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A pluggable hierarchical configuration management library with zero dependencies, for golang.

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With dep:

dep ensure -add

Or with go get:

go get

Why Venom?

This library aims to provide the basic building blocks of a configuration management library. It exposes as many aspects as the standard lib will reasonably support, such as:

  1. Explicitly set defaults or overrides.
  2. Find and load JSON config files.
  3. Load configuration files from a directory, with the option to recursively load configs from sub-directories.
  4. Load configs from environment variables.
  5. Load command line arguments using the flags library.

These mechanisms are exposed in a completely extendible way which will allow you to easily perform the following actions:

  1. Specify your own config level precedence (eg, environment variables can be made to override command line arguments).
  2. Default behavior can be replaced with custom behavior by writing a custom Resolver.
  3. Custom config levels can be easily implemented by writing a custom Resolver.

Loading Configs

Setting Defaults

You can easily set default values for configuration options that are not required to be specified by config levels with a higher precedence.

// simple values can easily be set as defaults
venom.SetDefault("verbose", false)
venom.SetDefault("hostname", "localhost")

// more complex objects can also be set
venom.SetDefault("log", map[string]interface{
    "level": "INFO",
    "fields": map[string]interface{
        "origin": "my_awesome_app", 
        "category": "categories",

Loading Configs From Files

Venom allows you to specify custom file loaders for specific file types. By default the only implementation is the JSONLoader which loads any file with an extension of .json using json.Unmarshal.

If you wish to implement your own type of config file reader you need only to implement the IOFileLoader interface:

type IOFileLoader func(io.Reader) (map[string]interface{}, error)

Once you have any custom IOFileLoaders registered via RegisterExtension, you can easily load a single file or all files in a directory.

// load a single file

// load all files within a directory
venom.LoadDirectory("/etc/conf.d", false)

// recursively load all files within a directory or it's sub-directories
venom.LoadDirectory("/etc/conf.d", true)

Setting Overrides

You can easily set values which overrides all other values for a single configuration.

venom.SetOverride("verbose", true)

Environment Variables

Configuration values may be loaded from any set environment variables, which is enabled by default in the global venom instance and when creating a custom venom instance via venom.Default().

Note: Environment variables are case sensitive and should be set as uppercase strings with words that are separated by an underscore ("_") character.

package main

import (


func main() {
    os.Setenv("LOG_LEVEL", "INFO")

    fmt.Println(venom.Get("log.level"))  // Output: "INFO"

To specify a custom environment variable prefix, you can simply create and register a new EnvironmentVariableResolver.

package main

import (


func main() {
    envVarResolver := &venom.EnvironmentVariableResolver{
        Prefix: "MYSERVICE",

    venom.RegisterResolver(venom.EnvironmentLevel, envVarResolver)

    os.Setenv("LOG_LEVEL", "IGNORED")
    os.Setenv("MYSERVICE_LOG_LEVEL", "INFO")

    fmt.Println(venom.Get("log.level"))  // Output: "INFO"

To perform specific character mapping translations for environment variables you can provide a custom KeyTranslator to an EnvironmentVariableResolver.

package main

import (


func main() {
    envVarResolver := &venom.EnvironmentVariableResolver{
        // Provide a custom KeyTranslator to an EnvironmentVariableResolver
        Translator: func(b byte) byte {
            switch b {
            case '-':
                // Convert all hypens to underscores.
                return '_'
                // And otherwise translate all other characters to their
                // uppercase equivalents.
                return byte(unicode.ToUpper(rune(b)))

    venom.RegisterResolver(venom.EnvironmentLevel, envVarResolver)

    os.Setenv("LOG_LEVEL", "IGNORED")
    os.Setenv("MYSERVICE_LOG_LEVEL", "INFO")

    fmt.Println(venom.Get("log.level"))  // Output: "INFO"


By default commandline flags can be parsed using the standard lib flag library. There are two different ways that you can approach loading command line configs, and they both involve creating and registering a new FlagsetResolver instance with Venom.

Note: Venom will parse flags if they have not already been parsed, but Venom will not attempt to re-parse an already parsed FlagSet.

Straight From os.Args

If you configure your commandline options to be loaded from the global flags FlagSet, you can pass a zero-value FlagsetResolver when registering the resolver with Venom.

fs.String("log-level", "WARNING", "set log level")

flagResolver := &venom.FlagsetResolver{}

venom.RegisterResolver(venom.FlagLevel, flagResolver)

Default and Custom Logging

You can initialize Venom to log to stdout (default) on reads and writes or provide a logging interface of your choice.

venDef := venom.NewLoggable()
venDef.SetDefault("baz", "bee")
// 2019-04-21T10:01:39.529Z[venom]: writing level=0 key=baz val=bee
// 2019-04-21T10:01:39.529Z[venom]: reading key=baz val=bee exist=true

There is a tiny bit of overhead to use a custom logger but is straight forward to do. For example, using sirupsen/logrus:

// create a wrapping struct to hold the logging mechanism
type LogrusLogger struct {
    log *logrus.Entry

// create read and write methods which have these signatures
// and configure the log line as you choose
func (l *LogrusLogger) LogWrite(level venom.ConfigLevel, key string, val interface{}) {
    l.log.WithFields(logrus.Fields{"level": level,"key": key,"val": val}).Info("writing config value")

func (l *LogrusLogger) LogRead(key string, val interface{}, bl bool) {
    l.log.WithFields(logrus.Fields{"key": key,"val": val,"exist": bl}).Info("read config value")

lgr := logrus.New()
l := logrus.NewEntry(lgr)
ven := venom.NewLoggableWith(&LogrusLogger{log: l})

ven.SetDefault("foo", "bar")
// INFO[0000] writing config value               fields.level=0 key=foo val=bar
// INFO[0000] read config value                  exist=true key=foo val=bar

Provide a Custom flag.FlagSet

You also have the option to provide a custom flag.FlagSet instance via a FlagsetResolver.

