context.go

package main

import (
	"fmt"
	"net/http"
	"time"
)

func hello(w http.ResponseWriter, req *http.Request) {

	// A context.Context is created for each request by
	// the net/http machinery, and is available with the
	// Context() method.
	ctx := req.Context()
	fmt.Println("server: hello handler started")
	defer fmt.Println("server: hello handler ended")

	// Wait for a few seconds before sending a reply to the client.
	// This could simulate some work the server is doing.
	// While working, keep an eye on the context’s Done() channel for
	// a signal that we should cancel the work and return as soon as possible.
	select {
	case <-time.After(10 * time.Second):
		fmt.Fprintf(w, "hello\n")
	case <-ctx.Done():

		// The context’s Err() method returns an error that explains why
		// the Done() channel was closed.
		err := ctx.Err()
		fmt.Println("server:", err)
		internalError := http.StatusInternalServerError
		http.Error(w, err.Error(), internalError)
	}
}

func main() {

	// We register our handler on the “/hello” route, and start serving.
	http.HandleFunc("/hello", hello)
	http.ListenAndServe(":8090", nil)
}