
  • default

Important: Viper configuration keys are case insensitive. There are ongoing discussions about making that optional.

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.


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, HCL, INI, envfile and Java Properties 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.

viper.SetConfigName("config") // name of config file (without extension) viper.SetConfigType("yaml") // REQUIRED if the config file does not have the extension in the name 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)) }

You can handle the specific case where no config file is found like this:

if err := viper.ReadInConfig(); err != nil { if \_, ok := err.(viper.ConfigFileNotFoundError); ok { // Config file not found; ignore error if desired } else { // Config file was found but another error was produced } } // Config file found and successfully parsed

NOTE [since 1.6]: You can also have a file without an extension and specify the format programmaticaly. For those configuration files that lie in the home of the user without any extension like


Writing Config Files

Reading from config files is useful, but at times you want to store all modifications made at run time. For that, a bunch of commands are available, each with its own purpose:

  • WriteConfig - writes the current viper configuration to the predefined path, if exists. Errors if no predefined path. Will overwrite the current config file, if it exists.
  • SafeWriteConfig - writes the current viper configuration to the predefined path. Errors if no predefined path. Will not overwrite the current config file, if it exists.
  • WriteConfigAs - writes the current viper configuration to the given filepath. Will overwrite the given file, if it exists.
  • SafeWriteConfigAs - writes the current viper configuration to the given filepath. Will not overwrite the given file, if it exists.

As a rule of the thumb, everything marked with safe won't overwrite any file, but just create if not existent, whilst the default behavior is to create or truncate.

A small examples section:

viper.WriteConfig() // writes current config to predefined path set by 'viper.AddConfigPath()' and 'viper.SetConfigName' viper.SafeWriteConfig() viper.WriteConfigAs("/path/to/my/.config") viper.SafeWriteConfigAs("/path/to/my/.config") // will error since it has already been written viper.SafeWriteConfigAs("/path/to/my/.other\_config")

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


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.

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.

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 five methods that exist to aid working with ENV:




BindEnv(string...) : error




SetEnvKeyReplacer(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


, you can tell Viper to use a prefix while reading from the environment variables. Both 




 will use this prefix.


 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 ENV variable matches the following format: prefix + "\_" + the key name in ALL CAPS. When you explicitly provide the ENV variable name (the second parameter), it **does not** automatically add the prefix. For example if the second parameter is "id", Viper will look for the ENV variable "ID".

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


 is called.


 is a powerful helper especially when combined with


. When called, Viper will check for an environment variable any time a 


 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 


 if set.


 allows you to use a 


 object to rewrite Env keys to an extent. This is useful if you want to use 


 or something in your


 calls, but want your environmental variables to use 


 delimiters. An example of using it can be found in 



Alternatively, you can use




 factory function. Unlike 


, it accepts a 


 interface allowing you to write custom string replacing logic.

By default empty environment variables are considered unset and will fall back to the next configuration source. To treat empty environment variables as set, use the


#### 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


as used in the [Cobra]( library.



, 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



For individual flags, the


 method provides this functionality.


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

You can also bind an existing set of pflags (pflag.FlagSet):


pflag.Int("flagname", 1234, "help message for flagname") pflag.Parse() viper.BindPFlags(pflag.CommandLine) i := viper.GetInt("flagname") // retrieve values from viper instead of pflag

The use of [pflag]( in Viper does not preclude the use of other packages that use the [flag]( 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().


package main import ( "flag" "" ) func main() { // using standard library "flag" package flag.Int("flagname", 1234, "help message for flagname") pflag.CommandLine.AddGoFlagSet(flag.CommandLine) pflag.Parse() viper.BindPFlags(pflag.CommandLine) i := viper.GetInt("flagname") // retrieve value from viper ... }

#### Flag interfaces

Viper provides two Go interfaces to bind other flag systems if you don’t use




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

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

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

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


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

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:

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



import _ ""

Viper will read a config string (as JSON, TOML, YAML, HCL or envfile) 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]( 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.


 has a command-line helper that you can use to put configurations in your K/V store. 


 defaults to etcd on

$ go get $ 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


 documentation for examples of how to set encrypted values, or how to use Consul.
### Remote Key/Value Store Example - Unencrypted

#### etcd

viper.AddRemoteProvider("etcd", "","/config/hugo.json") viper.SetConfigType("json") // because there is no file extension in a stream of bytes, supported extensions are "json", "toml", "yaml", "yml", "properties", "props", "prop", "env", "dotenv" err := viper.ReadRemoteConfig()

