true, "author"=>"björn", "date"=>2020-02-16 23:00:00 +0800, "layout"=>"presentation", "slug"=>"go-channels", "title"=>"Go channels", "subtitle"=>"…a means of communication", "description"=>"Takeaways from learning to use channels in Go.", "categories"=>["programming", "go"], "content"=>"
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Go Channels

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…a means of communication

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Björn Andersson

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Quick thing about Go

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What?

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No globals?

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Syntax and usage

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Passing channels

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Types of channels

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Unbuffered

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Unbuffered example

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\ngreetingChan := make(chan string)\n\ngo func (c <-chan string) {\n  fmt.Printf(\"Hello, %s\", <-c)\n}(greetingChan)\n\ngreetingChan <- \"World\"\n
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Buffered

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Buffered concurrent execution

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With channels you can share a limited resource across unlimited concurrency

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\nvar dbConns []*DB.Conn // assume it contains 5 connections\ndbPool := make(chan *DB.Conn, 5)\nfor i := 0; i < 5; i++ {\n  dbPool <-dbConns[i]\n}\n\nt.Run(\"parallel test run 1\", func (t *testing.T) {\n  t.Parallel()\n  db := <- dbPool\n  defer func () { dbPool <- db }() // returns to pool\n  // run lots of tests\n})\n
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Closed

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Closed example

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\nstopChan := make(chan struct{})\n\nclose(stopChan) // No value written to the channel\nv, ok := <- stopChan\nfmt.Printf(\"v=%v, open=%v\\n\", v, ok) // v=false, open=false\nv, ok = <- stopChan\nfmt.Printf(\"v=%v, open=%v\\n\", v, ok) // v=false, open=false\n
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Tips

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Example

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func getMessages(\n    ctx context.Context,\n    query Query,\n    errChan chan<- error,\n    stopChan <-chan struct{},\n) {}\n
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select statement

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\n", "dir"=>"/presentations/go-channels/", "excerpt"=>nil, "name"=>"go-channels.html", "path"=>"presentations/go-channels.html", "url"=>"/presentations/go-channels/"}">

Go Channels

…a means of communication

Björn Andersson

Quick thing about Go

  • Does not have threads
    • Instead has "go routines"
    • Kind of like a "green/lightweight" thread
    • Many go routines in one thread and many threads to a program
  • You have to write your code as if it could go into another thread at any point

What?

  • Channels are for letting different parts of the code communicate
  • In Go, you never know if you're in a thread or not
  • The fastest way of sharing sharing state is over a channel

No globals?

  • Of course, we still have globals and mutexes
  • But prefer to send a message on a channel!

Syntax and usage

  • stopChan := make(chan bool), create a channel
  • <-c: read from a channel
  • c <-: write to a channel

Passing channels

  • Used when specifying which channel you want in your functions and interfaces
  • name <-chan string: read-only
  • name chan<- string: write-only

Types of channels

  • Unbuffered
  • Buffered
  • Closed

Unbuffered

  • Holds a single value at a time
  • You only continue as soon as someone reads from the channel

Unbuffered example


greetingChan := make(chan string)

go func (c <-chan string) {
  fmt.Printf("Hello, %s", <-c)
}(greetingChan)

greetingChan <- "World"

Buffered

  • Can hold up to n values at a time
  • Before it has n values in it doesn't wait for someone to read
  • Is practically the same as a buffered with 1 as n

Buffered concurrent execution

With channels you can share a limited resource across unlimited concurrency


var dbConns []*DB.Conn // assume it contains 5 connections
dbPool := make(chan *DB.Conn, 5)
for i := 0; i < 5; i++ {
  dbPool <-dbConns[i]
}

t.Run("parallel test run 1", func (t *testing.T) {
  t.Parallel()
  db := <- dbPool
  defer func () { dbPool <- db }() // returns to pool
  // run lots of tests
})

Closed

  • Will return the default value immediately
  • You can always ask a channel if its open when reading by asking for a second boolean parameter

Closed example


stopChan := make(chan struct{})

close(stopChan) // No value written to the channel
v, ok := <- stopChan
fmt.Printf("v=%v, open=%v\n", v, ok) // v=false, open=false
v, ok = <- stopChan
fmt.Printf("v=%v, open=%v\n", v, ok) // v=false, open=false

Tips

  • Think about about your lines of communication
    • Only one Go routine writes and one reads — don't do both in the same
    • Who is leading the communication?
    • Who is charge of telling the routine to shut down?
  • Does a started Go routine have a reason to communicate something back?

Example

func getMessages(
    ctx context.Context,
    query Query,
    errChan chan<- error,
    stopChan <-chan struct{},
) {}
  • context.Context means external shutdown
  • errChan sounds like there can be errors that forces the function to stop work on its own
  • stopChan tells me that there are other reasons than context to stop working

select statement

  • Like a switch statement, but for channels
  • Reads from one channel at a time in random order
  • default evaluates immediately if no channel is readable
  • Will wait for a channel to return something before it continues, unless it has a default
  • Remember, a closed channel is always readable