Channel Use Cases

• asynchronous and concurrency programming with Go channels is easy and enjoyable.
• the channel synchronization technique has a wider range of uses and has more variations than the synchronization solutions used in some other languages, such as the actor model and the async/await pattern.

Use Channels as Futures/Promises

Return receive-only channels as results

package main

import (
	"time"
	"math/rand"
	"fmt"
)

func longTimeRequest() <-chan int32 {
	r := make(chan int32)

	go func() {
		// Simulate a workload.
		time.Sleep(time.Second * 3)
		r <- rand.Int31n(100)
	}()

	return r
}

func sumSquares(a, b int32) int32 {
	return a*a + b*b
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	a, b := longTimeRequest(), longTimeRequest()
	fmt.Println(sumSquares(<-a, <-b))
}

Pass send-only channels as arguments

package main

import (
	"time"
	"math/rand"
	"fmt"
)

func longTimeRequest(r chan<- int32)  {
	// Simulate a workload.
	time.Sleep(time.Second * 3)
	r <- rand.Int31n(100)
}

func sumSquares(a, b int32) int32 {
	return a*a + b*b
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	ra, rb := make(chan int32), make(chan int32)
	go longTimeRequest(ra)
	go longTimeRequest(rb)

	fmt.Println(sumSquares(<-ra, <-rb))
}

...

	// The channel can be buffered or not.
	results := make(chan int32, 2)
	go longTimeRequest(results)
	go longTimeRequest(results)

	fmt.Println(sumSquares(<-results, <-results))
}

The first response wins

package main

import (
	"fmt"
	"time"
	"math/rand"
)

func source(c chan<- int32) {
	ra, rb := rand.Int31(), rand.Intn(3) + 1
	// Sleep 1s/2s/3s.
	time.Sleep(time.Duration(rb) * time.Second)
	c <- ra
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	startTime := time.Now()
	// c must be a buffered channel.
	c := make(chan int32, 5)
	for i := 0; i < cap(c); i++ {
		go source(c)
	}
	// Only the first response will be used.
	rnd := <- c
	fmt.Println(time.Since(startTime))
	fmt.Println(rnd)
}

More request-response variants

Use Channels for Notifications

1-to-1 notification by sending a value to a channel

package main

import (
	"crypto/rand"
	"fmt"
	"os"
	"sort"
)

func main() {
	values := make([]byte, 32 * 1024 * 1024)
	if _, err := rand.Read(values); err != nil {
		fmt.Println(err)
		os.Exit(1)
	}

	done := make(chan struct{}) // can be buffered or not

	// The sorting goroutine
	go func() {
		sort.Slice(values, func(i, j int) bool {
			return values[i] < values[j]
		})
		// Notify sorting is done.
		done <- struct{}{}
	}()

	// do some other things ...

	<- done // waiting here for notification
	fmt.Println(values[0], values[len(values)-1])
}

1-to-1 notification by receiving a value from a channel

package main

import (
	"fmt"
	"time"
)

func main() {
	done := make(chan struct{})
		// The capacity of the signal channel can
		// also be one. If this is true, then a
		// value must be sent to the channel before
		// creating the following goroutine.

	go func() {
		fmt.Print("Hello")
		// Simulate a workload.
		time.Sleep(time.Second * 2)

		// Receive a value from the done
		// channel, to unblock the second
		// send in main goroutine.
		<- done
	}()

	// Blocked here, wait for a notification.
	done <- struct{}{}
	fmt.Println(" world!")
}

N-to-1 and 1-to-N notifications

package main

import "log"
import "time"

type T = struct{}

func worker(id int, ready <-chan T, done chan<- T) {
	<-ready // block here and wait a notification
	log.Print("Worker#", id, " starts.")
	// Simulate a workload.
	time.Sleep(time.Second * time.Duration(id+1))
	log.Print("Worker#", id, " job done.")
	// Notify the main goroutine (N-to-1),
	done <- T{}
}

func main() {
	log.SetFlags(0)

	ready, done := make(chan T), make(chan T)
	go worker(0, ready, done)
	go worker(1, ready, done)
	go worker(2, ready, done)

	// Simulate an initialization phase.
	time.Sleep(time.Second * 3 / 2)
	// 1-to-N notifications.
	ready <- T{}; ready <- T{}; ready <- T{}
	// Being N-to-1 notified.
	<-done; <-done; <-done
}

Broadcast (1-to-N) notifications by closing a channel

...
	close(ready) // broadcast notifications
...

