Clean Env

This is a simple configuration reading tool. It just does the following:

reads and parses configuration structure from the file
reads and overwrites configuration structure from environment variables
writes a detailed variable list to help output

Content

Installation
Usage
Model Format
Supported types
Custom Functions
- Custom Value Setter
- Custom Value Update
Supported File Formats
Integration
- Flag
Examples

Installation

To install the package run

go get -u github.com/zaidfadhil/cleanenv

Usage

The package is oriented to be simple in use and explicitness.

The main idea is to use a structured configuration variable instead of any sort of dynamic set of configuration fields like some libraries does, to avoid unnecessary type conversions and move the configuration through the program as a simple structure, not as an object with complex behavior.

There are just several actions you can do with this tool and probably only things you want to do with your config if your application is not too complicated.

read configuration file
read environment variables
read some environment variables again

Read Configuration

You can read a configuration file and environment variables in a single function call.

import "github.com/zaidfadhil/cleanenv"

type ConfigDatabase struct {
    Port     string `yaml:"port" env:"PORT" env-default:"5432"`
    Host     string `yaml:"host" env:"HOST" env-default:"localhost"`
    Name     string `yaml:"name" env:"NAME" env-default:"postgres"`
    User     string `yaml:"user" env:"USER" env-default:"user"`
    Password string `yaml:"password" env:"PASSWORD"`
}

var cfg ConfigDatabase

err := cleanenv.ReadConfig("config.yml", &cfg)
if err != nil {
    ...
}

This will do the following:

parse configuration file according to YAML format (yaml tag in this case);
reads environment variables and overwrites values from the file with the values which was found in the environment (env tag);
if no value was found on the first two steps, the field will be filled with the default value (env-default tag) if it is set.

Read Environment Variables Only

Sometimes you don't want to use configuration files at all, or you may want to use .env file format instead. Thus, you can limit yourself with only reading environment variables:

import "github.com/zaidfadhil/cleanenv"

type ConfigDatabase struct {
    Port     string `env:"PORT" env-default:"5432"`
    Host     string `env:"HOST" env-default:"localhost"`
    Name     string `env:"NAME" env-default:"postgres"`
    User     string `env:"USER" env-default:"user"`
    Password string `env:"PASSWORD"`
}

var cfg ConfigDatabase

err := cleanenv.ReadEnv(&cfg)
if err != nil {
    ...
}

Update Environment Variables

Some environment variables may change during the application run. To get the new values you need to mark these variables as updatable with the tag env-upd and then run the update function:

import "github.com/zaidfadhil/cleanenv"

type ConfigRemote struct {
    Port     string `env:"PORT" env-upd`
    Host     string `env:"HOST" env-upd`
    UserName string `env:"USERNAME"`
}

var cfg ConfigRemote

cleanenv.ReadEnv(&cfg)

// ... some actions in-between

err := cleanenv.UpdateEnv(&cfg)
if err != nil {
    ...
}

Here remote host and port may change in a distributed system architecture. Fields cfg.Port and cfg.Host can be updated in the runtime from corresponding environment variables. You can update them before the remote service call. Field cfg.UserName will not be changed after the initial read, though.

Description

You can get descriptions of all environment variables to use them in the help documentation.

import "github.com/zaidfadhil/cleanenv"

type ConfigServer struct {
    Port     string `env:"PORT" env-description:"server port"`
    Host     string `env:"HOST" env-description:"server host"`
}

var cfg ConfigRemote

help, err := cleanenv.GetDescription(&cfg, nil)
if err != nil {
    ...
}

You will get the following:

Environment variables:
  PORT  server port
  HOST  server host

Model Format

Library uses tags to configure the model of configuration structure. There are the following tags:

env="<name>" - environment variable name (e.g. env="PORT");
env-upd - flag to mark a field as updatable. Run UpdateEnv(&cfg) to refresh updatable variables from environment;
env-required - flag to mark a field as required. If set will return an error during environment parsing when the flagged as required field is empty (default Go value). Tag env-default is ignored in this case;
env-default="<value>" - default value. If the field wasn't filled from the environment variable default value will be used instead;
env-separator="<value>" - custom list and map separator. If not set, the default separator , will be used;
env-description="<value>" - environment variable description;
env-layout="<value>" - parsing layout (for types like time.Time);
env-prefix="<value>" - prefix for all fields of nested structure (only for nested structures);

Supported types

There are following supported types:

int (any kind);
float (any kind);
string;
boolean;
slices (of any other supported type);
maps (of any other supported type);
time.Duration;
time.Time (layout by default is RFC3339, may be overridden by env-layout);
*time.Location (time zone parsing depends on running machine)
any type implementing cleanenv.Setter interface.

Custom Functions

To enhance package abilities you can use some custom functions.

Custom Value Setter

To make custom type allows to set the value from the environment variable, you need to implement the Setter interface on the field level:

type MyField string

func (f *MyField) SetValue(s string) error  {
    if s == "" {
        return fmt.Errorf("field value can't be empty")
    }
    *f = MyField("my field is: "+ s)
    return nil
}

type Config struct {
    Field MyField `env="MY_VALUE"`
}

SetValue method should implement conversion logic from string to custom type.

Custom Value Update

You may need to execute some custom field update logic, e.g. for remote config load.

Thus, you need to implement the Updater interface on the structure level:

type Config struct {
    Field string
}

func (c *Config) Update() error {
    newField, err := SomeCustomUpdate()
    f.Field = newField
    return err
}

Supported File Formats

There are several most popular config file formats supported:

YAML (.yaml, .yml)
ENV (.env)

Note:

while using .env file the library will set corresponding data to process environment variables. It will override existing variables with the same keys in the process environment.

Integration

The package can be used with many other solutions. To make it more useful, we made some helpers.

Flag

You can use the cleanenv help together with Golang flag package.

// create some config structure
var cfg config 

// create flag set using `flag` package
fset := flag.NewFlagSet("Example", flag.ContinueOnError)

// get config usage with wrapped flag usage
fset.Usage = cleanenv.FUsage(fset.Output(), &cfg, nil, fset.Usage)

fset.Parse(os.Args[1:])

Examples

type Config struct {
    Port string `yaml:"port" env:"PORT" env-default:"8080"`
    Host string `yaml:"host" env:"HOST" env-default:"localhost"`
}

var cfg Config

err := ReadConfig("config.yml", &cfg)
if err != nil {
    ...
}