:ref:`genindex` | :ref:`modindex` | :ref:`search`

This project implements an Socket.IO server that can run standalone or integrated with a Python WSGI application. The following are some of its features:

• Fully compatible with the Javascript socket.io-client library, versions 1.3.5 and up.
• Compatible with Python 2.7 and Python 3.3+.
• Supports large number of clients even on modest hardware when used with an asynchronous server based on eventlet or gevent. For development and testing, any WSGI complaint multi-threaded server can be used.
• Includes a WSGI middleware that integrates Socket.IO traffic with standard WSGI applications.
• Broadcasting of messages to all connected clients, or to subsets of them assigned to "rooms".
• Uses an event-based architecture implemented with decorators that hides the details of the protocol.
• Support for HTTP long-polling and WebSocket transports.
• Support for XHR2 and XHR browsers.
• Support for text and binary messages.
• Support for gzip and deflate HTTP compression.
• Configurable CORS responses to avoid cross-origin problems with browsers.

Socket.IO is a transport protocol that enables real-time bidirectional event-based communication between clients (typically web browsers) and a server. The official implementations of the client and server components are written in JavaScript.

The following is a basic example of a Socket.IO server that uses Flask to deploy the client code to the browser:

import socketio
import eventlet
from flask import Flask, render_template

sio = socketio.Server()
app = Flask(__name__)

@app.route('/')
def index():
    """Serve the client-side application."""
    return render_template('index.html')

@sio.on('connect')
def connect(sid, environ):
    print('connect ', sid)

@sio.on('my message')
def message(sid, data):
    print('message ', data)

@sio.on('disconnect')
def disconnect(sid):
    print('disconnect ', sid)

if __name__ == '__main__':
    # wrap Flask application with socketio's middleware
    app = socketio.Middleware(sio, app)

    # deploy as an eventlet WSGI server
    eventlet.wsgi.server(eventlet.listen(('', 8000)), app)

The client-side application must include the socket.io-client library (versions 1.3.5 or newer recommended).

Each time a client connects to the server the connect event handler is invoked with the sid (session ID) assigned to the connection and the WSGI environment dictionary. The server can inspect authentication or other headers to decide if the client is allowed to connect. To reject a client the handler must return False.

When the client sends an event to the server the appropriate event handler is invoked with the sid and the message. The application can define as many events as needed and associate them with event handlers. An event is defined simply by a name.

When a connection with a client is broken, the disconnect event is called, allowing the application to perform cleanup.

Because Socket.IO is a bidirectional protocol, the server can send messages to any connected client at any time. To make it easy to address groups of clients, the application can put clients into rooms, and then address messages to all the clients in a room.

When clients first connect, they are assigned to their own rooms, named with the session ID (the sid argument passed to all event handlers). The application is free to create additional rooms and manage which clients are in them using the :func:`socketio.Server.enter_room` and :func:`socketio.Server.leave_room` methods. Clients can be in as many rooms as needed and can be moved between rooms as often as necessary. The individual rooms assigned to clients when they connect are not special in any way, the application is free to add or remove clients from them, though once it does that it will lose the ability to address individual clients.

@sio.on('enter room')
def enter_room(sid, data):
    sio.enter_room(sid, data['room'])

@sio.on('enter room')
def leave_room(sid, data):
    sio.leave_room(sid, data['room'])

The :func:`socketio.Server.emit` method takes an event name, a message payload of type str, bytes, list or dict, and the recipient room. To address an individual client, the sid of that client should be given as room (assuming the application did not alter these initial rooms). To address all connected clients, the room argument should be omitted.

@sio.on('my message')
def message(sid, data):
    print('message ', data)
    sio.emit('my reply', data, room='my room')

Often when broadcasting a message to group of users in a room, it is desirable that the sender does not receive its own message. The :func:`socketio.Server.emit` method provides an optional skip_sid argument to specify a client that should be skipped during the broadcast.

@sio.on('my message')
def message(sid, data):
    print('message ', data)
    sio.emit('my reply', data, room='my room', skip_sid=sid)

When a client sends an event to the server, it can optionally provide a callback function, to be invoked with a response provided by the server. The server can provide a response simply by returning it from the corresponding event handler.

@sio.on('my event', namespace='/chat')
def my_event_handler(sid, data):
    # handle the message
    return "OK", 123

The event handler can return a single value, or a tuple with several values. The callback function on the client side will be invoked with these returned values as arguments.

The server can also request a response to an event sent to a client. The :func:`socketio.Server.emit` method has an optional callback argument that can be set to a callable. When this argument is given, the callable will be invoked with the arguments returned by the client as a response.

Using callback functions when broadcasting to multiple clients is not recommended, as the callback function will be invoked once for each client that received the message.

The Socket.IO protocol supports multiple logical connections, all multiplexed on the same physical connection. Clients can open multiple connections by specifying a different namespace on each. A namespace is given by the client as a pathname following the hostname and port. For example, connecting to http://example.com:8000/chat would open a connection to the namespace /chat.

Each namespace is handled independently from the others, with separate event handlers and rooms. It is important that applications that use multiple namespaces specify the correct namespace when setting up their event handlers and rooms, using the optional namespace argument available in all the methods in the :class:`socketio.Server` class.

