73-logging.md

This program demonstrates the use of the standard log package and the log/slog package (structured logger) in Go. Let me break down the code and provide some explanations:

package main

import (
	"bytes"
	"fmt"
	"log"
	"os"
	"log/slog"
)

func main() {
	// Standard logger usage
	log.Println("standard logger")

	// Modify standard logger flags to include microseconds
	log.SetFlags(log.LstdFlags | log.Lmicroseconds)
	log.Println("with micro")

	// Modify standard logger flags to include file and line
	log.SetFlags(log.LstdFlags | log.Lshortfile)
	log.Println("with file/line")

	// Creating a custom logger (mylog) with a prefix and specific flags
	mylog := log.New(os.Stdout, "my:", log.LstdFlags)
	mylog.Println("from mylog")

	// Changing prefix for the custom logger
	mylog.SetPrefix("ohmy:")
	mylog.Println("from mylog")

	// Creating a logger (buflog) that writes to a buffer
	var buf bytes.Buffer
	buflog := log.New(&buf, "buf:", log.LstdFlags)
	buflog.Println("hello")

	// Printing the content of the buffer
	fmt.Print("from buflog:", buf.String())

	// Using the slog package to create a structured logger (myslog)
	jsonHandler := slog.NewJSONHandler(os.Stderr, nil)
	myslog := slog.New(jsonHandler)

	// Logging messages with additional key-value pairs
	myslog.Info("hi there")
	myslog.Info("hello again", "key", "val", "age", 25)
}

Output

2024/01/25 17:41:04 standard logger
2024/01/25 17:41:04.107521 with micro
2024/01/25 17:41:04 logging.go:20: with file/line
my:2024/01/25 17:41:04 from mylog
ohmy:2024/01/25 17:41:04 from mylog
from buflog:buf:2024/01/25 17:41:04 hello
{"time":"2024-01-25T17:41:04.1080462+05:30","level":"INFO","msg":"hi there"}
{"time":"2024-01-25T17:41:04.1080462+05:30","level":"INFO","msg":"hello again","key":"val","age":25}

Explanation:

1. Standard Logger (log package):

   • The program starts by using the standard logger (log.Println) with various log messages.
   • Flags for the standard logger are modified to include microseconds and file/line information.

2. Custom Logger (log.New):

   • A custom logger (mylog) is created using log.New with a specific output (os.Stdout), prefix ("my:"), and flags.
   • The prefix is later modified to "ohmy:".

3. Buffered Logger (log.New with bytes.Buffer):

   • Another logger (buflog) is created that writes log messages to a buffer (bytes.Buffer).
   • The content of the buffer is printed to the standard output.

4. Structured Logger (log/slog package):

   • The slog package is used to create a structured logger (myslog) with a JSON formatter.
   • Log messages are provided with additional key-value pairs.

You can run the program to observe the different log messages and how they are formatted based on the logger configurations:

go run main.go