Timer: scheduled notification

package main

import (
	"fmt"
	"time"
)

func AfterDuration(d time.Duration) <- chan struct{} {
	c := make(chan struct{}, 1)
	go func() {
		time.Sleep(d)
		c <- struct{}{}
	}()
	return c
}

func main() {
	fmt.Println("Hi!")
	<- AfterDuration(time.Second)
	fmt.Println("Hello!")
	<- AfterDuration(time.Second)
	fmt.Println("Bye!")
}

Use Channels as Mutex Locks

1. Lock through a send, unlock through a receive.
2. Lock through a receive, unlock through a send.

package main

import "fmt"

func main() {
	// The capacity must be one.
	mutex := make(chan struct{}, 1)

	counter := 0
	increase := func() {
		mutex <- struct{}{} // lock
		counter++
		<-mutex // unlock
	}

	increase1000 := func(done chan<- struct{}) {
		for i := 0; i < 1000; i++ {
			increase()
		}
		done <- struct{}{}
	}

	done := make(chan struct{})
	go increase1000(done)
	go increase1000(done)
	<-done; <-done
	fmt.Println(counter) // 2000
}

...
func main() {
	mutex := make(chan struct{}, 1)
	mutex <- struct{}{} // this line is needed.

	counter := 0
	increase := func() {
		<-mutex // lock
		counter++
		mutex <- struct{}{} // unlock
	}
...

Use Channels as Counting Semaphores

1. Acquire ownership through a send, release through a receive.
2. Acquire ownership through a receive, release through a send.

package main

import (
	"log"
	"time"
	"math/rand"
)

type Seat int
type Bar chan Seat

func (bar Bar) ServeCustomer(c int) {
	log.Print("customer#", c, " enters the bar")
	seat := <- bar // need a seat to drink
	log.Print("++ customer#", c, " drinks at seat#", seat)
	time.Sleep(time.Second * time.Duration(2 + rand.Intn(6)))
	log.Print("-- customer#", c, " frees seat#", seat)
	bar <- seat // free seat and leave the bar
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	// the bar has 10 seats.
	bar24x7 := make(Bar, 10)
	// Place seats in an bar.
	for seatId := 0; seatId < cap(bar24x7); seatId++ {
		// None of the sends will block.
		bar24x7 <- Seat(seatId)
	}

	for customerId := 0; ; customerId++ {
		time.Sleep(time.Second)
		go bar24x7.ServeCustomer(customerId)
	}

	// sleeping != blocking
	for {time.Sleep(time.Second)}
}

... // same code as the above example

func (bar Bar) ServeCustomerAtSeat(c int, seat Seat) {
	log.Print("++ customer#", c, " drinks at seat#", seat)
	time.Sleep(time.Second * time.Duration(2 + rand.Intn(6)))
	log.Print("-- customer#", c, " frees seat#", seat)
	bar <- seat // free seat and leave the bar
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	bar24x7 := make(Bar, 10)
	for seatId := 0; seatId < cap(bar24x7); seatId++ {
		bar24x7 <- Seat(seatId)
	}

	for customerId := 0; ; customerId++ {
		time.Sleep(time.Second)
		// Need a seat to serve next customer.
		seat := <- bar24x7
		go bar24x7.ServeCustomerAtSeat(customerId, seat)
	}
	for {time.Sleep(time.Second)}
}

... // same code as the above example

func (bar Bar) ServeCustomerAtSeat(consumers chan int) {
	for c := range consumers {
		seatId := <- bar
		log.Print("++ customer#", c, " drinks at seat#", seatId)
		time.Sleep(time.Second * time.Duration(2 + rand.Intn(6)))
		log.Print("-- customer#", c, " frees seat#", seatId)
		bar <- seatId // free seat and leave the bar
	}
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	bar24x7 := make(Bar, 10)
	for seatId := 0; seatId < cap(bar24x7); seatId++ {
		bar24x7 <- Seat(seatId)
	}

	consumers := make(chan int)
	for i := 0; i < cap(bar24x7); i++ {
		go bar24x7.ServeCustomerAtSeat(consumers)
	}
	
	for customerId := 0; ; customerId++ {
		time.Sleep(time.Second)
		consumers <- customerId
	}
}

