docs: separate configuration and logging documentation

Extracts logging documentation into its own file (docs/logging.md) and updates README.md to link to both configuration and logging guides.

README.md

@@ -41,6 +41,11 @@ make test
 make lint
 ```
 
+## Documentation
+
+- [Configuration Guide](docs/configuration.md): Details on YAML config, environment variables, and CLI flags.
+- [Logging Guide](docs/logging.md): Information about log levels and structured logging format.
+
 ## Docker
 
 Build the Docker image:

cmd/app/main.go

@@ -2,16 +2,33 @@ package main
 
 import (
 	"context"
+	"flag"
+	"fmt"
 	"log/slog"
 	"os"
 	"os/signal"
 	"syscall"
 	"time"
+
+	"github.com/placeholder/golang-template/internal/config"
+	"github.com/placeholder/golang-template/internal/logging"
 )
 
 func main() {
-	logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
-	slog.SetDefault(logger)
+	// Parse CLI flags
+	var configPath string
+	flag.StringVar(&configPath, "config", "config/config.yaml", "path to config file")
+	flag.Parse()
+
+	// Load Configuration
+	cfg, err := config.Load(configPath, "APP")
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to load config: %v\n", err)
+		os.Exit(1)
+	}
+
+	// Setup Logger
+	logging.Configure(cfg.Logging)
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()

config/config.yaml

@@ -0,0 +1,12 @@
+# Application Configuration
+# This file contains the default configuration for the application.
+# Values can be overridden by environment variables with the prefix "APP_".
+# Example: APP_LOGGING_LEVEL=debug
+
+logging:
+  # Log Level: debug, info, warn, error
+  level: "info"
+
+server:
+  host: "localhost"
+  port: 8080

docs/configuration.md

@@ -0,0 +1,45 @@
+# Configuration
+
+This application uses a flexible configuration system that supports YAML files and environment variable overrides.
+
+## Configuration File
+
+The configuration is loaded from a YAML file. By default, the application looks for `config/config.yaml`. You can specify a different configuration file using the `-config` CLI flag.
+
+```bash
+./app -config /path/to/my-config.yaml
+```
+
+### Structure
+
+The configuration file supports the following structure:
+
+```yaml
+# Logging configuration
+logging:
+  # Log Level: debug, info, warn, error
+  level: "info"
+
+# Server configuration
+server:
+  host: "localhost"
+  port: 8080
+```
+
+## Environment Variables
+
+All configuration values can be overridden using environment variables. The application uses the `APP_` prefix for all environment variables. The variable name is constructed by capitalizing the path to the configuration value and replacing dots/nesting with underscores.
+
+| YAML Path | Environment Variable | Description |
+| :--- | :--- | :--- |
+| `logging.level` | `APP_LOGGING_LEVEL` | Sets the logging level (debug, info, warn, error) |
+| `server.host` | `APP_SERVER_HOST` | Sets the server host address |
+| `server.port` | `APP_SERVER_PORT` | Sets the server listening port |
+
+**Example:**
+
+To start the application with `debug` logging and listening on port `9090`:
+
+```bash
+APP_LOGGING_LEVEL=debug APP_SERVER_PORT=9090 ./app
+```

docs/logging.md

@@ -0,0 +1,24 @@
+# Logging
+
+The application uses structured logging (JSON format) via the standard library's `log/slog` package.
+
+## Log Levels
+
+The supported log levels are:
+
+*   `debug`: Detailed information for debugging purposes.
+*   `info`: General operational events (startup, shutdown, etc.).
+*   `warn`: Non-critical errors or potential issues.
+*   `error`: Critical errors that require attention.
+
+The log level is configured via the `logging.level` setting in the configuration file or the `APP_LOGGING_LEVEL` environment variable.
+
+## Log Output
+
+Logs are output to `stdout` in JSON format to facilitate parsing by log management systems.
+
+**Example Log Entry:**
+
+```json
+{"time":"2023-10-27T10:00:00.123Z","level":"INFO","msg":"Starting application..."}
+```

go.mod

@@ -1,3 +1,5 @@
 module github.com/placeholder/golang-template
 
 go 1.23
+
+require gopkg.in/yaml.v3 v3.0.1

go.sum

@@ -0,0 +1,4 @@
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

internal/config/config.go

@@ -0,0 +1,144 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"reflect"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/placeholder/golang-template/internal/logging"
+	"gopkg.in/yaml.v3"
+)
+
+// Config holds the application configuration.
