oracular (1) python-socketio.1.gz
NAME
python-socketio - python-socketio Documentation
This projects implements Socket.IO clients and servers that can run standalone or integrated with a
variety of Python web frameworks.
GETTING STARTED
What is Socket.IO?
Socket.IO is a transport protocol that enables real-time bidirectional event-based communication between
clients (typically, though not always, web browsers) and a server. The official implementations of the
client and server components are written in JavaScript. This package provides Python implementations of
both, each with standard and asyncio variants.
Version compatibility
The Socket.IO protocol has been through a number of revisions, and some of these introduced backward
incompatible changes, which means that the client and the server must use compatible versions for
everything to work.
If you are using the Python client and server, the easiest way to ensure compatibility is to use the same
version of this package for the client and the server. If you are using this package with a different
client or server, then you must ensure the versions are compatible.
The version compatibility chart below maps versions of this package to versions of the JavaScript
reference implementation and the versions of the Socket.IO and Engine.IO protocols.
┌──────────────────┬─────────────────────┬─────────────────────┬─────────────────┬─────────────────┐
│JavaScript │ Socket.IO protocol │ Engine.IO protocol │ python-socketio │ python-engineio │
│Socket.IO version │ revision │ revision │ version │ version │
├──────────────────┼─────────────────────┼─────────────────────┼─────────────────┼─────────────────┤
│0.9.x │ 1, 2 │ 1, 2 │ Not supported │ Not supported │
├──────────────────┼─────────────────────┼─────────────────────┼─────────────────┼─────────────────┤
│1.x and 2.x │ 3, 4 │ 3 │ 4.x │ 3.x │
├──────────────────┼─────────────────────┼─────────────────────┼─────────────────┼─────────────────┤
│3.x and 4.x │ 5 │ 4 │ 5.x │ 4.x │
└──────────────────┴─────────────────────┴─────────────────────┴─────────────────┴─────────────────┘
Client Examples
The example that follows shows a simple Python client:
import socketio
sio = socketio.Client()
@sio.event
def connect():
print('connection established')
@sio.event
def my_message(data):
print('message received with ', data)
sio.emit('my response', {'response': 'my response'})
@sio.event
def disconnect():
print('disconnected from server')
sio.connect('http://localhost:5000')
sio.wait()
Below is a similar client, coded for asyncio (Python 3.5+ only):
import asyncio
import socketio
sio = socketio.AsyncClient()
@sio.event
async def connect():
print('connection established')
@sio.event
async def my_message(data):
print('message received with ', data)
await sio.emit('my response', {'response': 'my response'})
@sio.event
async def disconnect():
print('disconnected from server')
async def main():
await sio.connect('http://localhost:5000')
await sio.wait()
if __name__ == '__main__':
asyncio.run(main())
Client Features
• Can connect to other Socket.IO servers that are compatible with the JavaScript Socket.IO 1.x and 2.x
releases. Work to support release 3.x is in progress.
• Compatible with Python 3.6+.
• Two versions of the client, one for standard Python and another for asyncio.
• Uses an event-based architecture implemented with decorators that hides the details of the protocol.
• Implements HTTP long-polling and WebSocket transports.
• Automatically reconnects to the server if the connection is dropped.
Server Examples
The following application is a basic server example that uses the Eventlet asynchronous server:
import eventlet
import socketio
sio = socketio.Server()
app = socketio.WSGIApp(sio, static_files={
'/': {'content_type': 'text/html', 'filename': 'index.html'}
})
@sio.event
def connect(sid, environ):
print('connect ', sid)
@sio.event
def my_message(sid, data):
print('message ', data)
@sio.event
def disconnect(sid):
print('disconnect ', sid)
if __name__ == '__main__':
eventlet.wsgi.server(eventlet.listen(('', 5000)), app)
Below is a similar application, coded for asyncio (Python 3.5+ only) and the Uvicorn web server:
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.event
def connect(sid, environ):
print("connect ", sid)
@sio.event
async def chat_message(sid, data):
print("message ", data)
@sio.event
def disconnect(sid):
print('disconnect ', sid)
app.router.add_static('/static', 'static')
app.router.add_get('/', index)
if __name__ == '__main__':
web.run_app(app)
Server Features
• Can connect to servers running other Socket.IO clients that are compatible with the JavaScript client
versions 1.x and 2.x. Work to support the 3.x release is in progress.
