Logging

Create a logger with Go’s default *log.Logger package to provide feedback for background processes or error tracking. Go’s default logger is concurrency-safe, so you can use a single logger across multiple goroutines without worry.

The following code creates two leveled loggers: one that logs information messages to the console, and one that sends errors to STDERR:

	infoLog := log.New(os.Stdout, "INFO\t", log.Ldate|log.Ltime)
	errorLog := log.New(os.Stderr, "ERROR\t", log.Ldate|log.Ltime|log.Lshortfile)

The New() function accepts the following arguments:

• An io.Writer where you write the logs.
• Prefix for the log message.
• One or more log package flag constants. These constants include contextual information in the log message, such as the date (Ldate), the time (Ltime), and the file and line number where the error occurred (Lshortfile). You can OR multiple flags together.

Log to a file

By default, Go’s log library sends messages to STDERR, but you can configure it to write to a file. It adds the date and time to each log entry, and you can add a prefix to the string to help searchability

f, err := os.OpenFile("/path/to/logfile.log", os.O_RDWR|os.O_CREATE, 0666)
if err != nil {
    log.Fatal(err)
}
defer f.Close()

l := log.New(f, "LOGGER PREFIX: ", log.LstdFlags)

log.LstdFlags uses the default log flags, such as date and time.

Add logger to server

You can create an error logger and then assign it to a custom server’s ErrorLog field:

errorLog := log.New(os.Stderr, "ERROR\t", log.Ldate|log.Ltime|log.Lshortfile)

srv := &http.Server{
    ...
    ErrorLog: errorLog,
    ...
}