+type Config struct {
+	Logging logging.Config `yaml:"logging"`
+	Server  ServerConfig   `yaml:"server"`
+}
+
+// ServerConfig holds server-specific configuration.
+type ServerConfig struct {
+	Host string `yaml:"host"`
+	Port int    `yaml:"port"`
+}
+
+// Load reads configuration from a YAML file and overrides it with environment variables.
+// The envPrefix is used to namespace environment variables (e.g. "APP").
+// Env vars should be in the format: PREFIX_FIELD_NAME (uppercase).
+func Load(configPath string, envPrefix string) (*Config, error) {
+	cfg := &Config{}
+
+	// 1. Read YAML file
+	file, err := os.Open(configPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to open config file: %w", err)
+	}
+	defer file.Close()
+
+	decoder := yaml.NewDecoder(file)
+	if err := decoder.Decode(cfg); err != nil {
+		return nil, fmt.Errorf("failed to decode config file: %w", err)
+	}
+
+	// 2. Override from Environment Variables
+	if err := overrideFromEnv(cfg, envPrefix); err != nil {
+		return nil, fmt.Errorf("failed to process environment variables: %w", err)
+	}
+
+	return cfg, nil
+}
+
+// overrideFromEnv uses reflection to traverse the struct and update fields from env vars.
+func overrideFromEnv(cfg interface{}, prefix string) error {
+	v := reflect.ValueOf(cfg)
+	if v.Kind() == reflect.Ptr {
+		v = v.Elem()
+	}
+
+	return setFromEnv(v, prefix)
+}
+
+func setFromEnv(v reflect.Value, prefix string) error {
+	t := v.Type()
+
+	for i := 0; i < v.NumField(); i++ {
+		field := t.Field(i)
+		fieldVal := v.Field(i)
+		
+		// Determine the key for this field
+		// We use the yaml tag if available to guess the name, or just the field name
+		keyName := field.Name
+		if tag := field.Tag.Get("yaml"); tag != "" {
+			parts := strings.Split(tag, ",")
+			if parts[0] != "" {
+				keyName = parts[0]
+			}
+		}
+
+		// Sanitize key name for Env (uppercase, replace non-alphanum with _)
+		envKey := strings.ToUpper(fmt.Sprintf("%s_%s", prefix, keyName))
+		// Clean up common separators like hyphens in yaml keys
+		envKey = strings.ReplaceAll(envKey, "-", "_")
+
+		if fieldVal.Kind() == reflect.Struct {
+			// Recursive call for nested structs
+			if err := setFromEnv(fieldVal, envKey); err != nil {
+				return err
+			}
+			continue
+		}
+
+		if !fieldVal.CanSet() {
+			continue
+		}
+
+		val, present := os.LookupEnv(envKey)
+		if present {
+			if err := setField(fieldVal, val); err != nil {
+				return fmt.Errorf("failed to set field %s from env %s: %w", field.Name, envKey, err)
+			}
+		}
+	}
+	return nil
+}
+
+func setField(field reflect.Value, value string) error {
+	switch field.Kind() {
+	case reflect.String:
+		field.SetString(value)
+	case reflect.Bool:
+		b, err := strconv.ParseBool(value)
+		if err != nil {
+			return err
+		}
+		field.SetBool(b)
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		// Handle Duration specially if it's an int64 underlying type? 
+		// For now, let's assume standard ints. 
