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问题:

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.

回答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?

回答2:

Answering my own questions

It seems that indeed only Firefox supports CORS for SSE for now -> article
With help from Janus Troelsen I figured out how to keep the connection open and send several barcodes across the line (see code below)
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.