... // same code as the above example

func ServeCustomer(consumers chan int) {
	for c := range consumers {
		log.Print("++ customer#", c, " drinks at the bar")
		time.Sleep(time.Second * time.Duration(2 + rand.Intn(6)))
		log.Print("-- customer#", c, " leaves the bar")
	}
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	const BarSeatCount = 10
	consumers := make(chan int)
	for i := 0; i < BarSeatCount; i++ {
		go ServeCustomer(consumers)
	}
	
	for customerId := 0; ; customerId++ {
		time.Sleep(time.Second)
		consumers <- customerId
	}
}

package main

import (
	"log"
	"time"
	"math/rand"
)

type Customer struct{id int}
type Bar chan Customer

func (bar Bar) ServeCustomer(c Customer) {
	log.Print("++ customer#", c.id, " starts drinking")
	time.Sleep(time.Second * time.Duration(3 + rand.Intn(16)))
	log.Print("-- customer#", c.id, " leaves the bar")
	<- bar // leaves the bar and save a space
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	// The bar can serve most 10 customers
	// at the same time.
	bar24x7 := make(Bar, 10)
	for customerId := 0; ; customerId++ {
		time.Sleep(time.Second * 2)
		customer := Customer{customerId}
		// Wait to enter the bar.
		bar24x7 <- customer
		go bar24x7.ServeCustomer(customer)
	}
	for {time.Sleep(time.Second)}
}

Dialogue (Ping-Pong)

package main

import "fmt"
import "time"
import "os"

type Ball uint64

func Play(playerName string, table chan Ball) {
	var lastValue Ball = 1
	for {
		ball := <- table // get the ball
		fmt.Println(playerName, ball)
		ball += lastValue
		if ball < lastValue { // overflow
			os.Exit(0)
		}
		lastValue = ball
		table <- ball // bat back the ball
		time.Sleep(time.Second)
	}
}

func main() {
	table := make(chan Ball)
	go func() {
		table <- 1 // throw ball on table
	}()
	go Play("A:", table)
	Play("B:", table)
}

Channel Encapsulated in Channel

package main

import "fmt"

var counter = func (n int) chan<- chan<- int {
	requests := make(chan chan<- int)
	go func() {
		for request := range requests {
			if request == nil {
				n++ // increase
			} else {
				request <- n // take out
			}
		}
	}()

	// Implicitly converted to chan<- (chan<- int)
	return requests
}(0)

func main() {
	increase1000 := func(done chan<- struct{}) {
		for i := 0; i < 1000; i++ {
			counter <- nil
		}
		done <- struct{}{}
	}

	done := make(chan struct{})
	go increase1000(done)
	go increase1000(done)
	<-done; <-done

	request := make(chan int, 1)
	counter <- request
	fmt.Println(<-request) // 2000
}

Check Lengths and Capacities of Channels

// Assume the current goroutine is the only
// goroutine tries to receive values from
// the channel c at present.
for len(c) > 0 {
	value := <-c
	// use value ...
}

for len(c) < cap(c) {
	c <- aValue
}

Block the Current Goroutine Forever

package main

import "runtime"

func DoSomething() {
	for {
		// do something ...

		runtime.Gosched() // avoid being greedy
	}
}

func main() {
	go DoSomething()
	go DoSomething()
	select{}
}

Try-Send and Try-Receive

• If the operation following the case keyword is a send operation, then the select block is called as try-send operation. If the send operation would block, then the default branch will get executed (fail to send), otherwise, the send succeeds and the only case branch will get executed.
• If the operation following the case keyword is a receive operation, then the select block is called as try-receive operation. If the receive operation would block, then the default branch will get executed (fail to receive), otherwise, the receive succeeds and the only case branch will get executed.

package main

import "fmt"

func main() {
	type Book struct{id int}
	bookshelf := make(chan Book, 3)

	for i := 0; i < cap(bookshelf) * 2; i++ {
		select {
		case bookshelf <- Book{id: i}:
			fmt.Println("succeeded to put book", i)
		default:
			fmt.Println("failed to put book")
		}
	}

	for i := 0; i < cap(bookshelf) * 2; i++ {
		select {
		case book := <-bookshelf:
			fmt.Println("succeeded to get book", book.id)
		default:
			fmt.Println("failed to get book")
		}
	}
}

succeed to put book 0
succeed to put book 1
succeed to put book 2
failed to put book
failed to put book
failed to put book
succeed to get book 0
succeed to get book 1
succeed to get book 2
failed to get book
failed to get book
failed to get book

Check if a channel is closed without blocking the current goroutine

func IsClosed(c chan T) bool {
	select {
	case <-c:
		return true
	default:
	}
	return false
}

Peak/burst limiting

...
	// Can serve most 10 customers at the same time
	bar24x7 := make(Bar, 10)
	for customerId := 0; ; customerId++ {
		time.Sleep(time.Second)
		customer := Consumer{customerId}
		select {
		case bar24x7 <- customer: // try to enter the bar
			go bar24x7.ServeConsumer(customer)
		default:
			log.Print("customer#", customerId, " goes elsewhere")
		}
	}
...