+		// Real generic implementations often handle time.Duration
+		if field.Type() == reflect.TypeOf(time.Duration(0)) {
+			d, err := time.ParseDuration(value)
+			if err != nil {
+				return err
+			}
+			field.SetInt(int64(d))
+		} else {
+			i, err := strconv.ParseInt(value, 10, 64)
+			if err != nil {
+				return err
+			}
+			field.SetInt(i)
+		}
+	case reflect.Float32, reflect.Float64:
+		f, err := strconv.ParseFloat(value, 64)
+		if err != nil {
+			return err
+		}
+		field.SetFloat(f)
+	default:
+		return fmt.Errorf("unsupported type: %s", field.Kind())
+	}
+	return nil
+}

internal/config/config_test.go

@@ -0,0 +1,65 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoad(t *testing.T) {
+	// Create a temporary config file
+	tmpDir := t.TempDir()
+	configFile := filepath.Join(tmpDir, "config.yaml")
+	
+	yamlContent := []byte(`
+logging:
+  level: "info"
+server:
+  host: "localhost"
+  port: 8080
+`)
+	if err := os.WriteFile(configFile, yamlContent, 0644); err != nil {
+		t.Fatalf("failed to write temp config file: %v", err)
+	}
+
+	t.Run("Load from YAML", func(t *testing.T) {
+		cfg, err := Load(configFile, "TEST")
+		if err != nil {
+			t.Fatalf("Load() error = %v", err)
+		}
+		if cfg.Logging.Level != "info" {
+			t.Errorf("expected Logging.Level 'info', got '%s'", cfg.Logging.Level)
+		}
+		if cfg.Server.Host != "localhost" {
+			t.Errorf("expected Server.Host 'localhost', got '%s'", cfg.Server.Host)
+		}
+		if cfg.Server.Port != 8080 {
+			t.Errorf("expected Server.Port 8080, got %d", cfg.Server.Port)
+		}
+	})
+
+	t.Run("Override from Env", func(t *testing.T) {
+		os.Setenv("TEST_LOGGING_LEVEL", "debug")
+		os.Setenv("TEST_SERVER_PORT", "9090")
+		os.Setenv("TEST_SERVER_HOST", "0.0.0.0")
+		defer func() {
+			os.Unsetenv("TEST_LOGGING_LEVEL")
+			os.Unsetenv("TEST_SERVER_PORT")
+			os.Unsetenv("TEST_SERVER_HOST")
+		}()
+
+		cfg, err := Load(configFile, "TEST")
+		if err != nil {
+			t.Fatalf("Load() error = %v", err)
+		}
+		if cfg.Logging.Level != "debug" {
+			t.Errorf("expected Logging.Level 'debug', got '%s'", cfg.Logging.Level)
+		}
+		if cfg.Server.Port != 9090 {
+			t.Errorf("expected Server.Port 9090, got %d", cfg.Server.Port)
+		}
+		if cfg.Server.Host != "0.0.0.0" {
+			t.Errorf("expected Server.Host '0.0.0.0', got '%s'", cfg.Server.Host)
+		}
+	})
+}

internal/logging/logging.go

@@ -0,0 +1,37 @@
+package logging
+
+import (
+	"log/slog"
+	"os"
+	"strings"
+)
+
+// Config holds the logging configuration.
+type Config struct {
+	Level string `yaml:"level"`
+}
+
+// Configure sets up the global logger based on the provided configuration.
+func Configure(cfg Config) *slog.Logger {
+	opts := &slog.HandlerOptions{
+		Level: parseLogLevel(cfg.Level),
+	}
+	logger := slog.New(slog.NewJSONHandler(os.Stdout, opts))
+	slog.SetDefault(logger)
+	return logger
+}
+
+func parseLogLevel(level string) slog.Level {
+	switch strings.ToLower(level) {
+	case "debug":
+		return slog.LevelDebug
+	case "info":
+		return slog.LevelInfo
+	case "warn":
+		return slog.LevelWarn
+	case "error":
+		return slog.LevelError
+	default:
+		return slog.LevelInfo
+	}
+}

internal/logging/logging_test.go

@@ -0,0 +1,37 @@
+package logging
+
+import (
+	"log/slog"
+	"testing"
+)
+
+func TestParseLogLevel(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected slog.Level
+	}{
+		{"debug", slog.LevelDebug},
+		{"DEBUG", slog.LevelDebug},
+		{"info", slog.LevelInfo},
+		{"warn", slog.LevelWarn},
+		{"error", slog.LevelError},
+		{"unknown", slog.LevelInfo},
+		{"", slog.LevelInfo},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			if got := parseLogLevel(tt.input); got != tt.expected {
+				t.Errorf("parseLogLevel(%q) = %v, want %v", tt.input, got, tt.expected)
+			}
+		})
+	}
+}
+
+func TestConfigure(t *testing.T) {
+	cfg := Config{Level: "debug"}
+	logger := Configure(cfg)
+	if logger == nil {
+		t.Error("Configure() returned nil")
+	}
+}