Timeout reading from a channel with select

Receiving from a channel with <- chan or for range loop blocks.

Sometimes you want to limit time waiting for a value on a channel.

It’s possible with select:

func main() {
	chResult := make(chan int, 1)

	go func() {
		time.Sleep(1 * time.Second)
		chResult <- 5
		fmt.Printf("Worker finished")
	}()

	select {
	case res := <-chResult:
		fmt.Printf("Got %d from worker\n", res)
	case <-time.After(100 * time.Millisecond):
		fmt.Printf("Timed out before worker finished\n")
	}
}

Timed out before worker finished

Let's dig into how this works:

select {
	case res := <-chResult:
		fmt.Printf("Got %d from worker\n", res)
	case <-time.After(100 * time.Millisecond):
		fmt.Printf("Timed out before worker finished\n")
}

• time.After returns a channel on which a value will be enqueued after a given time (100 milliseconds in our example). It's worth nothing that it's at least 100 ms and can be more. Let's call it a timeout channel.
• we don't care about the value read from timeout channel. We only care that value was sent on the channel
• we use select to wait on 2 channels: chResult and a timeout channel
• select finishes when receive on one of the 2 channels completes
• we either get the value on chResult before timeout expires or we receive the value from timeout channel