When the namespace argument is omitted, set to None or to '/', the default namespace, representing the physical connection, is used.

The following sections describe a variety of deployment strategies for Socket.IO servers.

Eventlet is a high performance concurrent networking library for Python 2 and 3 that uses coroutines, enabling code to be written in the same style used with the blocking standard library functions. An Socket.IO server deployed with eventlet has access to the long-polling and WebSocket transports.

Instances of class socketio.Server will automatically use eventlet for asynchronous operations if the library is installed. To request its use explicitly, the async_mode option can be given in the constructor:

sio = socketio.Server(async_mode='eventlet')

A server configured for eventlet is deployed as a regular WSGI application, using the provided socketio.Middleware:

app = socketio.Middleware(sio)
import eventlet
eventlet.wsgi.server(eventlet.listen(('', 8000)), app)

An alternative to running the eventlet WSGI server as above is to use gunicorn, a fully featured pure Python web server. The command to launch the application under gunicorn is shown below:

$ gunicorn -k eventlet -w 1 module:app

Due to limitations in its load balancing algorithm, gunicorn can only be used with one worker process, so the -w 1 option is required. Note that a single eventlet worker can handle a large number of concurrent clients.

Another limitation when using gunicorn is that the WebSocket transport is not available, because this transport it requires extensions to the WSGI standard.

Note: Eventlet provides a monkey_patch() function that replaces all the blocking functions in the standard library with equivalent asynchronous versions. While python-socketio does not require monkey patching, other libraries such as database drivers are likely to require it.

Gevent is another asynchronous framework based on coroutines, very similar to eventlet. An Socket.IO server deployed with gevent has access to the long-polling transport. If project gevent-websocket is installed, the WebSocket transport is also available.

Instances of class socketio.Server will automatically use gevent for asynchronous operations if the library is installed and eventlet is not installed. To request gevent to be selected explicitly, the async_mode option can be given in the constructor:

sio = socketio.Server(async_mode='gevent')

A server configured for gevent is deployed as a regular WSGI application, using the provided socketio.Middleware:

app = socketio.Middleware(sio)
from gevent import pywsgi
pywsgi.WSGIServer(('', 8000), app).serve_forever()

If the WebSocket transport is installed, then the server must be started as follows:

from gevent import pywsgi
from geventwebsocket.handler import WebSocketHandler
app = socketio.Middleware(sio)
pywsgi.WSGIServer(('', 8000), app,
                  handler_class=WebSocketHandler).serve_forever()

An alternative to running the gevent WSGI server as above is to use gunicorn, a fully featured pure Python web server. The command to launch the application under gunicorn is shown below:

$ gunicorn -k gevent -w 1 module:app

Or to include WebSocket:

$ gunicorn -k geventwebsocket.gunicorn.workers.GeventWebSocketWorker -w 1 module: app

Same as with eventlet, due to limitations in its load balancing algorithm, gunicorn can only be used with one worker process, so the -w 1 option is required. Note that a single eventlet worker can handle a large number of concurrent clients.

Note: Gevent provides a monkey_patch() function that replaces all the blocking functions in the standard library with equivalent asynchronous versions. While python-socketio does not require monkey patching, other libraries such as database drivers are likely to require it.

While not comparable to eventlet and gevent in terms of performance, the Socket.IO server can also be configured to work with multi-threaded web servers that use standard Python threads. This is an ideal setup to use with development servers such as Werkzeug. Only the long-polling transport is currently available when using standard threads.

Instances of class socketio.Server will automatically use the threading mode if neither eventlet nor gevent are not installed. To request the threading mode explicitly, the async_mode option can be given in the constructor:

sio = socketio.Server(async_mode='threading')

A server configured for threading is deployed as a regular web application, using any WSGI complaint multi-threaded server. The example below deploys an Socket.IO application combined with a Flask web application, using Flask's development web server based on Werkzeug:

sio = socketio.Server(async_mode='threading')
app = Flask(__name__)
app.wsgi_app = socketio.Middleware(sio, app.wsgi_app)

# ... Socket.IO and Flask handler functions ...

if __name__ == '__main__':
    app.run(threaded=True)

When using the threading mode, it is important to ensure that the WSGI server can handle multiple concurrent requests using threads, since a client can have up to two outstanding requests at any given time. The Werkzeug server is single-threaded by default, so the threaded=True option is required.

Note that servers that use worker processes instead of threads, such as gunicorn, do not support a Socket.IO server configured in threading mode.

Socket.IO is a stateful protocol, which makes horizontal scaling more difficult. To deploy a cluster of Socket.IO processes (hosted on one or multiple servers), the following conditions must be met:

• Each Socket.IO process must be able to handle multiple requests, either by using eventlet, gevent, or standard threads. Worker processes that only handle one request at a time are not supported.
• The load balancer must be configured to always forward requests from a client to the same process. Load balancers call this sticky sessions, or session affinity.

A limitation in the current release of the Socket.IO server is that because the clients are randomly assigned to different server processes, any form of broadcasting is not supported. A storage backend that enables multiple processes to share information about clients is currently in development to address this important limitation.

.. toctree::
   :maxdepth: 2

.. module:: socketio

.. autoclass:: Middleware
   :members:

.. autoclass:: Server
   :members: