lixenwraith/config Quick Reference Guide

This guide details the lixenwraith/config package for thread-safe Go configuration. It supports multiple sources (files, environment, CLI), type-safe struct population, and live reconfiguration.

Quick Start: Recommended Usage

The recommended pattern uses the Builder with a target struct providing compile-time type safety and eliminating the need for runtime type assertions

package main

import (
	"fmt"
	"log"
	"os"
    
	"github.com/lixenwraith/config"
)

// 1. Define your application's configuration struct
type AppConfig struct {
	Server struct {
		Host string `toml:"host"`
		Port int    `toml:"port"`
	} `toml:"server"`
	Debug bool `toml:"debug"`
}

func main() {
	// 2. Create a target instance with defaults
	// Method A: Direct initialization (cleaner for simple defaults)
	target := &AppConfig{}
	target.Server.Host = "localhost"
	target.Server.Port = 8080

	// Method B: Use WithDefaults() for explicit separation (shown below)

	// 3. Use the builder to configure and load all sources
	cfg, err := config.NewBuilder().
		WithTarget(target).               // Enable type-safe mode and register struct fields
		// WithDefaults(&AppConfig{...}), // Optional: Override defaults from WithTarget
		WithFile("config.toml").          // Load from file (supports .toml, .json, .yaml)
		WithEnvPrefix("APP_").            // Load from environment (e.g., APP_SERVER_PORT)
		WithArgs(os.Args[1:]).            // Load from command-line flags (e.g., --server.port=9090)
		Build()                           // Build the final config object
	if err != nil {
		log.Fatalf("Config build failed: %v", err)
	}

	// 4. Access the fully populated, type-safe struct
	// The `target` variable is now populated with the final merged values
	fmt.Printf("Running on %s:%d\n", target.Server.Host, target.Server.Port)

    // Or, retrieve the updated struct at any time (e.g., after a live reload)
    latest, _ := cfg.AsStruct()
    latestConfig := latest.(*AppConfig)
    fmt.Printf("Debug mode: %v\n", latestConfig.Debug)
}

Supported Formats: TOML, JSON, YAML

The package supports multiple file formats. The format is auto-detected from the file extension (.toml, .json, .yaml, .yml) or file content

To specify a format explicitly, use Builder.WithFileFormat("json")
The default struct tag for field mapping is toml, but can be changed with Builder.WithTagName("json")

Builder Pattern

The Builder is the primary way to construct a Config instance

// NewBuilder creates a new configuration builder
func NewBuilder() *Builder
// Build finalizes configuration; returns the first of any accumulated errors
func (b *Builder) Build() (*Config, error)
// MustBuild is like Build but panics on fatal errors
func (b *Builder) MustBuild() *Config

Builder Methods

WithTarget(target any): (Recommended) Registers fields from the target struct and allows access via AsStruct()
WithDefaults(defaults any): Explicitly sets a struct containing default values, overriding any default values already in struct from WithTarget
WithFile(path string): Sets the configuration file path
WithFileDiscovery(opts FileDiscoveryOptions): Enables automatic config file discovery (searches CLI flags, env vars, XDG paths, and current directory)
WithArgs(args []string): Sets the command-line arguments to parse (e.g., --server.port=9090)
WithEnvPrefix(prefix string): Sets the global environment variable prefix (e.g., MYAPP_)
WithSources(sources ...Source): Overrides the default source precedence order
WithTypedValidator(fn any): (Recommended for validation) Adds a type-safe validation function with signature func(c *YourConfigType) error that runs after the target struct is populated
WithValidator(fn ValidatorFunc): Adds a validation function that runs before type-safe population, operating on the raw *Config object
WithTagName(tagName string): Sets the primary struct tag for field mapping ("toml", "json", "yaml")
WithPrefix(prefix string): Adds a prefix to all registered paths from a struct
WithFileFormat(format string): Explicitly sets the file format ("toml", "json", "yaml", "auto")
WithSecurityOptions(opts SecurityOptions): Sets security options for file loading (path traversal, file size limits)
WithEnvTransform(fn EnvTransformFunc): Sets a custom environment variable mapping function
WithEnvWhitelist(paths ...string): Limits environment variable loading to a specific set of paths

Type-Safe Access & Population

These are the preferred methods for accessing configuration data

AsStruct() (any, error): After using Builder.WithTarget(), this method returns the populated, type-safe target struct - This is the primary way to access config after initialization or live reload
Scan(basePath string, target any): Populates a struct with values from a specific config path (e.g., cfg.Scan("server", &serverConf))
ScanMap(configMap map[string]any, target any, tagName ...string): Decodes a map[string]any directly into a target struct, useful for plugins or custom config sources
GetTyped[T](c *Config, path string) (T, error): Retrieves a single value and decodes it to type T, handling type conversion automatically
ScanTyped[T](c *Config, basePath ...string) (*T, error): A generic wrapper around Scan that allocates, populates, and returns a pointer to a struct of type T

Live Reconfiguration

Enable automatic reloading of configuration when the source file changes

AutoUpdate(): Enables file watching and automatic reloading with default options
AutoUpdateWithOptions(opts WatchOptions): Enables reloading with custom options (e.g., poll interval, debounce)
StopAutoUpdate(): Stops the file watcher
Watch() <-chan string: Returns a channel that receives the paths of changed values
WatchFile(filePath string, formatHint ...string): Switches the watcher to a new file at runtime
IsWatching() bool: Returns true if the file watcher is active

The watch channel also receives special notifications: "file_deleted", "permissions_changed", "reload_error:..."

Source Precedence

The default order of precedence (highest to lowest) is:

CLI: Command-line arguments (--server.port=9090)
Env: Environment variables (MYAPP_SERVER_PORT=8888)
File: Configuration file (config.toml)
Default: Values registered from a struct

This order can be changed via Builder.WithSources() or Config.SetPrecedence()

Dynamic / Legacy Value Access

These methods are for dynamic key-value access that require type assertion and should be avoided when a type-safe struct can be used

Get(path string) (any, bool): Retrieves the final merged value. The bool indicates if the path was registered. Requires a type assertion, e.g., port := val.(int64)
Set(path string, value any): Updates a value in the highest priority source. The path must be registered first
GetSource(path string, source Source) (any, bool): Retrieves a value from a specific source layer
SetSource(path string, source Source, value any): Sets a value for a specific source layer

API Reference Summary

Core Types

Config: The primary thread-safe configuration manager
Source: A configuration source (SourceCLI, SourceEnv, SourceFile, SourceDefault)
LoadOptions: Options for loading configuration from multiple sources
Builder: Fluent API for constructing a Config instance

Core Methods

New() *Config: Creates a new Config instance.
Register(path string, defaultValue any): Registers a path with a default value
RegisterStruct(prefix string, structWithDefaults any): Recursively registers fields from a struct using toml tags
Validate(required ...string): Checks that all specified required paths have been set from a non-default source
Save(path string): Atomically saves the current merged configuration state to a file
Clone() *Config: Creates a deep copy of the configuration state
Debug() string: Returns a formatted string of all values for debugging

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