Note: When providing a FlagSet, you may also provide the arguments to be parsed if the FlagSet has not already been parsed. The arguments will default to os.Args[1:] if none are specified.

fs := flag.NewFlagSet("example", flag.ContinueOnError)
fs.String("log-level", "WARNING", "set log level")

flagResolver := &venom.FlagsetResolver{
    Flags: fs,
venom.RegisterResolver(venom.FlagLevel, flagResolver)

Custom ConfigLevels

Consuming applications are able to define their own ConfigLevels in order to define configuration values with higher or lower precedence. For example, to set configs at levels above Override (not recommended), you could do something similar to the following:

const MySuperImportantLevel venom.ConfigLevel = venom.OverrideLevel + 1

venom.SetLevel(MySuperImportantLevel, "verbose", true)

Reading Config Values

There are several ways to access config values from Venom:

  • Get(key string) interface{}
  • Find(key string) (interface{}, bool)

If you're unsure whether or not a config value has been set, Find will return an optional boolean value indicating whether or not the value has been specified. Otherwise, Get will return nil in the event that a config has not been specified.

Key Management

Venom automatically nests config values that are specified as separated by the value of Delim, which defaults to ".". This means that you can express more complex config structures when setting and reading variables.

venom.SetDefault("log.level", "INFO")
fmt.Printf("%v", venom.Get("log"))  // Output: map[string]interface{"level": "INFO"}
fmt.Printf("%v", venom.Get("log.level"))  // Output: "INFO"

Aliasing Keys

Venom exposes the ability to alias one key to another. This allows applications to more easily modify their configuration without breaking backwards compatibility when doing so.

venom.SetDefault("log.enabled", true)
venom.Alias("verbose", "log.enabled")
fmt.Println(venom.Get("verbose"))  // Output: true

Unmarshal Configs

Venom supports the ability to unmarshal configuration data into struct values and also introduces the new venom struct field tag, which allows callers to specify different configuration fields to set to fields. Nested structs

Note: that errors are returned if the stored type and the type of the struct field do not match.

Note: Nested structs are supported and will carry the context of the name of the parent config. See the following example for more on this

type LoggingConfig struct {
    // this field will be loaded from log.level if Unmarshalled via the 
    // following Config struct
    Level string `venom:"level"`

type Config struct {
    Log LoggingConfig `venom:"log"` 

var c Config
err := venom.Unmarshal(nil, &c)


As of 0.2.0, venom now exposes optional functions for defining Venom instances that are safe for concurrent goroutine access.

Two functions are exposed to provide new goroutine safe Venom instances, the behavior of which mimics the functions unsafe counterparts:

// generate a new, empty, venom instance that is safe for concurrent access 
ven := venom.NewSafe()

// or generate a new venom instance with some default levels applied to it
ven = venom.DefaultSafe()

Custom Venom Behavior

The above examples show how to use the global venom instance for the sake of brevity, but you can create your own venom instances to use directly.

ven := venom.New()

ven.SetDefault("verbose", false)

Additionally, as of 0.2.0, you can define your own ConfigStore to control how venom manages it's underlying configuration storage. This can be achieved by creating a new Venom instance with a predefined ConfigStore via the NewWithStore function.

ven := venom.NewWithStore(venom.NewSafeConfigStore())

ven.SetDefault("verbose", false)


goos: darwin
goarch: amd64
BenchmarkVenomGet/single_ConfigLevel_with_one_key/value_pair-8         	20000000	        84.6 ns/op	      16 B/op	       1 allocs/op
BenchmarkVenomGet/many_key/value_pairs_in_a_single_ConfigLevel-8       	20000000	       102 ns/op	      16 B/op	       1 allocs/op
BenchmarkVenomGet/many_key/value_pairs_spread_across_multiple_ConfigLevels-8         	20000000	        99.2 ns/op	      16 B/op	       1 allocs/op
BenchmarkVenomWrite/single_key/value_pair_in_one_ConfigLevel-8                       	20000000	        89.0 ns/op	      16 B/op	       1 allocs/op
BenchmarkVenomWrite/many_key/value_pairs_in_one_ConfigLevel-8                        	 2000000	       648 ns/op	     121 B/op	       3 allocs/op
BenchmarkVenomWrite/many_nested_key/value_pairs_in_one_ConfigLevel-8                 	 1000000	      2055 ns/op	     922 B/op	       7 allocs/op
BenchmarkVenomWrite/many_key/value_pairs_in_many_ConfigLevels-8                      	 2000000	       589 ns/op	     121 B/op	       3 allocs/op
BenchmarkVenomWrite/many_nested_key/value_pairs_in_many_ConfigLevels-8               	 1000000	      1679 ns/op	     922 B/op	       7 allocs/op


A pluggable configuration management library with zero dependencies





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