Solved

Go by default will send requests with the header Connection: Keep-Alive and persist connections for re-use. The problem that I ran into is that the server is responding with Connection: Keep-Alive in the response header and then immediately closing the connection.

As a little background as to how go implements connections in this case (you can look at the full code in net/http/transport.go). There are two goroutines, one responsible for writing and one responsible for reading (readLoop and writeLoop) In most circumstances readLoop will detect a close on the socket, and close down the connection. The problem here occurs when you initiate another request before the readLoop actually detects the close, and the EOF that it reads get interpreted as an error for that new request rather than a close that occurred prior to the request.

Solved by setting request.Close as True will prevent from being re-used!

client := &http.Client{}
req, err := http.NewRequest(method, url, httpBody)

// NOTE this !!
req.Close = true

req.Header.Set("Content-Type", "application/json")
req.SetBasicAuth("user", "pass")
resp, err := client.Do(req)
if err != nil {
    // whatever
}
defer resp.Body.Close()

response, err = ioutil.ReadAll(resp.Body)
if err != nil {
    // Whatever
}

Given that this is the case the reason why sleeping in between requests works is that it gives readLoop time to detect the close on the connection before your new request and shut it down, so that your new request will initiate a new connection. (And the reason why it would intermittently fail is because there is some amount code running between your requests and depending of scheduling of goroutines, sometimes the EOF will be properly handled before your next request, sometimes not). And the req.Close = true, solution works because it prevents the connection from being re-used.

https://stackoverflow.com/questions/17714494/golang-http-request-results-in-eof-errors-when-making-multiple-requests-successi

When I send multiple request throw my server with gotybench, I bump into error below.

First error : socket: too many open files

gyuminhwangbo@Gyuminui-MacBookPro testing % go run main.go -j "[userId,string,userPw,string,mail,string,userName,string]" -r 10000 -t 1000 -u http://127.0.0.1:8080/auth/user
	Request url: http://127.0.0.1:8080/auth/user
	The number of HTTP Requests: 10000
	The number of threads: 1000
	Proceeding! Please wait until getting all the responses
ERROR MESSAGE: Post "http://127.0.0.1:8080/auth/user": dial tcp 127.0.0.1:8080: socket: too many open files
ERROR MESSAGE: Post "http://127.0.0.1:8080/auth/user": dial tcp 127.0.0.1:8080: socket: too many open files
ERROR MESSAGE: Post "http://127.0.0.1:8080/auth/user": dial tcp 127.0.0.1:8080: socket: too many open files
ERROR MESSAGE: Post "http://127.0.0.1:8080/auth/user": dial tcp 127.0.0.1:8080: socket: too many open files

I think the file descriptors size is the main reason for this error.

To overcome this error, I use ulimit -n 2048 to resize my file descriptors(Which is originally 256).

After I type ulimit -n 2048 and re-run testing program, again, error comes.

Second error : read: connection reset by peer

gyuminhwangbo@Gyuminui-MacBookPro testing % go run main.go -j "[userId,string,userPw,string,mail,string,userName,string]" -r 10000 -t 1000 -u http://127.0.0.1:8080/auth/user
	Request url: http://127.0.0.1:8080/auth/user
	The number of HTTP Requests: 10000
	The number of threads: 1000
	Proceeding! Please wait until getting all the responses
ERROR MESSAGE: Post "http://127.0.0.1:8080/auth/user": read tcp 127.0.0.1:57615->127.0.0.1:8080: read: connection reset by peer