golang concurrent http request handling

Question

I am creating a simple http server using golang. I have two questions, one is more theoretic and another one about the real program.

1. Concurrent request handling

I create a server and use s.ListenAndServe() to handle the requests. As much as I understand the requests served concurrently. I use a simple handler to check it:

func  ServeHTTP(rw http.ResponseWriter, request *http.Request) {  
    fmt.Println("1")  
    time.Sleep(1 * time.Second)       //Phase 2 delete this line
    fmt.Fprintln(rw, "Hello, world.")
    fmt.Println("2")  
}

I see that if I send several requests, I will see all the "1" appear and only after a second all the "2" appear. But if I delete the Sleep line, I see that program never start a request before it finishes with the previous one (the output is 1 2 1 2 1 2 ...). So I don't understand, if they are concurrent or not really. If they are I would expect to see some mess in prints...

2. The real life problem

In the real handler, I send the request to another server and return the answer to the user (with some changes to request and the answer but in idea it is kind of a proxy). All this of course takes time and from what can see (by adding some prints to the handler), the requests are handled one by one, with no concurrency between them (my prints show me that a request starts, go through all the steps, ends and only then I see a new start....). What can I do to make them really concurrent?
Putting the handler function as goroutine gives an error, that body of the request is already closed. Also if it is already concurrent adding more goroutines will make things only worse.

Thank you!

Answer 1

Your example makes it very hard to tell what is happening.

The below example will clearly illustrate that the requests are run in parallel.

package main

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

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        if len(r.FormValue("case-two")) > 0 {
            fmt.Println("case two")
        } else {
            fmt.Println("case one start")
            time.Sleep(time.Second * 5)
            fmt.Println("case one end")
        }
    })

    if err := http.ListenAndServe(":8000", nil); err != nil {
        log.Fatal(err)
    }
}

Make one request to http://localhost:8000

Make another request to http://localhost:8000?case-two=true within 5 seconds

console output will be

case one start
case two
case one end