What is the Advantage of sync.WaitGroup over Channels?

Question

I'm working on a concurrent Go library, and I stumbled upon two distinct patterns of synchronization between goroutines whose results are similar:

Waitgroup

package main

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

var wg sync.WaitGroup

func main() {
    words := []string{"foo", "bar", "baz"}

    for _, word := range words {
        wg.Add(1)
        go func(word string) {
            time.Sleep(1 * time.Second)
            defer wg.Done()
            fmt.Println(word)
        }(word)
    }
    // do concurrent things here

    // blocks/waits for waitgroup
    wg.Wait()
}

Channel

package main

import (
    "fmt"
    "time"
)

func main() {
    words := []string{"foo", "bar", "baz"}
    done := make(chan bool)
    // defer close(done)
    for _, word := range words {
        // fmt.Println(len(done), cap(done))
        go func(word string) {
            time.Sleep(1 * time.Second)
            fmt.Println(word)
            done <- true
        }(word)
    }
    // Do concurrent things here

    // This blocks and waits for signal from channel
    for range words {
        <-done
    }
}

I was advised that sync.WaitGroup is slightly more performant, and I have seen it being used commonly. However, I find channels more idiomatic. What is the real advantage of using sync.WaitGroup over channels and/or what might be the situation when it is better?

Answer 1

Independently of the correctness of your second example (as explained in the comments, you aren't doing what you think, but it's easily fixable), I tend to think that the first example is easier to grasp.

Now, I wouldn't even say that channels are more idiomatic. Channels being a signature feature of the Go language shouldn't mean that it is idiomatic to use them whenever possible. What is idiomatic in Go is to use the simplest and easiest to understand solution: here, the WaitGroup convey both the meaning (your main function is Waiting for workers to be done) and the mechanic (the workers notify when they are Done).

Unless you're in a very specific case, I don't recommend using the channel solution here.