Alex

Select (Video 24)

• The select statement is used to multiplex channels, allowing any ready alternative to proceed among:
  • A channel we can read from
  • A channel we can write to
  • A default action that is always ready
• Selects are used to compose channels as synchronisation primitives, things like mutexes can’t be composed
• Selects often tend to be run in a loop so that we can keep trying
• It is common to put a timeout or “done” channel into a select
• Example:

func main() {
  chans := []chan int{
    make(chan int),
    make(chan int)
  }

  for i := range chans {
    go func(i int, ch chan<- int) {
      for {
        time.Sleep(time.Duration(i)*time.Second)
        ch<- i
      }
    }(i+1, chans[i])
  }

  for i := 0; i < 12; i ++ {
    // Select allows us to listen to both channels at the same time, and whichever
    // one is ready first will be read
    select {
    case m0 := <-chans[0]:
      fmt.Println("received", m0)
    case m1 := <-chans[1]:
      fmt.Println("received", m1)
    }
  }
}

• As mentioned a common pattern is to have a stopper channel that will be a separate select case that will trigger the select to stop waiting on any stuck channels
• We can do this using the time.After function
• time.After(5*time.Second) will create a new timer, this timer has a channel which is returned by this function. The timer will send the current time on this channel when the timer elapses
• This can be used in a select statement to trigger a timeout scenario for example
• There also exists time.Ticker which is similar but will tick indefinitely with a given tick rate

Default Case

• In select blocks, the default case is already and is chosen if no other case is ready
• You shouldn’t put a default in a select in a loop - this will cause busy waiting and high CPU usage
• An example pattern of using a default:

func sendOrDrop(data []byte) {
  select {
  case ch <- data;
    // sent ok; do nothing
  default:
    log.Printf("overflow, dropped %d bytes", len(data))
  }
}

• In this example if the channel is ready to receive data, it will send the data as expected
• In the case that the channel can’t receive data, the default case is run and a log message is sent