YuNing's Thought

Go Channel

4 min read

It provides a connection between two goroutines, allowing them to communicate (sends value to another)

ch := make(chan int)
ch <- v    // Send v to channel ch.
v, ok := <-ch  // Receive from ch, and assign value to v, test whether a channel has been closed
ch := make(chan int, 100) // Provide the buffer length as the second argument to `make` to initialize a buffered channel:
 
// The loop `for i := range c` receives values from the channel repeatedly until it is closed.
func fibonacci(n int, c chan int) {
	x, y := 0, 1
	for i := 0; i < n; i++ {
		c <- x
		x, y = y, x+y
	}
	close(c)
}
 
func main() {
	c := make(chan int, 10)
	go fibonacci(cap(c), c)
	for i := range c {
		fmt.Println(i)
	}
}

By default (unbuffered), sends and receives block until the other side is ready. This allows goroutines to synchronize without explicit Lock or condition variables. (Go sync)

  • Note: Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic.
  • Note: Channels aren’t like files; you don’t usually need to close them. Closing is only necessary when the receiver must be told there are no more values coming, such as to terminate a range loop.

Buffered vs Unbuffered

  • Unbuffered channels block the sender until the receiver receives the data, and vice versa. So you need another Goroutine to cooperate with.
  • Buffered channels, on the other hand, are non-blocking for the sender as long as there is still room in the buffer.

Select

The select statement lets a goroutine wait on multiple communication operations (handle multiple channels).

A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.

The default case in a select is run if no other case is ready.

Set timeout by time.After

func fibonacci(c, quit chan int) {
	x, y := 0, 1
	for {
		select {
		case c <- x:
			x, y = y, x+y
		case <-quit:
			fmt.Println("quit")
			return
		}
	}
}
 
func main() {
	c := make(chan int)
	quit := make(chan int)
	go func() {
		for i := 0; i < 10; i++ {
			fmt.Println(<-c)
		}
		quit <- 0
	}()
	fibonacci(c, quit)
}
 
select {
case i := <-c:
    // use i
default:
    // receiving from c would block
}

Channel VS Mutex

  • Channel:
    • passing ownership of data
    • distributing units of work
    • communicating async results
  • Mutex
    • caches
    • state

Examples

Merge Channel

package main
 
import (
	"fmt"
	"math/rand"
	"time"
)
 
// the boring function return a channel to communicate with it.
func boring(msg string) <-chan string { // <-chan string means receives-only channel of string.
	c := make(chan string)
	go func() { // we launch goroutine inside a function.
		for i := 0; ; i++ {
			c <- fmt.Sprintf("%s %d", msg, i)
			time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
		}
 
	}()
	return c // return a channel to caller.
}
 
// <-chan string only get the receive value
// fanIn spawns 2 goroutines to reads the value from 2 channels
// then it sends to value to result channel( `c` channel)
func fanIn(c1, c2 <-chan string) <-chan string {
	c := make(chan string)
	go func() {
		for { // infinite loop to read value from channel.
			v1 := <-c1 // read value from c2. This line will wait when receiving value.
			c <- v1
		}
	}()
	go func() {
		for {
			c <- <-c2 // read value from c2 and send it to c
		}
	}()
	return c
}
 
func fanInSimple(cs ...<-chan string) <-chan string {
	c := make(chan string)
	for _, ci := range cs { // spawn channel based on the number of input channel
 
		go func(cv <-chan string) { // cv is a channel value
			for {
				c <- <-cv
			}
		}(ci) // send each channel to
 
	}
	return c
}
 
func main() {
	// merge 2 channels into 1 channel
	// c := fanIn(boring("Joe"), boring("Ahn"))
	c := fanInSimple(boring("Joe"), boring("Ahn"))
 
	for i := 0; i < 5; i++ {
		fmt.Println(<-c) // now we can read from 1 channel
	}
	fmt.Println("You're both boring. I'm leaving")
}

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