pyserial reading serial port in flask (maybe using gevent)

Question

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, an

Answer 1

Here are some gists that may help (I've been meaning to release something like 'flask-sse' based on 'django-sse):

https://gist.github.com/3680055

https://gist.github.com/3687523

also useful - https://github.com/jkbr/chat/blob/master/app.py

The 'RedisSseStream' class uses redis as a backend to communicate between threads (although maybe gevent can do this?), and 'listens' for redis publish events.

While the 'PeriodicSseStream' doesn't need redis, it cannot communicate between flask threads i.e. use information from another response; Without something like redis, the seperate threads (the stream, and the one serving another user) cannot communicate.

As Janus says, the generator only return one result - it must yield multiple, and in this case it must be enclosed in a loop that endlessly yields after each serial poll; You also need to decide what will limit polling, will it be limited by time (periodically poll), or something else (e.g. if it already take awhile to read the serial port)?

I don't really know much about the performance of sse, or how well supported it is (and wrt cross-domain), but if you consider socket.io, You could maybe use this to improve web-socket performance?

Answer 2

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.