Previously in Golang Concurrency we talked about how goroutines in Go language works. Today we will focus on providing robustness to our previous talk by using semaphores from Golang sync package.

Here is redone example from our previous blogpost

package main


var wait sync.WaitGroup // This assign a WaitGroup, to be used later

func Printer(a int){
  defer wait.Done() // Done signals that this gorutine is done doing work, decrementing semaphore counter
  time.Sleep(time.Millisecond * time.Duration(rand.Intn(100)))
  fmt.Println("a is: ", a)

func main(){
  max_routine := 10
  wait.Add(max_routine) // This adds maximum counter to waithgroup should wait before terminating main program
  fmt.Println("At start")
  for i := 0; i < max_routine; i++ {
    fmt.Println("Sending to routine: ", i)
    go Printer(i)
  wait.Wait() // This waits for semaphore counter to be 0 and then terminates current program
  fmt.Println("At end!")

// time.Duration(rand.Intn(100)) will make goroutine sleep for varying time
//At the start
//Sending to routine:  0
//Sending to routine:  1
//Sending to routine:  2
//Sending to routine:  3
//Sending to routine:  4
//Sending to routine:  5
//Sending to routine:  6
//Sending to routine:  7
//Sending to routine:  8
//Sending to routine:  9
//a is:  9
//a is:  5
//a is:  6
//a is:  7
//a is:  2
//a is:  8
//a is:  3
//a is:  0
//a is:  4
//a is:  1
//At the end!

Results may vary from machine to machine as. But in essence what we did is, we imported sync package. sync package contains WaitGroup type, we declared variable wait to be of WaitGroup type. Waitgroup provides three methods namely Add, Done and Wait.

1.) Add method takes integer as a parameter delta to the counter, and once counter becomes zero, all blocking goroutines, waiting onto counter would be released. We provided no of goroutines we are going to wait upon

2.) Wait methods waits for semaphore counter to become zero, before releasing goroutines, waiting onto other goroutines to terminate/finish. Our main program is waiting for other goroutines to finish.

3.) Done method basically indicates that current goroutine, in which this is being called is done/finished, while decrementing semaphore counter. Our goroutines calls to Done method indicating that goroutine is done executing its task.

Happy Going!

About The Author

I am Pankaj Baagwan, a System Design Architect. A Computer Scientist by heart, process enthusiast, and open source author/contributor/writer. Advocates Karma. Love working with cutting edge, fascinating, open source technologies.

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Stay tuned <3. Signing off for RAAM