• Compatible with Python 3.6+.
• Two versions of the server, one for standard Python and another for asyncio.
• Supports large number of clients even on modest hardware due to being asynchronous.
• Can be hosted on any WSGI and ASGI web servers including Gunicorn, Uvicorn, eventlet and gevent.
• Can be integrated with WSGI applications written in frameworks such as Flask, Django, etc.
• Can be integrated with aiohttp, sanic and tornado asyncio 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.
THE SOCKET.IO CLIENT
This package contains two Socket.IO clients:
• The socketio.Client() class creates a client compatible with the standard Python library.
• The socketio.AsyncClient() class creates a client compatible with the asyncio package.
The methods in the two clients are the same, with the only difference that in the asyncio client most
methods are implemented as coroutines.
Installation
To install the standard Python client along with its dependencies, use the following command:
pip install "python-socketio[client]"
If instead you plan on using the asyncio client, then use this:
pip install "python-socketio[asyncio_client]"
Creating a Client Instance
To instantiate an Socket.IO client, simply create an instance of the appropriate client class:
import socketio
# standard Python
sio = socketio.Client()
# asyncio
sio = socketio.AsyncClient()
Defining Event Handlers
The Socket.IO protocol is event based. When a server wants to communicate with a client it emits an
event. Each event has a name, and a list of arguments. The client registers event handler functions with
the socketio.Client.event() or socketio.Client.on() decorators:
@sio.event
def message(data):
print('I received a message!')
@sio.on('my message')
def on_message(data):
print('I received a message!')
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 the asyncio client, event handlers can be regular functions as above, or can also be coroutines:
@sio.event
async def message(data):
print('I received a message!')
If the server includes arguments with an event, those are passed to the handler function as arguments.
Catch-All Event Handlers
A "catch-all" event handler is invoked for any events that do not have an event handler. You can define a
catch-all handler using '*' as event name:
@sio.on('*')
def catch_all(event, data):
pass
Asyncio clients can also use a coroutine:
@sio.on('*')
async def catch_all(event, data):
pass
A catch-all event handler receives the event name as a first argument. The remaining arguments are the
same as for a regular event handler.
Connect, Connect Error and Disconnect Event Handlers
The connect, connect_error and disconnect events are special; they are invoked automatically when a
client connects or disconnects from the server:
@sio.event
def connect():
print("I'm connected!")
@sio.event
def connect_error(data):
print("The connection failed!")
@sio.event
def disconnect():
print("I'm disconnected!")
The connect_error handler is invoked when a connection attempt fails. If the server provides arguments,
these are passed on to the handler. The server can use an argument to provide information to the client
regarding the connection failure.
The disconnect handler is invoked for application initiated disconnects, server initiated disconnects, or
accidental disconnects, for example due to networking failures. In the case of an accidental
disconnection, the client is going to attempt to reconnect immediately after invoking the disconnect
handler. As soon as the connection is re-established the connect handler will be invoked once again.
The connect, connect_error and disconnect events have to be defined explicitly and are not invoked on a
catch-all event handler.
Connecting to a Server
The connection to a server is established by calling the connect() method:
sio.connect('http://localhost:5000')
In the case of the asyncio client, the method is a coroutine:
await sio.connect('http://localhost:5000')
Upon connection, the server assigns the client a unique session identifier. The applicaction can find
this identifier in the sid attribute:
print('my sid is', sio.sid)
Emitting Events
The client can emit an event to the server using the emit() method:
sio.emit('my message', {'foo': 'bar'})
Or in the case of asyncio, as a coroutine:
await sio.emit('my message', {'foo': 'bar'})
The single argument provided to the method is the data that is passed on to the server. The data can be
of type str, bytes, dict, list or tuple. 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
server-side event handler function.
The emit() method can be invoked inside an event handler as a response to a server event, or in any other
part of the application, including in background tasks.
Event Callbacks
When a server emits an event to a client, 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
server, the client can provide a list of return values that are to be passed on to the callback function
set up by the server. This is achieved simply by returning the desired values from the handler function:
@sio.event
def my_event(sid, data):
# handle the message
return "OK", 123
Likewise, the client can request a callback function to be invoked after the server 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 server has processed the event, and any
values returned by the server handler will be passed as arguments to this function.
