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POpyserial reading serial port in flask (maybe using gevent)
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copied!<p>I'm building a webserver that would need to read (and keep reading) the serial port of the machine it's running on.<br> The purpose is to be able to read a barcode scanner, and using Server-Sent Events to update a browser with the read barcode. </p> <p>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. </p> <p>I've tried to base my code on a very simple example I found on stackoverflow <a href="https://stackoverflow.com/questions/12232304/how-to-implement-server-push-in-flask-framework">here</a>. I have it mostly working, but I am stuck with some questions;</p> <ul> <li>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.</li> <li>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.</li> <li>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?</li> </ul> <p>My code is as follow (shortened for clarity)</p> <p>Server-side:</p> <pre><code>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) </code></pre> <p>Client-side:</p> <pre><code> <!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> </code></pre> <p>There is some more information I was looking at here: <a href="http://www.socketubs.net/2012/10/28/Websocket_with_flask_and_gevent/" rel="nofollow noreferrer">http://www.socketubs.net/2012/10/28/Websocket_with_flask_and_gevent/</a> <a href="http://sdiehl.github.com/gevent-tutorial/#chat-server" rel="nofollow noreferrer">http://sdiehl.github.com/gevent-tutorial/#chat-server</a></p> <p>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.</p> <p>Thanks.</p>

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