Logging to Files

Logging is essential for monitoring the behavior of an application in development and production environments. This snippet demonstrates how to perform logging to files in Go using the standard library.

Basic File Logging

Here's a simple example that shows how to log messages to a file using the built-in log package:

package main

import (
	"log"
	"os"
)

func main() {
	// Open file for writing logs.
	f, err := os.OpenFile("app.log", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0666)
	if err != nil {
		log.Fatalf("Error opening file: %v", err)
	}
	defer f.Close()

	// Set output of logs to file.
	log.SetOutput(f)

	// Example log entries.
	log.Println("This is an info message")
	log.Println("This is another log entry")
}

Customizing Logger Output

The log package allows you to customize the logger output format:

package main

import (
	"log"
	"os"
)

func main() {
	f, err := os.OpenFile("custom.log", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0666)
	if err != nil {
		log.Fatalf("Error opening file: %v", err)
	}
	defer f.Close()

	logger := log.New(f, "CUSTOM PREFIX: ", log.Ldate|log.Ltime|log.Lshortfile)
	logger.Println("This is a log entry with a custom prefix and details")
}

Rotating Log Files

For log rotation, consider using popular libraries like lumberjack:

package main

import (
	"log"

	"gopkg.in/natefinch/lumberjack.v2"
)

func main() {
	logger := &lumberjack.Logger{
		Filename:   "rotating.log",
		MaxSize:    5,  // megabytes
		MaxBackups: 3,
		MaxAge:     28, //days
	}
	log.SetOutput(logger)

	// Example log entry.
	log.Println("This is an entry in a rotating log file")
}

Best Practices

• Use structured logging with consistent formats for easier parsing and analysis.
• Implement log rotation using libraries like lumberjack to prevent log files from consuming too much disk space.
• Always use defer f.Close() when opening log files to ensure the file is closed when done.

Common Pitfalls

• Not setting a proper file mode when opening a log file can lead to permission issues.
• Forgetting to set the logger's output to the file can lead to logs being written to stderr instead.
• Ignoring errors from file operations can lead to silent failures in log file writes.

Performance Tips

• Adjust log.SetFlags to control the granularity of each log entry for performance tuning.
• Use a buffered writer to improve IO performance if logging is intensive.
• Minimize log entry size during critical operations to reduce latency.