understand-context.md

Understand Context

Before understanding Context, make sure you have learned Goroutines and Channels. Context is mainly used to transfer context information between Goroutines, including: cancellation signal, timeout, deadline, k-v, etc.

For example, we have a long-running function heartbeat that prints beat every second:

package main

import (
	"fmt"
	"time"
)

func main() {
	heartbeat()
}

func heartbeat() {
	tick := time.Tick(time.Second)

	for {
		<-tick
		fmt.Println("beat")
	}
}

If we want to abort the heartbeat whenever we press the enter key, we may code like this:

func main() {
	stop := make(chan struct{})
	go func() {
		fmt.Scanln()
		close(stop)
	}()

	heartbeat(stop)
}

func heartbeat(stop chan struct{}) {
	tick := time.Tick(time.Second)

	for {
		select {
		case <-tick:
		case <-stop:
			return
		}
		fmt.Println("beat")
	}
}

Because this kind of code is so often used, Golang abstracted a helper package to handle it, it's called Context. If we use Context, the code above will become something like this:

func main() {
	ctx, stop := context.WithCancel(context.Background())
	go func() {
		fmt.Scanln()
		stop()
	}()

	heartbeat(ctx)
}

func heartbeat(ctx context.Context) {
	tick := time.Tick(time.Second)

	for {
		select {
		case <-tick:
		case <-ctx.Done():
			return
		}
		fmt.Println("beat")
	}
}