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Asked: 2026-06-16T12:04:59+00:00

I’m building a webserver that would need to read (and keep reading) the serial

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I’m building a webserver that would need to read (and keep reading) the serial port of the machine it’s running on.
The purpose is to be able to read a barcode scanner, and using Server-Sent Events to update a browser with the read barcode.

I’m using flask to do this. I’ve browsed around and some implementations require only flask, some say I would need an async library like Gevent, and some others even say I’d need Gevent and some sort of queue like Redis or RabbitMQ.

I’ve tried to base my code on a very simple example I found on stackoverflow here. I have it mostly working, but I am stuck with some questions;

  • In Chrome there is a cross-origin error, by adding an
    Access-Control-Allow-Origin header I can get it to work in FireFox,
    but Chrome still doesn’t work. Is it correct that only FF supports
    SSE cross-origin? I need it to support CORS because the browser will
    need to load the barcode data from a separate machine.
  • After each message, the browser shows the barcode in the console, but
    it then closes the connection and only opens it again after about 3
    seconds. It seems that this originates in Flask, it gives me the data
    and then just stops.
  • Also, I’m wondering how this will perform under load. I mean, flask
    keeps a connection open for the text/event-stream mimetype. If
    multiple clients connect, won’t it block flask after a while, because
    all of the connections will be saturated?

My code is as follow (shortened for clarity)

Server-side:

from flask import Flask
import flask
import serial

app = Flask(__name__)
app.debug = True

def event_barcode():
    ser = serial.Serial()
    ser.port = 0
    ser.baudrate = 9600
    ser.bytesize = 8
    ser.parity = serial.PARITY_NONE
    ser.stopbits = serial.STOPBITS_ONE
    ser.open()
    s = ser.read(7)
    yield 'data: %s\n\n' % s

@app.route('/barcode')
def barcode():
    newresponse = flask.Response(event_barcode(), mimetype="text/event-stream")
    newresponse.headers.add('Access-Control-Allow-Origin', '*')
    return newresponse

if __name__ == '__main__':
    app.run(port=8080, threaded=True)

Client-side:

    <!DOCTYPE HTML>
<html>
<head>
    <meta http-equiv=Content-Type content="text/html; charset=utf-8">
    <title>TEST</title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js" type="text/javascript" charset="utf-8"></script>
    <script>

        $(document).ready(function(){
            if (!!window.EventSource) {
                console.log('SSE supported.');
                var source = new EventSource('http://localhost:8080/barcode');

                source.addEventListener('message', function(e) {
                  console.log(e.data);
                }, false);

                source.addEventListener('open', function(e) {
                  console.log('Connection was opened.');
                }, false);

                source.addEventListener('error', function(e) {
                  if (e.readyState == EventSource.CLOSED) {
                    console.log('Connection was closed.');
                  }
                }, false);

            } else {
                console.log('SSE notsupported.');
            }
        });

    </script>
</head>

<body>

</body>
</html>

There is some more information I was looking at here:
http://www.socketubs.net/2012/10/28/Websocket_with_flask_and_gevent/
http://sdiehl.github.com/gevent-tutorial/#chat-server

I hope someone can clear up my questions, and maybe point me towards some solutions, for the cross-origin and the 3 second delay problem.

Thanks.

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1 Answer

  1. Editorial Team

    Answering my own questions

    1. It seems that indeed only Firefox supports CORS for SSE for now ->
      article
    2. With help from Janus Troelsen I figured out how to keep the
      connection open and send several barcodes across the line (see code
      below)
    3. Performance-wise it seems that I can only make one connection, but
      that might be because I have only one serial port, the subsequent
      connections can’t open the serial port anymore. I think it will work
      from flask, but something with socketio and gevents will perfom
      better because it’s more suited for the job. There’s an interesting
      article here.

    For the code:

    import flask
import serial
from time import sleep

app = flask.Flask(__name__)
app.debug = True

def event_barcode():
    messageid = 0
    ser = serial.Serial()
    ser.port = 0
    ser.baudrate = 9600
    ser.bytesize = 8
    ser.parity = serial.PARITY_NONE
    ser.stopbits = serial.STOPBITS_ONE
    ser.timeout = 0
    try:
        ser.open()
    except serial.SerialException, e:
         yield 'event:error\n' + 'data:' + 'Serial port error({0}): {1}\n\n'.format(e.errno, e.strerror)
         messageid = messageid + 1
    str_list = []
    while True:
        sleep(0.01)
        nextchar = ser.read()
        if nextchar:
            str_list.append(nextchar)
        else:
            if len(str_list) > 0:
                yield 'id:' + str(messageid) + '\n' + 'data:' + ''.join(str_list) + '\n\n'
                messageid = messageid + 1
                str_list = []

@app.route('/barcode')
def barcode():
    newresponse = flask.Response(event_barcode(), mimetype="text/event-stream")
    newresponse.headers.add('Access-Control-Allow-Origin', '*')
    newresponse.headers.add('Cache-Control', 'no-cache')
    return newresponse

if __name__ == '__main__':
    app.run(port=8080, threaded=True)

    Because I want to support multiple browsers, SSE is not the way to go for me right now. I will look into websockets and try and work from that.

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