Second Golang Meetup - Go Channels 101

gistfile1.txt

# Go Channels 

Dennis - [email protected] or @denderello
Stephan - [email protected] or @zeisss_public


> "Don't communicate by sharing memory, share memory by communicating." -- Rob Pike


## Problem

Unsynchronized access can corrupt internal states of variables, 
e.g. maps, slices etc.

Solution:
* Synchronise access via Mutex or
* Use channels


## Channels

Used for communication between Goroutines.

```
# Create a channel (can be any type)
c := make(chan int)
# Write a value (blocking!)
c <- 1
# Read it (other goroutine!)
i := <- c
```

## Example: Simple Sequence 1/2

```
func Seq(max int) chan int {
  c := make(chan int)
  go func() {
    for i := 0;i < max; i++ {
    	c <- i
    }
  }()
  return c
}
```

## Example: Simple Sequence 2/2

```
func main() {
  c := Seq(10)
  for {
    fmt.Println(<-c)
  }	
}
```

### Output

http://play.golang.org/p/8Orwt59nOR

```
[...]
8
9
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.main()
	/tmp/sandbox195620736/main.go:18 +0x80
```

## Channels blocking!

* Reading and writing channels is a blocking operation!
* To prevent readers for blocking forever, close channels

   close(c)

  Best practive: Close in writer! Writing a closed channel panics.

* Closed channels return the empty value (http://play.golang.org/p/Ki3cG9FoZ4)

   c := make(chan int)
   close(c)
   i := <- c // i == 0

* Use special receive to detect closed channels

   i, ok := <- c // i == 0, ok == false


## Better Version

```
  close(c)
[...]
func main() {
  c := Seq(10)
  for {
    i, ok := <- c
    if !ok {
      return 
    }
    fmt.Println(i)
  }	
```

http://play.golang.org/p/yd2BwL28SJ

## Even easier: Use for-loops and range to consume channels #mindblown

```
c := Seq(10)
for i := range c {
	fmt.Println(i)
}
```

For-range loops until channel is closed.

http://play.golang.org/p/KmuvVuVVWJ


## More: Read/Write-Only types

```
var myReadOnlyChannel <-chan int
var myWriteOnlyChannel chan<- int

```

## More: Buffered Channel

```
# Can hold up to 2 values
func main() {
	c := make(chan int, 2)
	c <- 1 // Does not block!
	c <- 2
	fmt.Println(<- c) // = 1
	fmt.Println(<- c) // = 2
	<- c // channel empty, blocks => deadlock!
}
```

http://play.golang.org/p/SrI2j0-QxY

## More: Multi-Channel operations (select)

```
for {
	select {
	case job := <-jobQueue:
		job()
	case <-cancelChannel:
		fmt.Println("Ok, I am done here. Quitting..")
		return
	}
}
```
http://play.golang.org/p/oTUbJymNE9

## Timeouts

```
select {
case job := <-jobQueue:
	job()
case <- time.After(30 * time.Second):
	fmt.Println("Ok, I am done here. Timeout reached. Quitting..")
	doneChannel <- struct{}{}
}
```

http://play.golang.org/p/uQsrgY2fo_

## Writes + Reads together

```
select {
case <-cancel:
	return

case result <- i:
	i += 1
	if i > max {
		return
	}
}
```

http://play.golang.org/p/BQ6CKtQW_2

## Code-Review

parallel() - Execute the following batch of tasks, but only `n` tasks in parallel. Returns a channel for errors and a cancel-channel

https://gist.github.com/ZeissS/a2bad0bdb9f2f921933d

## Off-Topic: sync.Mutex

```
lock := sync.Mutex{}
...
lock.Lock()
defer lock.Unlock()
```

## links

 * Rob Pike - https://talks.golang.org/2012/concurrency.slide#26
 * Effective Go - https://golang.org/doc/effective_go.html#concurrency