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<!DOCTYPE html>
<html>
<head>
<title>Go Contexts</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
<style type="text/css">
@import url(https://fonts.googleapis.com/css?family=Yanone+Kaffeesatz);
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body { font-family: 'Droid Serif'; }
h1, h2, h3 {
font-family: 'Yanone Kaffeesatz';
font-weight: normal;
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<body>
<textarea id="source">
class: center, middle
# Go Contexts
---
# Goroutine
Starting a goroutine in Go is *very* easy todo and cheap to run.
```go
package main
import "fmt"
func main() {
// Run a function synchronously.
fmt.Print("Hello")
// Run a function asynchronously, ie. in a goroutine.
go fmt.Println(" world!")
}
```
---
# Goroutine
Starting a goroutine in Go is *very* easy todo and cheap to run.
```go
// see src/goroutine/main.go
// [...]
package main
import "fmt"
func main() {
// Run a function synchronously.
fmt.Print("Hello")
// Run a function asynchronously, ie. in a goroutine.
go fmt.Println(" world!")
}
```
```sh
$ go run goroutine/main.go
Hello
```
---
# Goroutine
```go
// see src/wait/main.go
// [...]
func main() {
// Run a function synchronously.
fmt.Print("Hello")
// done is a channel used to synchronize the goroutine and the main program.
done := make(chan struct{})
// Run a function asynchronously, ie. in a goroutine.
go func() {
fmt.Println(" world!")
// Once the function stops, we send a message in the channel.
done <- struct{}{}
}()
// Wait for the end message from the goroutine.
<-done
}
```
---
# Goroutine
```go
// see src/wait/main.go
// [...]
func main() {
// Run a function synchronously.
fmt.Print("Hello")
// done is a channel used to synchronize the goroutine and the main program.
done := make(chan struct{})
// Run a function asynchronously, ie. in a goroutine.
go func() {
fmt.Println(" world!")
// Once the function stops, we send a message in the channel.
done <- struct{}{}
}()
// Wait for the end message from the goroutine.
<-done
}
```
```sh
$ go run src/wait/main.go
Hello world!
```
---
# Goroutines can leak easily
> Never start a goroutine without knowing how it will stop
>
> [Dave Cheney](https://dave.cheney.net/2016/12/22/never-start-a-goroutine-without-knowing-how-it-will-stop)
---
# What if I want to cancel a goroutine?
You can't kill a running goroutine.
```go
// see src/slow/main.go
// [...]
// slow simulates a slow operation, like an HTTP request.
func slow() {
time.Sleep(2 * time.Second)
fmt.Println(" world!")
}
// ...
```
---
# What if I want to cancel a goroutine?
```go
// ...
func main() {
// Run a function synchronously.
fmt.Print("Hello")
// done is a channel used to synchronize the goroutine and the main program.
done := make(chan struct{})
// Run a function asynchronously, ie. in a goroutine.
go func() {
slow()
// Once the function stops, we send a message in the channel.
done <- struct{}{}
}()
// Wait for the end message from the goroutine.
<-done
}
```
```sh
$ go run src/slow/main.go
Hello world!
```
---
# What if I want to cancel a goroutine?
```go
// see src/cancel/main.go
// [...]
// slow a simulates a slow operation, like an HTTP request.
func slow(cancel <-chan struct{}) {
select {
// Simulate a long operation, like an HTTP request.
case <-time.After(2 * time.Second):
// nothing to do here
case <-cancel:
// operation is canceled, return early.
return
}
fmt.Println(" world!")
}
// ...
```
---
# What if I want to cancel a goroutine?
```go
// ...
func main() {
// Run a function synchronously.
fmt.Print("Hello")
// done is a channel used to synchronize the goroutine and the main program.
done := make(chan struct{})
// cancel is a channel used to send a cancelation message from the main
// program to the go routine.
cancel := make(chan struct{})
// Run a function asynchronously, ie. in a goroutine.
go func() {
slow(cancel)
// Once the function stops, we send a message in the channel.
done <- struct{}{}
}()
// Cancel the goroutine immediately.
cancel <- struct{}{}
// Wait for the end message from the goroutine.
<-done
}
```
---
# What if I want to cancel a goroutine?
```sh
$ go run src/cancel/main.go
Hello
```
---
# Context: a standardized way for cancelation.
```go
// A Context carries a deadline, cancellation signal, and request-scoped values
// across API boundaries. Its methods are safe for simultaneous use by multiple
// goroutines.
type Context interface {
// Done returns a channel that is closed when this Context is canceled
// or times out.
Done() <-chan struct{}
// Err indicates why this context was canceled, after the Done channel
// is closed.
Err() error
// Deadline returns the time when this Context will be canceled, if any.
Deadline() (deadline time.Time, ok bool)
// Value returns the value associated with key or nil if none.
Value(key interface{}) interface{}
}
```
[Context](https://pkg.go.dev/context#Context)
---
# Context: a standardized way for cancelation.
```go
// see src/context/main.go
// [...]
// slow simulates a slow operation, like an HTTP request.
func slow(ctx context.Context) {
select {
// Simulate a long operation, like an HTTP request.
case <-time.After(2 * time.Second):
// nothing to do here
case <-ctx.Done():
// operation is canceled, return early.
return
}
fmt.Println(" world!")
}
// ...
```
---
# Context: a standardized way for cancelation.
```go
// ...
func main() {
// create a cancelable context for our main program.
ctx, cancel := context.WithCancel(context.Background())
// Run a function synchronously.
fmt.Print("Hello")
// done is a channel used to synchronize the goroutine and the main program.
done := make(chan struct{})
// Run a function asynchronously, ie. in a goroutine.
go func() {
slow(ctx)
// Once the function stops, we send a message in the channel.
done <- struct{}{}
}()
// Cancel the context.
cancel()
// Wait for the end message from the goroutine.
<-done
}
```
---
# Good practices
By convention, context is the first function's argument.
```go
func F(ctx context.Context, ...) {
```
Do not store a context, pass it per function.
[Contexts and structs](https://go.dev/blog/context-and-structs)
```go
type Cli struct {}
func (cli Cli) List(ctx context.Context, ...) (error, []Items) {
func (cli Cli) Get(ctx context.Context, ...) (error, Item) {
```
Do not pass a nil context, use `context.TODO` if unsure.
Always call a created cancel context function.
```go
func F(ctx context.Context) {
ctx, cancel := context.WithCancel(ctx)
defer cancel()
// ....
```
---
# Use Context with incoming HTTP request
```go
// see src/hserver/main.go
// [...]
func main() {
http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
ctx := r.Context()
// Simulate a long operation, like an HTTP request.
select {
case <-ctx.Done():
fmt.Println("cancel", r.RemoteAddr)
return
case <-time.After(5 * time.Second):
// nothing to do here.
}
fmt.Fprintln(w, "done")
})
fmt.Fprintln(os.Stderr, http.ListenAndServe("127.0.0.1:4567", nil))
}
```
* [Request.Context](https://pkg.go.dev/net/http#Request.Context)
* [Server.BaseContext](https://pkg.go.dev/net/http#Server) to create an HTTP server with a base context
---
# Use Context with outgoing HTTP request
```go
req, err := http.NewRequestWithContext(
ctx, "GET", "https://clermontech.org/", nil,
)
if err != nil {
// handle error
}
```
* [NewRequestWithContext](https://pkg.go.dev/net/http#NewRequestWithContext)
---
class: bottom, left
# Thank you 🙏
Pierre Tachoire<br>
pierre@tch.re<br>
[mamot.fr/@krichprollsch](https://mamot.fr/@krichprollsch)
</textarea>
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