The Socket.IO Server

This package contains two Socket.IO servers:

The methods in the two servers are the same, with the only difference that in the asyncio server most methods are implemented as coroutines.

Installation

To install the Socket.IO server along with its dependencies, use the following command:

pip install python-socketio

In addition to the server, you will need to select an asynchronous framework or server to use along with it. The list of supported packages is covered in the Deployment Strategies section.

Creating a Server Instance

A Socket.IO server is an instance of class socketio.Server. This instance can be transformed into a standard WSGI application by wrapping it with the socketio.WSGIApp class:

import socketio

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

# wrap with a WSGI application
app = socketio.WSGIApp(sio)

For asyncio based servers, the socketio.AsyncServer class provides the same functionality, but in a coroutine friendly format. If desired, The socketio.ASGIApp class can transform the server into a standard ASGI application:

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

# wrap with ASGI application
app = socketio.ASGIApp(sio)

These two wrappers can also act as middlewares, forwarding any traffic that is not intended to the Socket.IO server to another application. This allows Socket.IO servers to integrate easily into existing WSGI or ASGI applications:

from wsgi import app  # a Flask, Django, etc. application
app = socketio.WSGIApp(sio, app)

Serving Static Files

The Engine.IO server can be configured to serve static files to clients. This is particularly useful to deliver HTML, CSS and JavaScript files to clients when this package is used without a companion web framework.

Static files are configured with a Python dictionary in which each key/value pair is a static file mapping rule. In its simplest form, this dictionary has one or more static file URLs as keys, and the corresponding files in the server as values:

static_files = {
    '/': 'latency.html',
    '/static/socket.io.js': 'static/socket.io.js',
    '/static/style.css': 'static/style.css',
}

With this example configuration, when the server receives a request for / (the root URL) it will return the contents of the file latency.html in the current directory, and will assign a content type based on the file extension, in this case text/html.

Files with the .html, .css, .js, .json, .jpg, .png, .gif and .txt file extensions are automatically recognized and assigned the correct content type. For files with other file extensions or with no file extension, the application/octet-stream content type is used as a default.

If desired, an explicit content type for a static file can be given as follows:

static_files = {
    '/': {'filename': 'latency.html', 'content_type': 'text/plain'},
}

It is also possible to configure an entire directory in a single rule, so that all the files in it are served as static files:

static_files = {
    '/static': './public',
}

In this example any files with URLs starting with /static will be served directly from the public folder in the current directory, so for example, the URL /static/index.html will return local file ./public/index.html and the URL /static/css/styles.css will return local file ./public/css/styles.css.

If a URL that ends in a / is requested, then a default filename of index.html is appended to it. In the previous example, a request for the /static/ URL would return local file ./public/index.html. The default filename to serve for slash-ending URLs can be set in the static files dictionary with an empty key:

static_files = {
    '/static': './public',
    '': 'image.gif',
}

With this configuration, a request for /static/ would return local file ./public/image.gif. A non-standard content type can also be specified if needed:

static_files = {
    '/static': './public',
    '': {'filename': 'image.gif', 'content_type': 'text/plain'},
}

The static file configuration dictionary is given as the static_files argument to the socketio.WSGIApp or socketio.ASGIApp classes:

# for standard WSGI applications
sio = socketio.Server()
app = socketio.WSGIApp(sio, static_files=static_files)

# for asyncio-based ASGI applications
sio = socketio.AsyncServer()
app = socketio.ASGIApp(sio, static_files=static_files)

The routing precedence in these two classes is as follows:

  • First, the path is checked against the Socket.IO endpoint.
  • Next, the path is checked against the static file configuration, if present.
  • If the path did not match the Socket.IO endpoint or any static file, control is passed to the secondary application if configured, else a 404 error is returned.

Note: static file serving is intended for development use only, and as such it lacks important features such as caching. Do not use in a production environment.

Defining Event Handlers

The Socket.IO protocol is event based. When a client wants to communicate with the server it emits an event. Each event has a name, and a list of arguments. The server registers event handler functions with the socketio.Server.event() or socketio.Server.on() decorators:

@sio.event
def my_event(sid, data):
    pass

@sio.on('my custom event')
def another_event(sid, data):
    pass

In the first example the event name is obtained from the name of the handler function. The second example is slightly more verbose, but it allows the event name to be different than the function name or to include characters that are illegal in function names, such as spaces.