Another way to implement the first-response-wins use case

package main

import (
	"fmt"
	"math/rand"
	"time"
)

func source(c chan<- int32) {
	ra, rb := rand.Int31(), rand.Intn(3)+1
	// Sleep 1s, 2s or 3s.
	time.Sleep(time.Duration(rb) * time.Second)
	select {
	case c <- ra:
	default:
	}
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	// The capacity should be at least 1.
	c := make(chan int32, 1)
	for i := 0; i < 5; i++ {
		go source(c)
	}
	rnd := <-c // only the first response is used
	fmt.Println(rnd)
}

The third way to implement the first-response-wins use case

package main

import (
	"fmt"
	"math/rand"
	"time"
)

func source() <-chan int32 {
	// c must be a buffered channel.
	c := make(chan int32, 1)
	go func() {
		ra, rb := rand.Int31(), rand.Intn(3)+1
		time.Sleep(time.Duration(rb) * time.Second)
		c <- ra
	}()
	return c
}

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	var rnd int32
	// Blocking here until one source responses.
	select{
	case rnd = <-source():
	case rnd = <-source():
	case rnd = <-source():
	}
	fmt.Println(rnd)
}

Timeout

func requestWithTimeout(timeout time.Duration) (int, error) {
	c := make(chan int)
	// May need a long time to get the response.
	go doRequest(c)

	select {
	case data := <-c:
		return data, nil
	case <-time.After(timeout):
		return 0, errors.New("timeout")
	}
}

Ticker

package main

import "fmt"
import "time"

func Tick(d time.Duration) <-chan struct{} {
	// The capacity of c is best set as one.
	c := make(chan struct{}, 1)
	go func() {
		for {
			time.Sleep(d)
			select {
			case c <- struct{}{}:
			default:
			}
		}
	}()
	return c
}

func main() {
	t := time.Now()
	for range Tick(time.Second) {
		fmt.Println(time.Since(t))
	}
}

Rate Limiting

package main

import "fmt"
import "time"

type Request interface{}
func handle(r Request) {fmt.Println(r.(int))}

const RateLimitPeriod = time.Minute
const RateLimit = 200 // most 200 requests in one minute

func handleRequests(requests <-chan Request) {
	quotas := make(chan time.Time, RateLimit)

	go func() {
		tick := time.NewTicker(RateLimitPeriod / RateLimit)
		defer tick.Stop()
		for t := range tick.C {
			select {
			case quotas <- t:
			default:
			}
		}
	}()

	for r := range requests {
		<-quotas
		go handle(r)
	}
}

func main() {
	requests := make(chan Request)
	go handleRequests(requests)
	// time.Sleep(time.Minute)
	for i := 0; ; i++ {requests <- i}
}

Switches

package main

import "fmt"
import "time"
import "os"

type Ball uint8
func Play(playerName string, table chan Ball, serve bool) {
	var receive, send chan Ball
	if serve {
		receive, send = nil, table
	} else {
		receive, send = table, nil
	}
	var lastValue Ball = 1
	for {
		select {
		case send <- lastValue:
		case value := <- receive:
			fmt.Println(playerName, value)
			value += lastValue
			if value < lastValue { // overflow
				os.Exit(0)
			}
			lastValue = value
		}
		// Switch on/off.
		receive, send = send, receive
		time.Sleep(time.Second)
	}
}

func main() {
	table := make(chan Ball)
	go Play("A:", table, false)
	Play("B:", table, true)
}

package main

import "fmt"
import "time"

func main() {
	for c := make(chan struct{}, 1); true; {
		select {
		case c <- struct{}{}:
			fmt.Print("1")
		case <-c:
			fmt.Print("2")
		}
		time.Sleep(time.Second)
	}
}

Control code execution possibility weights

package main

import "fmt"

func main() {
	foo, bar := make(chan struct{}), make(chan struct{})
	close(foo); close(bar) // for demo purpose
	x, y := 0.0, 0.0
	f := func(){x++}
	g := func(){y++}
	for i := 0; i < 100000; i++ {
		select {
		case <-foo: f()
		case <-foo: f()
		case <-bar: g()
		}
	}
	fmt.Println(x/y) // about 2
}

Select from dynamic number of cases

Data Flow Manipulations

• data generation/collecting/loading.
• data serving/saving.
• data calculation/analysis.
• data validation/filtering.
• data aggregation/division
• data composition/decomposition.
• data duplication/proliferation.

