How to deal with exceptions inside a event-stream using Flask

Question

This is my first application using Flask and SSE, I need to find a way to return to the page some exceptions that could happen during the execution of the event. This application uses Netconf to get some realtime information from a router.

I've created some exceptions that only show me in the console of Flask the messages, but I need to it show in the HTML interface and interrupt the stream.

@app.route('/data')
def data():
    def generate():
        try:
            conn = manager.connect(
                    host='10.0.0.1',
                    port='22',
                    username='admin',
                    password='123',
                    timeout=10,
                    device_params={'name':'junos'},
                    hostkey_verify=False
            def getInterface(command):
                interface = command.xpath('//system-information')
                return interface

            try:
                interface = getInterface(conn.command('code to get information'))
            except:
                print('Error, interface not found!')
                conn.close_session()

        except Exception as e:
            print('Error! ' + str(e))

    return Response(generate(), mimetype= 'text/event-stream')

This code above if gets an exception it doesn't stop the SSE 'looping' and keep logging into the console repeatedly the message "Error, interface not found!"

How can I handle it by sending these exceptions to the HTML interface and stop the execution of SSE when it happens?

I changed your tag from _[tag:SSE]_ to _[tag:Server-Sent-Events]_, because they mean totally different things. — zx485, Aug 08 '19 at 18:30

Kostas Charitidis · Answer 1 · 2019-08-08T13:59:54.477

0

Simply in the exceptions make a new Response with the exception message.

return Response('Error, interface not found!')

edited Aug 08 '19 at 13:59

answered Aug 08 '19 at 13:54

Kostas Charitidis

2,991
1
12
23

Unfortunately, it didn't work as I wanted. The SSE does not stop and this message will be sent to the EventSource (if I put it in yield), I just want to print the error in the html page and stop the event. – Leandro Fabris Milani Aug 08 '19 at 15:17

score 0 · Answer 2 · answered Aug 08 '19 at 14:47

0

from flask import Response

@app.route('/data')
def data():
    def generate():
        try:
            conn = manager.connect(
                host='10.0.0.1',
                port='22',
                username='admin',
                password='123',
                timeout=10,
                device_params={'name':'junos'},
                hostkey_verify=False
            def getInterface(command):
                interface = command.xpath('//system-information')
                return interface

            try:
                interface = getInterface(conn.command('code to get information'))
            except:
                # Close the session before the return statement
                conn.close_session()
                return Response('Error, interface not found!')

        except Exception as e:
            # Simply return the Response as shown in the above answer.
            # as posted by @Kostas Charitidis
            return Response('Error! ' + str(e))

    return Response(generate(), mimetype= 'text/event-stream')

answered Aug 08 '19 at 14:47

Sammit

169
1
8

It doesn't stop the stream, it keeping sending the same message. – Leandro Fabris Milani Aug 08 '19 at 14:59
`return 'Error, interface not found!'` try this. – Sammit Aug 08 '19 at 15:02
The same thing... Maybe there's a way to redirect to another page? Sending some variable to say what was the exception? I tried to use return redirect(url_for('realtime')) or just redirect, but it also didn't worked. – Leandro Fabris Milani Aug 08 '19 at 15:11
See if these links are helpful - [how-to-stop-server-sent-events](https://stackoverflow.com/questions/30645664/how-to-stop-server-sent-events) OR [Handling Connection Closure](https://auth0.com/blog/developing-real-time-web-applications-with-server-sent-events/) – Sammit Aug 08 '19 at 15:24
Thank you. It gives me some ideas. – Leandro Fabris Milani Aug 08 '19 at 16:06

How to deal with exceptions inside a event-stream using Flask

2 Answers2