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. Namespaces
use a path syntax starting with a forward slash. A list of namespaces can be given by the client in the
connect() call. For example, this example creates two logical connections, the default one plus a second
connection under the /chat namespace:
sio.connect('http://localhost:5000', namespaces=['/chat'])
To define event handlers on a namespace, the namespace argument must be added to the corresponding
decorator:
@sio.event(namespace='/chat')
def my_custom_event(sid, data):
pass
@sio.on('connect', namespace='/chat')
def on_connect():
print("I'm connected to the /chat namespace!")
Likewise, the client can emit an event to the server on a namespace by providing its in the emit() call:
sio.emit('my message', {'foo': 'bar'}, namespace='/chat')
If the namespaces argument of the connect() call isn't given, any namespaces used in event handlers are
automatically connected.
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.ClientNamespace:
class MyCustomNamespace(socketio.ClientNamespace):
def on_connect(self):
pass
def on_disconnect(self):
pass
def on_my_event(self, data):
self.emit('my_response', data)
sio.register_namespace(MyCustomNamespace('/chat'))
For asyncio based servers, namespaces must inherit from socketio.AsyncClientNamespace, and can define
event handlers as coroutines if desired:
class MyCustomNamespace(socketio.AsyncClientNamespace):
def on_connect(self):
pass
def on_disconnect(self):
pass
async def on_my_event(self, data):
await self.emit('my_response', data)
sio.register_namespace(MyCustomNamespace('/chat'))
When class-based namespaces are used, any events received by the client 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.Client and socketio.AsyncClient 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.
Disconnecting from the Server
At any time the client can request to be disconnected from the server by invoking the disconnect()
method:
sio.disconnect()
For the asyncio client this is a coroutine:
await sio.disconnect()
Managing Background Tasks
When a client connection to the server is established, a few background tasks will be spawned to keep the
connection alive and handle incoming events. The application running on the main thread is free to do any
work, as this is not going to prevent the functioning of the Socket.IO client.
If the application does not have anything to do in the main thread and just wants to wait until the
connection with the server ends, it can call the wait() method:
sio.wait()
Or in the asyncio version:
await sio.wait()
For the convenience of the application, a helper function is provided to start a custom background task:
def my_background_task(my_argument):
# do some background work here!
pass
task = sio.start_background_task(my_background_task, 123)
The arguments passed to this method are the background function and any positional or keyword arguments
to invoke the function with.
Here is the asyncio version:
async def my_background_task(my_argument):
# do some background work here!
pass
task = sio.start_background_task(my_background_task, 123)
Note that this function is not a coroutine, since it does not wait for the background function to end.
The background function must be a coroutine.
The sleep() method is a second convenience function that is provided for the benefit of applications
working with background tasks of their own:
sio.sleep(2)
Or for asyncio:
await sio.sleep(2)
The single argument passed to the method is the number of seconds to sleep for.
Debugging and Troubleshooting
To help you debug issues, the client can be configured to output logs to the terminal:
import socketio
# standard Python
sio = socketio.Client(logger=True, engineio_logger=True)
# asyncio
sio = socketio.AsyncClient(logger=True, engineio_logger=True)
The logger argument controls logging related to the Socket.IO protocol, while engineio_logger controls
logs that originate in the low-level Engine.IO transport. These arguments can be set to True to output
logs to stderr, or to an object compatible with Python's logging package where the logs should be emitted
to. A value of False disables logging.
Logging can help identify the cause of connection problems, unexpected disconnections and other issues.
THE SOCKET.IO SERVER
This package contains two Socket.IO servers:
• The socketio.Server() class creates a server compatible with the Python standard library.
• The socketio.AsyncServer() class creates a server compatible with the asyncio package.
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 Socket.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.
Catch-All Event Handlers
A "catch-all" event handler is invoked for any events that do not have an event handler. You can define a
catch-all handler using '*' as event name:
@sio.on('*')
def catch_all(event, sid, data):
pass
Asyncio servers can also use a coroutine:
@sio.on('*')
async def catch_all(event, sid, data):
pass
A catch-all event handler receives the event name as a first argument. The remaining arguments are the
same as for a regular event handler.
The connect and disconnect events have to be defined explicitly and are not invoked on a catch-all event
handler.
Connect and Disconnect Event Handlers
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, auth):
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. The auth
argument contains any authentication details passed by the client, or None if the client did not pass
anything. 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 servers, 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.
