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

• Fully compatible with the Javascript, Swift, C++ and Java official Socket.IO clients, plus any third party clients that comply with the Socket.IO specification.
• 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 asyncio (sanic and aiohttp), eventlet or gevent. For development and testing, any WSGI compliant multi-threaded server can also 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".
• Optional support for multiple servers, connected through a messaging queue such as Redis or RabbitMQ.
• Send messages to clients from external processes, such as Celery workers or auxiliary scripts.
• 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 original implementations of the client and server components are written in JavaScript.

The Socket.IO server can be installed with pip:

pip install python-socketio

The following is a basic example of a Socket.IO server that uses the aiohttp framework for asyncio (Python 3.5+ only):

from aiohttp import web
import socketio

sio = socketio.AsyncServer()
app = web.Application()
sio.attach(app)

async def index(request):
    """Serve the client-side application."""
    with open('index.html') as f:
        return web.Response(text=f.read(), content_type='text/html')

@sio.on('connect', namespace='/chat')
def connect(sid, environ):
    print("connect ", sid)

@sio.on('chat message', namespace='/chat')
async def message(sid, data):
    print("message ", data)
    await sio.emit('reply', room=sid)

@sio.on('disconnect', namespace='/chat')
def disconnect(sid):
    print('disconnect ', sid)

app.router.add_static('/static', 'static')
app.router.add_get('/', index)

if __name__ == '__main__':
    web.run_app(app)

And below is a similar example, but using Flask and Eventlet. This example is compatible with Python 2.7 and 3.3+:

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, which can be a single or multiple arguments. 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.

Socket.IO servers are instances of class :class:`socketio.Server`, which can be combined with a WSGI compliant application using :class:`socketio.Middleware`:

# create a Socket.IO server
sio = socketio.Server()

# wrap WSGI application with socketio's middleware
app = socketio.Middleware(sio, app)

For asyncio based servers, the :class:`socketio.AsyncServer` class provides a coroutine friendly server:

# create a Socket.IO server
sio = socketio.AsyncServer()

# attach server to application
sio.attach(app)

Event handlers for servers are register using the :func:`socketio.Server.on` method:

@sio.on('my custom event')
def my_custom_event():
    pass

For asyncio servers, event handlers can be regular functions or coroutines:

@sio.on('my custom event')
async def my_custom_event():
    await sio.emit('my reply')

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 the entire 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('leave 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, dict or tuple, and the recipient room. When sending a tuple, the elements in it need to be of any of the other four allowed types. The elements of the tuple will be passed as multiple arguments to the client-side callback function. 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 session IDs (sid``s), 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 '/', a default namespace is used.

As an alternative to the decorator-based event handlers, the event handlers that belong to a namespace can be created as methods of a subclass of :class:`socketio.Namespace`:

class MyCustomNamespace(socketio.Namespace):
    def on_connect(sid, environ):
        pass

    def on_disconnect(sid):
        pass

    def on_my_event(sid, data):
        self.emit('my_response', data)

sio.register_namespace(MyCustomNamespace('/test'))

For asyncio based severs, namespaces must inherit from :class:`socketio.AsyncNamespace`, and can define event handlers as regular methods or coroutines:

class MyCustomNamespace(socketio.AsyncNamespace):
    def on_connect(sid, environ):
        pass

    def on_disconnect(sid):
        pass

    async def on_my_event(sid, data):
        await self.emit('my_response', data)

sio.register_namespace(MyCustomNamespace('/test'))

When class-based namespaces are used, any events received by the server are dispatched to a method named as the event name with the on_ prefix. For example, event my_event will be handled by a method named on_my_event. If an event is received for which there is no corresponding method defined in the namespace class, then the event is ignored. All event names used in class-based namespaces must used characters that are legal in method names.

As a convenience to methods defined in a class-based namespace, the namespace instance includes versions of several of the methods in the :class:`socketio.Server` and :class:`socketio.AsyncServer` classes that default to the proper namespace when the namespace argument is not given.

In the case that an event has a handler in a class-based namespace, and also a decorator-based function handler, only the standalone function handler is invoked.

It is important to note that class-based namespaces are singletons. This means that a single instance of a namespace class is used for all clients, and consequently, a namespace instance cannot be used to store client specific information.

The Socket.IO server owns the socket connections to all the clients, so it is the only process that can emit events to them. Unfortunately this becomes a limitation for many applications that use more than one process. A common need is to emit events to clients from a process other than the server, for example a Celery worker.

To enable these auxiliary processes to emit events, the server can be configured to listen for externally issued events on a message queue such as Redis or RabbitMQ. Processes that need to emit events to client then post these events to the queue.

Another situation in which the use of a message queue is necessary is with high traffic applications that work with large number of clients. To support these clients, it may be necessary to horizontally scale the Socket.IO server by splitting the client list among multiple server processes. In this type of installation, each server processes owns the connections to a subset of the clients. To make broadcasting work in this environment, the servers communicate with each other through the message queue.

