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;
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.
Answering my own questions
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.