Data generation/collecting/loading

• by loading a file, reading a database, or crawling the web.
• by collecting all kinds of metrics from a software system or all kinds of hardware.
• by generating random numbers.
• etc.

import (
	"crypto/rand"
	"encoding/binary"
)

func RandomGenerator() <-chan uint64 {
	c := make(chan uint64)
	go func() {
		rnds := make([]byte, 8)
		for {
			_, err := rand.Read(rnds)
			if err != nil {
				close(c)
				break
			}
			c <- binary.BigEndian.Uint64(rnds)
		}
	}()
	return c
}

Data aggregation

func Aggregator(inputs ...<-chan uint64) <-chan uint64 {
	out := make(chan uint64)
	for _, in := range inputs {
		go func(in <-chan uint64) {
			for {
				out <- <-in // <=> out <- (<-in)
			}
		}(in)
	}
	return out
}

import "sync"

func Aggregator(inputs ...<-chan uint64) <-chan uint64 {
	output := make(chan uint64)
	var wg sync.WaitGroup
	for _, in := range inputs {
		wg.Add(1)
		go func(int <-chan uint64) {
			defer wg.Done()
			// If in is closed, then the
			// loop will ends eventually.
			for x := range in {
				output <- x
			}
		}(in)
	}
	go func() {
		wg.Wait()
		close(output)
	}()
	return output
}

// Assume the number of input stream is two.
...
	output := make(chan uint64)
	go func() {
		inA, inB := inputs[0], inputs[1]
		for {
			select {
			case v := <- inA: output <- v
			case v := <- inB: output <- v
			}
		}
	}
...

Data division

func Divisor(input <-chan uint64, outputs ...chan<- uint64) {
	for _, out := range outputs {
		go func(o chan<- uint64) {
			for {
				o <- <-input // <=> o <- (<-input)
			}
		}(out)
	}
}

Data composition

func Composer(inA, inB <-chan uint64) <-chan uint64 {
	output := make(chan uint64)
	go func() {
		for {
			a1, b, a2 := <-inA, <-inB, <-inA
			output <- a1 ^ b & a2
		}
	}()
	return output
}

Data decomposition

Data duplication/proliferation

func Duplicator(in <-chan uint64) (<-chan uint64, <-chan uint64) {
	outA, outB := make(chan uint64), make(chan uint64)
	go func() {
		for x := range in {
			outA <- x
			outB <- x
		}
	}()
	return outA, outB
}

Data calculation/analysis

func Calculator(in <-chan uint64, out chan uint64) (<-chan uint64) {
	if out == nil {
		out = make(chan uint64)
	}
	go func() {
		for x := range in {
			out <- ^x
		}
	}()
	return out
}

Data validation/filtering

import "math/big"

func Filter0(input <-chan uint64, output chan uint64) <-chan uint64 {
	if output == nil {
		output = make(chan uint64)
	}
	go func() {
		bigInt := big.NewInt(0)
		for x := range input {
			bigInt.SetUint64(x)
			if bigInt.ProbablyPrime(1) {
				output <- x
			}
		}
	}()
	return output
}

func Filter(input <-chan uint64) <-chan uint64 {
	return Filter0(input, nil)
}

Data serving/saving

import "fmt"

func Printer(input <-chan uint64) {
	for x := range input {
		fmt.Println(x)
	}
}

Data flow system assembling

package main

... // the worker functions declared above.

func main() {
	Printer(
		Filter(
			Calculator(
				RandomGenerator(), nil,
			),
		),
	)
}

package main

... // the worker functions declared above.

func main() {
	filterA := Filter(RandomGenerator())
	filterB := Filter(RandomGenerator())
	filterC := Filter(RandomGenerator())
	filter := Aggregator(filterA, filterB, filterC)
	calculatorA := Calculator(filter, nil)
	calculatorB := Calculator(filter, nil)
	calculator := Aggregator(calculatorA, calculatorB)
	Printer(calculator)
}

package main

... // the worker functions declared above.

func main() {
	c1 := make(chan uint64, 100)
	Filter0(RandomGenerator(), c1) // filterA
	Filter0(RandomGenerator(), c1) // filterB
	Filter0(RandomGenerator(), c1) // filterC
	c2 := make(chan uint64, 100)
	Calculator(c1, c2) // calculatorA
	Calculator(c1, c2) // calculatorB
	Printer(c2)
}

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