One of the messaging options offered by this package to access the message queue is Kombu , which means that any message queue supported by this package can be used. Kombu can be installed with pip:

pip install kombu

To use RabbitMQ or other AMQP protocol compatible queues, that is the only required dependency. But for other message queues, Kombu may require additional packages. For example, to use a Redis queue, Kombu needs the Python package for Redis installed as well:

pip install redis

The appropriate message queue service, such as RabbitMQ or Redis, must also be installed. To configure a Socket.IO server to connect to a Kombu queue, the client_manager argument must be passed in the server creation. The following example instructs the server to connect to a Redis service running on the same host and on the default port:

mgr = socketio.KombuManager('redis://')
sio = socketio.Server(client_manager=mgr)

For a RabbitMQ queue also running on the local server with default credentials, the configuration is as follows:

mgr = socketio.KombuManager('amqp://')
sio = socketio.Server(client_manager=mgr)

The URL passed to the :class:`KombuManager` constructor is passed directly to Kombu's Connection object, so the Kombu documentation should be consulted for information on how to connect to the message queue appropriately.

Note that Kombu currently does not support asyncio, so it cannot be used with the :class:`socketio.AsyncServer` class.

To use a Redis message queue, the Python package for Redis must also be installed:

# WSGI server
pip install redis

# asyncio server
pip install aioredis

Native Redis support is accessed through the :class:`socketio.RedisManager` and :class:`socketio.AsyncRedisManager` classes. These classes connect directly to the Redis store and use the queue's pub/sub functionality:

# WSGI server
mgr = socketio.RedisManager('redis://')
sio = socketio.Server(client_manager=mgr)

# asyncio server
mgr = socketio.AsyncRedisManager('redis://')
sio = socketio.AsyncServer(client_manager=mgr)

If multiple Socket.IO servers are connected to the same message queue, they automatically communicate with each other and manage a combined client list, without any need for additional configuration. When a load balancer such as nginx is used, this provides virtually unlimited scaling capabilities for the server.

To have a process other than a server connect to the queue to emit a message, the same client manager classes can be used as standalone objects. In this case, the write_only argument should be set to True to disable the creation of a listening thread, which only makes sense in a server. For example:

# connect to the redis queue through Kombu
external_sio = socketio.KombuManager('redis://', write_only=True)

# emit an event
external_sio.emit('my event', data={'foo': 'bar'}, room='my room')

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

Sanic is a very efficient asynchronous web server for Python 3.5 and newer.

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

sio = socketio.AsyncServer(async_mode='sanic')

A server configured for aiohttp must be attached to an existing application:

app = web.Application()
sio.attach(app)

The Sanic application can define regular routes that will coexist with the Socket.IO server. A typical pattern is to add routes that serve a client application and any associated static files.

The Sanic application is then executed in the usual manner:

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

Aiohttp is a framework with support for HTTP and WebSocket, based on asyncio. Support for this framework is limited to Python 3.5 and newer.

Instances of class socketio.AsyncServer will automatically use aiohttp 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.AsyncServer(async_mode='aiohttp')

A server configured for aiohttp must be attached to an existing application:

app = web.Application()
sio.attach(app)

The aiohttp application can define regular routes that will coexist with the Socket.IO server. A typical pattern is to add routes that serve a client application and any associated static files.

The aiohttp application is then executed in the usual manner:

if __name__ == '__main__':
    web.run_app(app)

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 option cannot be set to a value higher than 1. A single eventlet worker can handle a large number of concurrent clients, each handled by a greenlet.

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 option cannot be higher than 1. A single gevent worker can handle a large number of concurrent clients through the use of greenlets.

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.

When using the uWSGI server in combination with gevent, the Socket.IO server can take advantage of uWSGI's native WebSocket support.

Instances of class socketio.Server will automatically use this option for asynchronous operations if both gevent and uWSGI are installed and eventlet is not installed. To request this asynchoronous mode explicitly, the async_mode option can be given in the constructor:

# gevent with uWSGI
sio = socketio.Server(async_mode='gevent_uwsgi')

A complete explanation of the configuration and usage of the uWSGI server is beyond the scope of this documentation. The uWSGI server is a fairly complex package that provides a large and comprehensive set of options. It must be compiled with WebSocket and SSL support for the WebSocket transport to be available. As way of an introduction, the following command starts a uWSGI server for the latency.py example on port 5000:

$ uwsgi --http :5000 --gevent 1000 --http-websockets --master --wsgi-file latency.py --callable app

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 asyncio, 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 worker process. Load balancers call this sticky sessions, or session affinity.
• The worker processes communicate with each other through a message queue, which must be installed and configured. See the section on using message queues above for instructions.

.. module:: socketio

.. autoclass:: Middleware
   :members:

.. autoclass:: Server
   :members:

.. autoclass:: AsyncServer
   :members:
   :inherited-members:

.. autoclass:: Namespace
   :members:

.. autoclass:: AsyncNamespace
   :members:
   :inherited-members:

.. autoclass:: BaseManager
   :members:

.. autoclass:: PubSubManager
   :members:

.. autoclass:: KombuManager
   :members:

.. autoclass:: RedisManager
   :members:

.. autoclass:: AsyncManager
   :members:
   :inherited-members:

.. autoclass:: AsyncRedisManager
   :members: