What is the benefit of sending to a channel by using select in Go?

Question

In the example directory of Gorilla websocket there is a file called hub.go.

https://github.com/gorilla/websocket/blob/master/examples/chat/hub.go

Here you can find a method on the type hub that does this.

func (h *hub) run() {
    for {
        select {
        case c := <-h.register:
            h.connections[c] = true
        case c := <-h.unregister:
            if _, ok := h.connections[c]; ok {
                delete(h.connections, c)
                close(c.send)
            }
        case m := <-h.broadcast:
            for c := range h.connections {
                select {
                case c.send <- m:
                default:
                    close(c.send)
                    delete(h.connections, c)
                }
            }
        }
    }
}

Why does it not just send to the c.send channel straight up in the last case like this?

case m := <-h.broadcast:
    for c := range h.connections {
        c.send <- m
    }

It tries to send and if that doesn't work out it does something else (close and delete). — Volker, Sep 21 '15 at 11:25

sisoft · Accepted Answer · 2015-09-21T12:06:02.603

6

It's method of guaranted nonblocking send to channel. In case of c.send chan can't accept new messages now, a default branch will be executed. Without select{} block sending to unbuffered or fully filled buffered channel can be blocked.

edited Sep 21 '15 at 12:06

answered Sep 21 '15 at 11:31

sisoft

953
10
22

1

It's not just useful for unbuffered channels. Select also works for buffered, closed, and nil channels. – JimB Sep 21 '15 at 11:56
1

It doesn't work for closed channels and send operation. Trying to send to closed channel will panic() in any case. But for nil and fully filled buffered channels it's true. – sisoft Sep 21 '15 at 12:02
1

I know, but this same pattern works for receives too, where you often have a closed channel. – JimB Sep 21 '15 at 13:19

score 1 · Answer 2 · answered Sep 21 '15 at 13:04

https://gobyexample.com/non-blocking-channel-operations

// Basic sends and receives on channels are blocking.
// However, we can use `select` with a `default` clause to
// implement _non-blocking_ sends, receives, and even
// non-blocking multi-way `select`s.

package main

import "fmt"

func main() {
    messages := make(chan string)
    //[...]

    // Here's a non-blocking receive. If a value is
    // available on `messages` then `select` will take
    // the `<-messages` `case` with that value. If not
    // it will immediately take the `default` case.
    select {
    case msg := <-messages:
        fmt.Println("received message", msg)
    default:
        fmt.Println("no message received")
    }

    // A non-blocking send works similarly.
    msg := "hi"
    select {
    case messages <- msg:
        fmt.Println("sent message", msg)
    default:
        fmt.Println("no message sent")
    }
    //[...]
}

And to be more clear: for a non-blocking "send", the first case will happen if there is a receiver already waiting for a message. Else it falls back to default

What is the benefit of sending to a channel by using select in Go?

2 Answers2

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