cfgbuild

cfgbuild

A Lightweight Golang Library for loading app configs

Introduction

The purpose of cfgbuild is to provide an easy, lightweight package for loading application configuration settings. It is able to build a struct and initialize the fields with associated environment variable values (see examples for loading from a .env file). The main package does not have any external dependencies (but the tests and examples do require external projects). This library is published under the BSD 2-Clause License which provides a lot of flexiblity for usage.

Example Config

A Config is just a struct with fields for different application settings. Fields can be of most types that have a string representation. Each field should have an envvar tag specifying the environment variable that will provide the value.

// Config struct defines fields for application settings.
// It can be called "Config" or anything else.
type Config struct {
    // Field names can be anything and many different types
    // are supported.  The cfgBuild.Builder will use the 
    // envvar tag to know which environment variable to use.
	// If the tag also contains the "required" flag, then 
	// calls to Build() will fail if the value has not been
	// set.  If the tag contains the "default" flag then the
	// provided value will be applied as the default value.
	MyInt    int     `envvar:"MY_INT,required"`
	MyFloat  float64 `envvar:"MY_FLOAT"`
	MyString string  `envvar:"MY_STRING"`
	MyBool   bool    `envvar:"MY_BOOL,default=TRUE"`
}

Usage

There are three primary ways to use the cfgbuild library to build configs:

NewConfig function

The NewConfig function creates, initializes, and returns a config with the type being specified when calling the function. Here's a simple example:

package main

import "github.com/NathanBak/cfgbuild"

func main() {
	cfg, err := cfgbuild.New[*Config]()
	// ...
    // Handle errors, use cfg, ... , profit
    //...
}

InitConfig function

If a config was created but needs to be initialized, the InitConfig function can be used. It accepts the config as an argument and will initialize the various values. Here's an example:

package main

import "github.com/NathanBak/cfgbuild"

func main() {
	cfg := &Config{}
	err := InitConfig(cfg)
	// ...
    // Handle errors, use cfg, ... , profit
    //...
}

Builder

To use the builder first create a new builder (providing the Config type) and then run the Builder.Build() function. Using a builder allows additional configuration such as specifying a non-default list separator. Here's some example code:

package main

import "github.com/NathanBak/cfgbuild"

func main() {
	builder := cfgbuild.Builder[*Config]{ListSeparator: ";"}
	cfg, err := builder.Build()
	// ...
    // Handle errors, use cfg, ... , profit
    //...
}

Here are the options that can be set on a Builder:

Tags

Tags are used to mark the fields in a config so that cfgbuild knows how to properly create and initialize a config instance. Tags follow the format

`TAG_KEY:"TAG_VALUE"`

By default the tag key is envvar, but a Builder can be configured to used a different tag key if desired.

The tag value follows the format

"ENV_VAR_NAME[,ATTRIBUTE_NAME[=ATTRIBUTE_VALUE]]"

For the example below, when cfgbuild creates a new config it will set MyString to the value of environment variable MY_STRING_. If MY_STRING is not set then MyString will be set to the default value "ahoy".

type Config struct {
	MyString string `envvar:"MY_STRING,default=ahoy"`
}

EnvVarName

The EnvVarName portion of the tag value specifies the name of the environment variable to be read when setting the tagged field. In addition, the EnvVarName can be "-" to mean there is no environment variable to be read or ">" to indicate the field is a nested config to be recursively initialized.

Attributes

• required If a value is required (must be set), the required flag can be added.

  MyString string `envvar:"MY_STRING,required"`
  

  In the above example, the cfgbuild.Builder.Build() function will return an error if MyString is not set (because the MY_STRING environment variable isn't set). The required attribute does not have an attribute value.

• default If there is a default value, it can be set using the default attribute.

  MyNumber int `envvar:"MY_NUMBER,default=1234"`
  

  If the environment variable is not set then the default value specified as the attribute value (after the equals sign) will be used instead. There is no compile time validation of the default value so if an integer has something like default=abc specified then the cfgbuild.Builder.Build() function will always fail.

• prefix The prefix attribute is used when a nested Config should have a preflix applied to all environment variable names.

  ChildConfig AnotherConfig `envvar:">,prefix=ANOTHER_"`
  

  In the above example, if "AnotherConfig" had field associated with the environment variable PORT when when initializing the nested config it would read the environment variable ANOTHER_PORT. Also note that that the tag for a nested Config does not have an environment variable name but instead uses >.

• unmarshalJSON The unmarshalJSON attribute is used when the environment variable is in JSON and that should be unmarshaled into a nested struct.

  type Child struct {
  	MyInt int `json:"i"`
  }
  
  type Config struct {
  	Nested Child `envvar:"NESTED_CHILD,unmarshalJSON,default={\"i\":3}"`
  }
  

  In the above example, the default for Nested Child MyInt would be 3 and it would apply any JSON snippet in the NESTED_CHILD envirnonment variable on top. The unmarshalJSON attribute does not have an attribute value.

Functions

Additional flexibility and customization can be achieved by adding implementations of specific functions to the Config struct.

CfgBuildInit()

The CfgBuildInit() function can be used to perform any special initialization logic. This can include things such as specifying complex default values or initializing special fields. The function should have a signature like func (cfg *Config) CfgBuildInit() error. It will be invoked during the Build() right after the new instance is created.

CfgBuildValidate()

The CfgBuildValidate() function can be used to perform special validation the config. This can include things such as verifying that set values are within certain ranges. The function should have a signature like func (cfg *Config) CfgBuildInit() error. It will be invoked as the final step during the Build().

Examples

The examples directory includes:

• simple which shows a simple use case of loading a config from environment variables
• fromdotenv which shows how to load a config from a .env file
• bootstrap which shows how to wrap a Builder into a Config constructor
• enumparse which shows how a config field can be an enum

FAQ

Q - Can this library read configuration information from .env files?
A - The cfgbuild package does not know how to read .env packages, but can easily be paired with godotenv. The examples show two different ways to use godotenv. Note that godetenv (created by John Barton) uses an MIT License.

Q - How does cfgbuild compare with Viper?
A - Viper has a lot more whistles and bells and is overall more flexible and more powerful, but is also more complicated and pulls in numerous dependencies. If you're looking for something to implement a complex CLI, then Viper is your friend. But if you have a microservice that will be running in a container and needs an easy way to get configuration information, it's worth considering cfgbuild.

Q - Does cfgbuild support enums?
A - A config can have an enum field if the enum implements the TextUnmarshaler interface. See the Color enum for an example.

Q - How doce cfgbuild compare with caarlos0/env?
A - Although caarlos0/env was written first, cfgbuild was developed independently. There is a high degree of overlap between basic functionality, but there seem to be difference when it comes to more complex use cases. For example, caarlos0/env supports reading values from files and cfgbuild supports nested structs.