Imagine that you have to write a few emails. They are going to be long and laborious, and the best way to keep yourself entertained is to listen to music while writing them, that is, listening to music "in parallel" to writing the emails. If we wanted to write a program that simulates this scenario, the following is one possible implementation:

package main 
 
import ( 
    "fmt" 
    "sync" 
    "time" 
) 
 
func printTime(msg string) { 
    fmt.Println(msg, time.Now().Format("15:04:05")) 
} 
 
// Task that will be done over time 
func writeMail1(wg *sync.WaitGroup) { 
    printTime("Done writing mail #1.") 
    wg.Done() 
} 
func writeMail2(wg *sync.WaitGroup) { 
    printTime("Done writing mail #2.") 
    wg.Done() 
} 
func writeMail3(wg *sync.WaitGroup) { 
    printTime("Done writing mail #3.") 
    wg.Done() 
} 
 
// Task done in parallel 
func listenForever() { 
    for { 
        printTime("Listening...") 
    } 
} 
 
func main() { 
    var waitGroup sync.WaitGroup 
    waitGroup.Add(3) 
 
    go listenForever() 
 
    // Give some time for listenForever to start 
    time.Sleep(time.Nanosecond * 10) 
 
    // Let's start writing the mails 
    go writeMail1(&waitGroup) 
    go writeMail2(&waitGroup) 
    go writeMail3(&waitGroup) 
 
    waitGroup.Wait() 
} 

The output of the program might be as follows:

Done writing mail #3. 19:32:57
Listening... 19:32:57
Listening... 19:32:57
Done writing mail #1. 19:32:57
Listening... 19:32:57
Listening... 19:32:57
Done writing mail #2. 19:32:57

The numbers represent the time in terms of Hour:Minutes:Seconds and, as can be seen, they are being executed in parallel. You might have noticed that the code for parallelism looks almost identical to the code for the final concurrency example. However, in the function listenForever, we are printing Listening... in an infinite loop. If the preceding example was written without goroutines, the output would keep printing Listening... and never reach the writeMail function calls.

Now that we understand how goroutine can be used to run concurrent programs, let's look at how Go is allowing us to do this. We shall next look at the scheduler used by Go runtime.