Overview

Flask-SocketIO 5.0, released on March 15, 2021, upgrades to the Socket.IO 5.x protocol and adds native async mode.

Main Features

Socket.IO 5.x protocol

Flask-SocketIO 5.0 adopts the Socket.IO 5 protocol which improves connection reliability, binary support, and reconnection handling.

python 
from flask import Flask
from flask_socketio import SocketIO, emit

app = Flask(__name__)
socketio = SocketIO(app, cors_allowed_origins='*')

@socketio.on('message')
def handle_message(data):
    print(f'Received: {data}')
    emit('response', {'reply': 'Message received!'})

@socketio.on('connect')
def handle_connect():
    print('Client connected')
    emit('welcome', {'message': 'Connection established'})

# socketio.run(app, host='0.0.0.0', port=5000)

Async mode

Native async mode allows using async/await in event handlers, improving performance for concurrent connections.

python 
from flask import Flask
from flask_socketio import SocketIO, emit

app = Flask(__name__)
socketio = SocketIO(app, async_mode='eventlet')

@socketio.on('process')
def long_process(data):
    """Processing with progressive emission."""
    import time
    for i in range(5):
        time.sleep(1)
        emit('progress', {'step': i + 1, 'total': 5})
    emit('done', {'result': 'OK'})

# Supported async modes: eventlet, gevent, threading

Sources