For asyncio servers, event handlers can optionally be given as coroutines:

@sio.event
async def my_event(sid, data):
    pass

The sid argument is the Socket.IO session id, a unique identifier of each client connection. All the events sent by a given client will have the same sid value.

The connect and disconnect events are special; they are invoked automatically when a client connects or disconnects from the server:

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

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

The connect event is an ideal place to perform user authentication, and any necessary mapping between user entities in the application and the sid that was assigned to the client. The environ argument is a dictionary in standard WSGI format containing the request information, including HTTP headers. After inspecting the request, the connect event handler can return False to reject the connection with the client.

Sometimes it is useful to pass data back to the client being rejected. In that case instead of returning False socketio.exceptions.ConnectionRefusedError can be raised, and all of its arguments will be sent to the client with the rejection message:

@sio.event
def connect(sid, environ):
    raise ConnectionRefusedError('authentication failed')

Emitting Events

Socket.IO is a bidirectional protocol, so at any time the server can send an event to its connected clients. The socketio.Server.emit() method is used for this task:

sio.emit('my event', {'data': 'foobar'})

Sometimes the server may want to send an event just to a particular client. This can be achieved by adding a room argument to the emit call:

sio.emit('my event', {'data': 'foobar'}, room=user_sid)

The 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 event handler function. The room argument is used to identify the client that should receive the event, and is set to the sid value assigned to that client’s connection with the server. When omitted, the event is broadcasted to all connected clients.

Event Callbacks

When a client sends an event to the server, it can optionally provide a callback function, to be invoked as a way of acknowledgment that the server has processed the event. While this is entirely managed by the client, the server can provide a list of values that are to be passed on to the callback function, simply by returning them from the handler function:

@sio.event
def my_event(sid, data):
    # handle the message
    return "OK", 123

Likewise, the server can request a callback function to be invoked after a client has processed an event. The socketio.Server.emit() method has an optional callback argument that can be set to a callable. If this argument is given, the callable will be invoked after the client has processed the event, and any values returned by the client will be passed as arguments to this function. 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.

Namespaces

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 (sids), 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 socketio.Server class:

@sio.event(namespace='/chat')
def my_custom_event(sid, data):
    pass

@sio.on('my custom event', namespace='/chat')
def my_custom_event(sid, data):
    pass

When emitting an event, the namespace optional argument is used to specify which namespace to send it on. When the namespace argument is omitted, the default Socket.IO namespace, which is named /, is used.

Class-Based Namespaces

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 socketio.Namespace:

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

    def on_disconnect(self, sid):
        pass

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

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

For asyncio based severs, namespaces must inherit from socketio.AsyncNamespace, and can define event handlers as coroutines if desired:

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

    def on_disconnect(self, sid):
        pass

    async def on_my_event(self, 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 use 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 socketio.Server and 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.

Rooms

To make it easy for the server to emit events to groups of related clients, the application can put its clients into “rooms”, and then address messages to these rooms.

In the previous section the room argument of the socketio.SocketIO.emit() method was used to designate a specific client as the recipient of the event. This is because upon connection, a personal room for each client is created and named with the sid assigned to the connection. The application is then free to create additional rooms and manage which clients are in them using the socketio.Server.enter_room() and socketio.Server.leave_room() methods. Clients can be in as many rooms as needed and can be moved between rooms as often as necessary.

@sio.event
def begin_chat(sid):
   sio.enter_room(sid, 'chat_users')

 @sio.event
 def exit_chat(sid):
     sio.leave_room(sid, 'chat_users')

In chat applications it is often desired that an event is broadcasted to all the members of the room except one, which is the originator of the event such as a chat message. The socketio.Server.emit() method provides an optional skip_sid argument to indicate a client that should be skipped during the broadcast.

@sio.event
def my_message(sid, data):
    sio.emit('my reply', data, room='chat_users', skip_sid=sid)

User Sessions

The server can maintain application-specific information in a user session dedicated to each connected client. Applications can use the user session to write any details about the user that need to be preserved throughout the life of the connection, such as usernames or user ids.

The save_session() and get_session() methods are used to store and retrieve information in the user session:

@sio.event
def connect(sid, environ):
    username = authenticate_user(environ)
    sio.save_session(sid, {'username': username})

@sio.event
def message(sid, data):
    session = sio.get_session(sid)
    print('message from ', session['username'])

For the asyncio server, these methods are coroutines:

@sio.event
async def connect(sid, environ):
    username = authenticate_user(environ)
    await sio.save_session(sid, {'username': username})

@sio.event
async def message(sid, data):
    session = await sio.get_session(sid)
    print('message from ', session['username'])

The session can also be manipulated with the session() context manager:

@sio.event
def connect(sid, environ):
    username = authenticate_user(environ)
    with sio.session(sid) as session:
        session['username'] = username

@sio.event
def message(sid, data):
    with sio.session(sid) as session:
        print('message from ', session['username'])

For the asyncio server, an asynchronous context manager is used:

@sio.event
def connect(sid, environ):
    username = authenticate_user(environ)
    async with sio.session(sid) as session:
        session['username'] = username

@sio.event
def message(sid, data):
    async with sio.session(sid) as session:
        print('message from ', session['username'])

The get_session(), save_session() and session() methods take an optional namespace argument. If this argument isn’t provided, the session is attached to the default namespace.

Using a Message Queue

When working with distributed applications, it is often necessary to access the functionality of the Socket.IO from multiple processes. There are two specific use cases:

  • Applications that use a work queues such as Celery may need to emit an event to a client once a background job completes. The most convenient place to carry out this task is the worker process that handled this job.
  • Highly available applications may want to use horizontal scaling of the Socket.IO server to be able to handle very large number of concurrent clients.

As a solution to the above problems, the Socket.IO server can be configured to connect to a message queue such as Redis or RabbitMQ, to communicate with other related Socket.IO servers or auxiliary workers.

Redis

To use a Redis message queue, a Python Redis client must be installed:

# socketio.Server class
pip install redis

# socketio.AsyncServer class
pip install aioredis

The Redis queue is configured through the socketio.RedisManager and socketio.AsyncRedisManager classes. These classes connect directly to the Redis store and use the queue’s pub/sub functionality:

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

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

The client_manager argument instructs the server to connect to the given message queue, and to coordinate with other processes connected to the queue.

Kombu

Kombu is a Python package that provides access to RabbitMQ and many other message queues. It 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 via Kombu, the Python package for Redis needs to be installed as well:

pip install redis

The queue is configured through the socketio.KombuManager:

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

The connection URL passed to the KombuManager constructor is passed directly to Kombu’s Connection object, so the Kombu documentation should be consulted for information on how to build the correct URL for a given message queue.

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

Emitting from external processes

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 as an external process
external_sio = socketio.RedisManager('redis://', write_only=True)

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

Deployment Strategies

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

Aiohttp

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)

Tornado

Tornado is a web framework with support for HTTP and WebSocket. Support for this framework requires Python 3.5 and newer. Only Tornado version 5 and newer are supported, thanks to its tight integration with asyncio.

Instances of class socketio.AsyncServer will automatically use tornado 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='tornado')

A server configured for tornado must include a request handler for Socket.IO:

app = tornado.web.Application(
    [
        (r"/socketio.io/", socketio.get_tornado_handler(sio)),
    ],
    # ... other application options
)

The tornado application can define other 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 tornado application is then executed in the usual manner:

app.listen(port)
tornado.ioloop.IOLoop.current().start()

Sanic

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 = Sanic()
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()

It has been reported that the CORS support provided by the Sanic extension sanic-cors is incomaptible with this package’s own support for this protocol. To disable CORS support in this package and let Sanic take full control, initialize the server as follows:

sio = socketio.AsyncServer(async_mode='sanic', cors_allowed_origins=[])

On the Sanic side you will need to enable the CORS_SUPPORTS_CREDENTIALS setting in addition to any other configuration that you use:

app.config['CORS_SUPPORTS_CREDENTIALS'] = True

Uvicorn, Daphne, and other ASGI servers

The socketio.ASGIApp class is an ASGI compatible application that can forward Socket.IO traffic to an socketio.AsyncServer instance:

sio = socketio.AsyncServer(async_mode='asgi')
app = socketio.ASGIApp(sio)

The application can then be deployed with any ASGI compatible web server.

Eventlet

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.WSGIApp:

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

Eventlet with Gunicorn

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

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.WSGIApp:

app = socketio.WSGIApp(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.WSGIApp(sio)
pywsgi.WSGIServer(('', 8000), app,
                  handler_class=WebSocketHandler).serve_forever()

Gevent with Gunicorn

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.

uWSGI

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 asynchronous 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

Standard Threads

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.WSGIApp(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.

Scalability Notes

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 concurrently. This is required because long-polling clients send two requests in parallel. Worker processes that can 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 need to communicate with each other to coordinate complex operations such as broadcasts. This is done through a configured message queue. See the section on using message queues for details.