Note: the contents of the user session are destroyed when the client disconnects. In particular, user
session contents are not preserved when a client reconnects after an unexpected disconnection from the
server.
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 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.
Kafka
Apache Kafka is supported through the kafka-python package:
pip install kafka-python
Access to Kafka is configured through the socketio.KafkaManager class:
mgr = socketio.KafkaManager('kafka://')
sio = socketio.Server(client_manager=mgr)
Note that Kafka currently does not support asyncio, so it cannot be used with the socketio.AsyncServer
class.
AioPika
A RabbitMQ message queue is supported in asyncio applications through the AioPika package:: You need to
install aio_pika with pip:
pip install aio_pika
The RabbitMQ queue is configured through the socketio.AsyncAioPikaManager class:
mgr = socketio.AsyncAioPikaManager('amqp://')
sio = socketio.AsyncServer(client_manager=mgr)
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')
A limitation of the write-only client manager object is that it cannot receive callbacks when emitting.
When the external process needs to receive callbacks, using a client to connect to the server with read
and write support is a better option than a write-only client manager.
Debugging and Troubleshooting
To help you debug issues, the server can be configured to output logs to the terminal:
import socketio
# standard Python
sio = socketio.Server(logger=True, engineio_logger=True)
# asyncio
sio = socketio.AsyncServer(logger=True, engineio_logger=True)
The logger argument controls logging related to the Socket.IO protocol, while engineio_logger controls
logs that originate in the low-level Engine.IO transport. These arguments can be set to True to output
logs to stderr, or to an object compatible with Python's logging package where the logs should be emitted
to. A value of False disables logging.
Logging can help identify the cause of connection problems, 400 responses, bad performance and other
issues.
Deployment Strategies
The following sections describe a variety of deployment strategies for Socket.IO servers.
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)
If desired, the socketio.ASGIApp class can forward any traffic that is not Socket.IO to another ASGI
application, making it possible to deploy a standard ASGI web application and the Socket.IO server as a
bundle:
sio = socketio.AsyncServer(async_mode='asgi')
app = socketio.ASGIApp(sio, other_app)
The ASGIApp instance is a fully complaint ASGI instance that can be deployed with an ASGI compatible web
server.
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"/socket.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
Note: Due to some backward incompatible changes introduced in recent versions of Sanic, it is currently
recommended that a Sanic application is deployed with the ASGI integration instead.
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 incompatible
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
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.
Instances of class socketio.Server will automatically use the threading mode if neither eventlet nor
gevent are 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()
The example that follows shows how to start an Socket.IO application using Gunicorn's threaded worker
class:
$ gunicorn -w 1 --threads 100 module:app
With the above configuration the server will be able to handle up to 100 concurrent clients.
When using standard threads, WebSocket is supported through the simple-websocket package, which must be
installed separately. This package provides a multi-threaded WebSocket server that is compatible with
Werkzeug and Gunicorn's threaded worker. Other multi-threaded web servers are not supported and will not
enable the WebSocket transport.
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.
Cross-Origin Controls
For security reasons, this server enforces a same-origin policy by default. In practical terms, this
means the following:
• If an incoming HTTP or WebSocket request includes the Origin header, this header must match the scheme
and host of the connection URL. In case of a mismatch, a 400 status code response is returned and the
connection is rejected.
• No restrictions are imposed on incoming requests that do not include the Origin header.
If necessary, the cors_allowed_origins option can be used to allow other origins. This argument can be
set to a string to set a single allowed origin, or to a list to allow multiple origins. A special value
of '*' can be used to instruct the server to allow all origins, but this should be done with care, as
this could make the server vulnerable to Cross-Site Request Forgery (CSRF) attacks.
API REFERENCE
Client class
AsyncClient class
Server class
AsyncServer class
ConnectionRefusedError class
WSGIApp class
ASGIApp class
Middleware class (deprecated)
ClientNamespace class
Namespace class
AsyncClientNamespace class
AsyncNamespace class
BaseManager class
PubSubManager class
KombuManager class
RedisManager class
KafkaManager class
AsyncManager class
AsyncRedisManager class
AsyncAioPikaManager class
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AUTHOR
Miguel Grinberg
COPYRIGHT
2018, Miguel Grinberg
Jan 14, 2023 PYTHON-SOCKETIO(1)