#### Consul

You need to set a key to Consul key/value storage with JSON value containing your desired config. For example, create a Consul key/value store key


 with value:

{ "port": 8080, "hostname": "" }

viper.AddRemoteProvider("consul", "localhost:8500", "MY_CONSUL_KEY") viper.SetConfigType("json") // Need to explicitly set this to json err := viper.ReadRemoteConfig() fmt.Println(viper.Get("port")) // 8080 fmt.Println(viper.Get("hostname")) //

#### Firestore

viper.AddRemoteProvider("firestore", "google-cloud-project-id", "collection/document") viper.SetConfigType("json") // Config's format: "json", "toml", "yaml", "yml" err := viper.ReadRemoteConfig()

Of course, you're allowed to use


### Remote Key/Value Store Example - Encrypted

viper.AddSecureRemoteProvider("etcd","","/config/hugo.json","/etc/secrets/mykeyring.gpg") viper.SetConfigType("json") // because there is no file extension in a stream of bytes, supported extensions are "json", "toml", "yaml", "yml", "properties", "props", "prop", "env", "dotenv" err := viper.ReadRemoteConfig()

### Watching Changes in etcd - Unencrypted

// alternatively, you can create a new viper instance. var runtime_viper = viper.New() runtime_viper.AddRemoteProvider("etcd", "", "/config/hugo.yml") runtime_viper.SetConfigType("yaml") // because there is no file extension in a stream of bytes, supported extensions are "json", "toml", "yaml", "yml", "properties", "props", "prop", "env", "dotenv" // 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


GetIntSlice(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


AllSettings() : map[string]interface{}

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


 method has been provided.


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:

{ "host": { "address": "localhost", "port": 5799 }, "datastore": { "metric": { "host": "", "port": 3099 }, "warehouse": { "host": "", "port": 2112 } } }

Viper can access a nested field by passing a


 delimited path of keys:

GetString("") // (returns "")

This obeys the precedence rules established above; the search for the path will cascade through the remaining configuration registries until found.

For example, given this configuration file, both



 are already defined (and may be overridden). If in addition


 was defined in the defaults, Viper would also find it.

However, if


 was overridden (by a flag, an environment variable, the 


 method, …) with an immediate value, then all sub-keys of


 become undefined, they are “shadowed” by the higher-priority configuration level.

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

{ "": "", "host": { "address": "localhost", "port": 5799 }, "datastore": { "metric": { "host": "", "port": 3099 }, "warehouse": { "host": "", "port": 2112 } } } GetString("") // returns ""

### Extract sub-tree

Extract sub-tree from Viper.

For example,



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

After executing:

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



max-items: 100 item-size: 64

Suppose we have:

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

which creates a cache based on config information formatted as


. Now it’s easy to create these 2 caches separately as:

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


type config struct { Port int Name string PathMap string mapstructure:"path_map" } var C config err := viper.Unmarshal(&C) if err != nil { t.Fatalf("unable to decode into struct, %v", err) }

If you want to unmarshal configuration where the keys themselves contain dot (the default key delimiter), you have to change the delimiter:

v := viper.NewWithOptions(viper.KeyDelimiter("::")) v.SetDefault("chart::values", map[string]interface{}{ "ingress": map[string]interface{}{ "annotations": map[string]interface{}{ "traefik.frontend.rule.type": "PathPrefix", "": "true", }, }, }) type config struct { Chart struct{ Values map[string]interface{} } } var C config v.Unmarshal(&C)

Viper also supports unmarshaling into embedded structs:

/* Example config: module: enabled: true token: 89h3f98hbwf987h3f98wenf89ehf */ type config struct { Module struct { Enabled bool moduleConfig mapstructure:",squash" } } // moduleConfig could be in a module specific package type moduleConfig struct { Token string } var C config err := viper.Unmarshal(&C) if err != nil { t.Fatalf("unable to decode into struct, %v", err) }

Viper uses []( under the hood for unmarshaling values which uses


 tags by default.
### Marshalling to string

You may need to marshal all the settings held in viper into a string rather than write them to a file. You can use your favorite format's marshaller with the config returned by



import ( yaml "" // ... ) func yamlStringSettings() string { c := viper.AllSettings() bs, err := yaml.Marshal(c) if err != nil { log.Fatalf("unable to marshal config to YAML: %v", err) } return string(bs) }

## 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 its 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.


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 is it called “Viper”?

A: Viper is designed to be a companionto 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?

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