Demonstration of how to coordinate many long-running goroutines using channels

longjob.go

package main

import (
	"bufio"
	"fmt"
	"math/rand"
	"os"
	"time"
)

func main() {
	// Keeping jobs in a map so we can look them up later. We happen to
	// be using int keys, but they could be strings or whatever else.
	jobs := make(map[int]*longJob)

	// number of jobs. play with it. it can be larger than the number of
	// goroutines, but there will be blocking.
	n := 100

	// create the jobs, giving each their own chan chan int.
	for i := 0; i < n; i++ {
		job := &longJob{i, make(chan chan int, 1)}
		jobs[i] = job
	}
	fmt.Printf("Made %d channels\n", len(jobs))

	// start them all
	for _, job := range jobs {
		job.start()
	}

	// Wait for user input, and when we get it, send quit message to all
	// jobs
	bufio.NewReader(os.Stdin).ReadBytes('\n')
	rsvp := make(chan int, 1)

	// write the each job's quit channel giving them the rsvp
	// channel. they'll write their ID to it when they finish.
	for _, job := range jobs {
		job.quit <- rsvp
	}

	// listen for the right number of responses
	for i := 0; i < n; i++ {
		jobId := <-rsvp
		delete(jobs, jobId)
		fmt.Printf("\n%d: Job %d is done. %d jobs still running.\n", i, jobId, len(jobs))
	}

	fmt.Printf("\n\nAll jobs complete. Closing rsvp channel.\n")
	close(rsvp)
}

type longJob struct {
	jobId int           // which job this represents
	quit  chan chan int // when job gets msg on this channel it writes done to the channel and quits
}

func (j *longJob) start() {
	fmt.Printf("Starting job %d...\n", j.jobId)
	go j.process()
	fmt.Printf("Started job %d.\n", j.jobId)
}

func (j *longJob) process() {
	naptime := time.Duration(rand.Int63n(6000)) * time.Millisecond
	keepRunning := true
	for keepRunning {
		fmt.Printf("[%d] ", j.jobId)
		time.Sleep(naptime)
		select {
		case rsvp := <-j.quit:
			fmt.Printf("[%d quits] ", j.jobId)
			close(j.quit)
			keepRunning = false
			rsvp <- j.jobId
		default:
			// the default case is necessary, otherwise it would block on
			// the <-j.quit case for ever.
		}
	}
	fmt.Printf("[%d complete] ", j.jobId)
}