mu/config

config is a package for declaring and reading configuration variables.

To use config, create a Config object with New, and then call its NewInt, NewString, or NewBool methods to declare configuration variables. You can then call Int, String, or Bool to read the value of a variable.

Motivation

Cloud-native applications are usually configured using environment variables. However, accessing environment variables directly with os.Getenv is primitive and error-prone. config provides a simple, type-safe interface for declaring and reading configuration variables.

• Configuration variables are loaded, type-checked, and validated when they are registered, not when they are accessed. This ensures that configuration errors are caught early, instead of causing application errors, crashes, or misbehavior at runtime.
• Because validation happens at registration time, accessor methods like Int and String do not need to return an error. This simplifies the application code, and makes it easier to use configuration variables in places where error handling is not convenient.
• All configuration variables are declared in a single place, instead of scattered throughout the application code. This makes it easy to see all configuration variables, and to find the code that uses them.
• Description strings are attached to each configuration variable. This enables automatic documentation generation, and makes it easy to see what each variable is used for.

Example

package main

import (
    "fmt"
    "os"

    "github.com/neuralnorthwest/mu/config"
)

func main() {
    c := config.New()
    c.NewInt("PORT", 8080, "The port to listen on")
    c.NewString("MESSAGE", "Hello world!", "The message to print")
    c.NewBool("DEBUG", false, "Enable debug mode")

    fmt.Printf("Listening on port %d\n", c.Int("PORT"))
    fmt.Printf("Message: %s\n", c.String("MESSAGE"))
    if c.Bool("DEBUG") {
        fmt.Println("Debug mode is enabled")
    }
}

Options

Various options are available for customizing the behavior of config. Some options can be passed to New, while others are used with NewInt, NewString, and NewBool.

Options for New

• WithEnvPrefix - Sets the prefix for environment variables. For example, if the prefix is MYAPP_, then the environment variable MYAPP_PORT will be used to set the value of the PORT configuration variable. Example:

package main

import (
    "fmt"
    "os"

    "github.com/neuralnorthwest/mu/config"
)

func main() {
    c := config.New(config.WithEnvPrefix("MYAPP_"))
    c.NewInt("PORT", 8080, "The port to listen on")
    fmt.Printf("Listening on port %d\n", c.Int("PORT"))
}

• WithSource - Sets the source for configuration variables. The default source is os.Getenv, but you can also use a custom source that reads configuration variables from a file, or from a database.

package main

import (
    "fmt"
    "os"

    "github.com/neuralnorthwest/mu/config"
)

type customSource struct {
    prefix string
    // Other fields
}

func newCustomSource() *customSource {
    return &customSource{}
}

// Implement the Source interface
func (s *customSource) SetPrefix(prefix string) {
    s.prefix = prefix
}

func (s *customSource) LoadInt(name string) (int, error) {
    // Read the value of the configuration variable from a file or database
    value := ...
    return value, nil
}

func (s *customSource) LoadString(name string) (string, error) {
    // Read the value of the configuration variable from a file or database
    value := ...
    return value, nil
}

func (s *customSource) LoadBool(name string) (bool, error) {
    // Read the value of the configuration variable from a file or database
    value := ...
    return value, nil
}

func main() {
    c := config.New(config.WithSource(newCustomSource()))
    c.NewInt("PORT", 8080, "The port to listen on")
    fmt.Printf("Listening on port %d\n", c.Int("PORT"))
}

Options for NewInt

• WithMinimumValue and WithMaximumValue - Sets the minimum and maximum allowed values for the configuration variable. If the value is outside the allowed range, NewInt will either return an error, or clamp the value to the nearest allowed value.
• WithClamping - Enables clamping for the configuration variable. If the value is outside the allowed range, it will be clamped to the nearest allowed value. If clamping is disabled, NewInt will return an error if the value is outside the allowed range.
• WithIntValidator - Sets a custom validator for the configuration variable. The validator is a function that takes an integer value and returns an error if the value is invalid. If the validator returns an error, NewInt will return the same error.