Correct approach to global logging in Golang

• Create a single log.Logger and pass it around?

That is possible. A log.Logger can be used concurrently from multiple goroutines.

• Pass around a pointer to that log.Logger?

log.New returns a *Logger which is usually an indication that you should pass the object around as a pointer. Passing it as value would create a copy of the struct (i.e. a copy of the Logger) and then multiple goroutines might write to the same io.Writer concurrently. That might be a serious problem, depending on the implementation of the writer.

• Should each goroutine or function create a logger?

I wouldn't create a separate logger for each function or goroutine. Goroutines (and functions) are used for very lightweight tasks that will not justify the maintenance of a separate logger. It's probably a good idea to create a logger for each bigger component of your project. For example, if your project uses a SMTP service for sending mails, creating a separate logger for the mail service sounds like a good idea so that you can filter and turn off the output separately.

• Should I create the logger as a global variable?

That depends on your package. In the previous mail service example, it would be probably a good idea to have one logger for each instance of your service, so that users can log failures while using the gmail mail service differently than failures that occured while using the local MTA (e.g. sendmail).

For simple cases, there is a global logger defined in the log package, log.Logger. This global logger can be configured through log.SetFlags.

Afterwards one can just call the top level functions of the log package like log.Printf and log.Fatalf, which use that global instance.

This is a simple logger

package customlogger

import (
    "log"
    "os"
    "sync"
)

type logger struct {
    filename string
    *log.Logger
}

var logger *logger
var once sync.Once

// start loggeando
func GetInstance() *logger {
    once.Do(func() {
        logger = createLogger("mylogger.log")
    })
    return logger
}

func createLogger(fname string) *logger {
    file, _ := os.OpenFile(fname, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0777)

    return &logger{
        filename: fname,
        Logger:   log.New(file, "My app Name ", log.Lshortfile),
    }
}

You can use it in this way

package main

import (
    "customlogger"
    "fmt"
    "net/http"
)

func main() {
    logger := customlogger.GetInstance()
    logger.Println("Starting")

    http.HandleFunc("/", sroot)
    http.ListenAndServe(":8080", nil)
}

func sroot(w http.ResponseWriter, r *http.Request) {
    logger := customlogger.GetInstance()

    fmt.Fprintf(w, "welcome")
    logger.Println("Starting")
}