the_art_of_select_go_channle.md

Golang Channle

1. 队列满则保存到backend

如果当前的channel的memoryChan已经满了，则默认写入到backend的缓存中

func (c *Channel) put(m *Message) error {
	select {
	case c.memoryMsgChan <- m: // 当前缓存队列未满，则放入
	default:                   // 如果队列满，则保存到backend
		b := bufferPoolGet()                         
		err := writeMessageToBackend(b, m, c.backend)
		bufferPoolPut(b)
		c.ctx.nsqd.SetHealth(err)
		if err != nil {
			c.ctx.nsqd.logf("CHANNEL(%s) ERROR: failed to write message to backend - %s",
				c.name, err)
			return err
		}
	}
	return nil
}

2. Polling Channle

select {
case <-exit:
	fmt.Println("It's done")

default:
	// do nothing
}

3. Polling Timeout

tick := time.Tick(1*time.Second)
select {
case <-tick:
	fmt.Println("Timeout")

case <- exit:
	fmt.Println("Exit")
	
// do nothing
}

4. 通过Channel实现随机概率分发

select {
		case b := <-backendMsgChan:
		if sampleRate > 0 && rand.Int31n(100) > sampleRate {
			continue
		} 
}

5. A trick

ch := make(chan int, 1)

for i := 0; i < 10; i++ {
	select {
	case x:= <-ch:
		fmt.Println(x) // 0 2 4 6 8

	case ch<-i:
	}
}

6. Range on Closed Channel

channel 关闭会导致range返回

 queue := make(chan string, 2)
queue <- "one"
queue <- "two"
close(queue)

for elem := range queue {
    fmt.Println(elem)
}

7. Check Closed Channel

queue := make(chan int, 1)

value, ok := <-queue
if !ok {
	fmt.Println("queue is closed")
}

8. Select skip nil Channel

for {
    select {
    case x, ok := <-ch: // if ch is nil, just skip
        fmt.Println("ch1", x, ok)
        if !ok {
            ch = nil
        }
}