Job queue

Distribute jobs to workers over a channel.

Job queue and dispatch

The code sending jobs (the producer) runs in its own goroutine and closes the channel when done. Closing signals "no more jobs" to the workers; their range loops end by themselves.

go func() {
    for i, yol := range dosyalar {
        isler <- Is{ID: i, Yol: yol}
    }
    close(isler)
}()

Only the sending side closes a channel; the receiving side doesn't.

Buffer size

Giving the job channel a small buffer (make(chan Is, 100)) lets the producer run a bit ahead of the workers and smooths short waits. But a buffer isn't a concurrency fix; a correct program must also work unbuffered. A huge buffer consumes memory and hides backpressure.

Exercise Dispatch

In a separate goroutine send the files []string{"a", "bb", "ccc"} to the isler channel and close it. Three workers send the length of each path to sonuclar. main reads the three results and prints the total.

Show example solution

package main

import "fmt"

func main() {
    dosyalar := []string{"a", "bb", "ccc"}
    isler := make(chan string)
    sonuclar := make(chan int)
    for w := 0; w < 3; w++ {
        go func() {
            for yol := range isler {
                sonuclar <- len(yol)
            }
        }()
    }
    go func() {
        for _, d := range dosyalar {
            isler <- d
        }
        close(isler)
    }()
    toplam := 0
    for i := 0; i < len(dosyalar); i++ {
        toplam += <-sonuclar
    }
    fmt.Println(toplam)
}

Test yourself

Why does the producer run in its own goroutine?
- Because closing a channel requires it
- To catch errors
- So sending on an unbuffered channel doesn't deadlock before results are read
- So it compiles faster
Show answer

So sending on an unbuffered channel doesn't deadlock before results are read — If main both sends and reads results, workers may wait on the result channel while main waits on the job channel; a separate producer breaks this cycle.

Signal

for _, is := range liste {
    isler <- is
}
____(isler)

Show answer

for _, is := range liste {
    isler <- is
}
close(isler)

Who does what?

Producer

Sends jobs and closes
Worker

Reads with range
Arabellek

Smooths short waits
close

"No more jobs" signal

Next lesson: Collecting results

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