Golang : Wait and sync.WaitGroup example

There are time when we need to get multiple things done concurrently first and make sure the processes result are synchronized before moving on to the next process. This can be done with the Sync package http://golang.org/pkg/sync/ in Golang.

For example, a template HTML page must wait for the database queries to be completed before being rendered to PDF and write to http.ResponseWriter.

This example below will simulate a waitgroup with some time delay. You can modify it to implement your own waitgroup for your code.

 package main

 import (
 "fmt"
 "sync"
 "time"
 )

 func f(w *sync.WaitGroup, sec int) {
 fmt.Println("Time : ", time.Now())
 time.Sleep(time.Duration(sec) * time.Second)
 fmt.Println("Time : ", time.Now())
 w.Done()
 fmt.Println("Wait completed")
 }

 func main() {

 // send query to database and wait

 fmt.Println("Starting to wait")

 var wg sync.WaitGroup

 wg.Add(2) // for example, wait for 2 concurrent queries before this wait group is done

 go func(swg *sync.WaitGroup, sec int) {
 fmt.Println("Time : ", time.Now())
 <-time.After(time.Duration(sec) * time.Millisecond)
 swg.Done()
 }(&wg, 2500)

 go f(&wg, 4) // simulate a concurrent wait 4 second

 wg.Wait()

 fmt.Println("Finished")
 }

References :

https://www.socketloop.com/tutorials/golang-how-to-stream-file-to-client-browser-or-write-to-http-responsewriter

http://golang.org/pkg/sync/