How to Stream Responses in Golang and Overcome `http.ResponseWriter` Buffering Limitations?

Streaming Responses in Golang: A Buffered ResponseWriter Hitch

When creating web applications in Golang, it's essential to understand the behavior of the http.ResponseWriter. By default, Responses are buffered, which means data is collected and sent in blocks once the request is fully processed. However, in scenarios where you want to stream responses to clients line by line or handle large outputs that exceed buffering capacity, this behavior becomes a hindrance.

Consider the following example:

func handle(res http.ResponseWriter, req *http.Request) {
  fmt.Fprintf(res, "sending first line of data")
  sleep(10) // Simulation of a long-running process
  fmt.Fprintf(res, "sending second line of data")
}

From the client's perspective, the "sending first line of data" and "sending second line of data" messages should be received separately. However, due to buffering, both lines will be aggregated and sent simultaneously.

To resolve this issue, one can manually flush the ResponseWriter after each write operation. This can be achieved using the Flusher interface, as demonstrated below:

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()
  }
  sleep(10) // Simulation of a long-running process
  fmt.Fprintf(res, "sending second line of data")
}

With this modification, the response will be progressively streamed to the client, as desired.

Advanced Scenario: Piping External Commands

However, in certain situations, manual flushing may not be sufficient. Consider a scenario where you want to pipe the output of an external command to the client. The command generates a large amount of data, exceeding the buffering capacity.

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

// Function to write command output to ResponseWriter
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
    }
  }
}

In this case, it becomes necessary to "autoflush" the ResponseWriter to ensure the data is streamed to the client without delay. This can be achieved using the code snippet provided.

Alternative Solutions

As an alternative to directly piping the external command's output, one can use a channel-based approach:

// Create a channel to communicate with the goroutine
outputChan := make(chan string)

// Start a goroutine to process the command output
go func() {
  scanner := bufio.NewScanner(cmd.Stdout)
  for scanner.Scan() {
    outputChan <- scanner.Text()
  }
  if err := scanner.Err(); err != nil {
    log.Fatal(err)
  }
  close(outputChan) // Notify that all output has been processed
}()

// Stream output to ResponseWriter lazily
func handleCmdOutput(res http.ResponseWriter, req *http.Request) {
  if f, ok := res.(http.Flusher); ok {
    for {
      select {
      case output := <-outputChan:
        res.Write([]byte(output + "\n"))
        f.Flush()
      default:
        time.Sleep(10 * time.Millisecond)
      }
    }
  }
}

In this approach, the goroutine asynchronously processes the command output and sends it to the channel. The handleCmdOutput function then lazily streams the output to the ResponseWriter, flushing after each write operation.

By leveraging the Flusher interface and exploring alternative approaches, you can effectively stream data to clients and overcome the buffering limitation in Golang's ResponseWriter.

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