Context

This program is a simple HTTP server written in Go that defines an HTTP handler function called hello. This handler uses the request's context to implement a timeout for processing the request.

Here's a breakdown of the code:

package main

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

// hello is an HTTP handler function that responds with "hello\n".
func hello(w http.ResponseWriter, req *http.Request) {
	// Get the request context.
	ctx := req.Context()

	fmt.Println("server: hello handler started")
	// Defer a statement to print a message when the handler ends.
	defer fmt.Println("server: hello handler ended")

	// Use a select statement to implement a timeout using time.After.
	select {
	case <-time.After(10 * time.Second):
		// Respond with "hello\n" if the operation completes within 10 seconds.
		fmt.Fprintf(w, "hello\n")
	case <-ctx.Done():
		// If the context is canceled (due to a timeout or cancellation),
		// handle the error and respond with an internal server error.
		err := ctx.Err()
		fmt.Println("server:", err)
		internalError := http.StatusInternalServerError
		http.Error(w, err.Error(), internalError)
	}
}

func main() {
	// Register the /hello path with the hello handler.
	http.HandleFunc("/hello", hello)
	// Start the HTTP server on port 8090.
	http.ListenAndServe(":8090", nil)
}

Explanation:

1. hello is an HTTP handler function that uses the request's context (ctx). It prints messages when the handler starts and ends.

2. Inside the select statement, there are two cases:

   • If the operation (responding with "hello\n") completes within 10 seconds, the response is sent.

   • If the context is canceled (due to a timeout or cancellation), an error message is printed, and an internal server error response is sent.

3. In the main function:

   • The /hello path is registered with the hello handler using http.HandleFunc.

   • The server is started using http.ListenAndServe on port 8090.

This program demonstrates a simple way to handle timeouts in an HTTP server using the request context.