stream - Not buffered http.ResponseWritter in Golang

Question

I'm writing a simple web app in Go and I want my responses to be streamed to the client (i.e. not buffered and sent in blocks once the request is fully processed) :

func handle(res http.ResponseWriter, req *http.Request) {
  fmt.Fprintf(res, "sending first line of data")
  sleep(10) //not real code
  fmt.Fprintf(res, "sending second line of data")
}

From the client point of view, the two lines will be sent at the same time. Any suggestions are appreciated :)

Edit after @dystroy answer

It's possible to flush after each write I personally make, but in my use case it's not enough:

cmd := exec.Command("a long command that outputs lots of lines")
cmd.Stdout = res //where res is a http.ResponseWritter
cmd.Stderr = res
err := cmd.Run()

I want the output of my cmd to be flushed as well. Anyway to "autoflush" the ResponseWritter ?

Solution

I found help on golang's mailing list. There is 2 way to achieve this: using hijacker that allow to take over the underlying TCP connection of HTTP, or piping the stdout and stderr of the command in a go routine that will write and flush :

pipeReader, pipeWriter := io.Pipe()
cmd.Stdout = pipeWriter
cmd.Stderr = pipeWriter
go writeCmdOutput(res, pipeReader)
err := cmd.Run()
pipeWriter.Close()

//---------------------
func writeCmdOutput(res http.ResponseWriter, pipeReader *io.PipeReader) {
  buffer := make([]byte, BUF_LEN)
  for {
    n, err := pipeReader.Read(buffer)
    if err != nil {
      pipeReader.Close()
      break
    }

    data := buffer[0:n]
    res.Write(data)
    if f, ok := res.(http.Flusher); ok {
      f.Flush()
    }
    //reset buffer
    for i := 0; i < n; i++ {
      buffer[i] = 0
    }
  } 
}

Last update

Even nicer: http://play.golang.org/p/PpbPyXbtEs

question from:https://stackoverflow.com/questions/19292113/not-buffered-http-responsewritter-in-golang

Answer 1

As implied in the documentation, some ResponseWriter may implement the Flusher interface.

This means you can do something like this :

func handle(res http.ResponseWriter, req *http.Request) {
  fmt.Fprintf(res, "sending first line of data")
  if f, ok := res.(http.Flusher); ok {
     f.Flush()
  } else {
     log.Println("Damn, no flush");
  }
  sleep(10) //not real code
  fmt.Fprintf(res, "sending second line of data")
}

Be careful that buffering can occur in many other places in the network or client side.