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 theasyncio
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
If you plan to build an asynchronous web server based on the asyncio
package, then you can install this package and some additional dependencies
that are needed with:
pip install "python-socketio[asyncio]"
Creating a Server Instance¶
A Socket.IO server is an instance of class socketio.Server
:
import socketio
# create a Socket.IO server
sio = socketio.Server()
For asyncio based servers, the socketio.AsyncServer
class provides
the same functionality, but in a coroutine friendly format:
import socketio
# create a Socket.IO server
sio = socketio.AsyncServer()
Running the Server¶
To run the Socket.IO application it is necessary to configure a web server to receive incoming requests from clients and forward them to the Socket.IO server instance. To simplify this task, several integrations are available, including support for the WSGI and ASGI standards.
Running as a WSGI Application¶
To configure the Socket.IO server as a WSGI application wrap the server
instance with the socketio.WSGIApp
class:
# wrap with a WSGI application
app = socketio.WSGIApp(sio)
The resulting WSGI application can be executed with supported WSGI servers such as Werkzeug for development and Gunicorn for production.
When combining Socket.IO with a web application written with a WSGI framework
such as Flask or Django, the WSGIApp
class can wrap both applications
together and route traffic to them:
from mywebapp import app # a Flask, Django, etc. application
app = socketio.WSGIApp(sio, app)
Running as an ASGI Application¶
To configure the Socket.IO server as an ASGI application wrap the server
instance with the socketio.ASGIApp
class:
# wrap with ASGI application
app = socketio.ASGIApp(sio)
The resulting ASGI application can be executed with an ASGI compliant web server, for example Uvicorn.
Socket.IO can also be combined with a web application written with an ASGI
web framework such as FastAPI. In that case, the ASGIApp
class can wrap
both applications together and route traffic to them:
from mywebapp import app # a FastAPI or other ASGI application
app = socketio.ASGIApp(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.
Events¶
The Socket.IO protocol is event based. When a client wants to communicate with the server, or the server wants to communicate with one or more clients, they emit an event to the other party. Each event has a name, and an optional list of arguments.
Listening to Events¶
To receive events from clients, the server application must register event
handler functions. These functions are invoked when the corresponding events
are emitted by clients. To register a handler for an event, the
socketio.Server.event()
or socketio.Server.on()
decorators are used:
@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 that is passed to all handlers is the Socket.IO session
id, a unique identifier that Socket.IO assigns to each client connection. All
the events sent by a given client will have the same sid
value.
Connect and Disconnect Events¶
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.
In addition to the sid
, the connect handler receives environ
as an
argument, with the request information in standard WSGI format, including HTTP
headers. The connect handler also receives the auth
argument with any
authentication details passed by the client, or None
if the client did not
pass any authentication.
After inspecting the arguments, 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
a socketio.exceptions.ConnectionRefusedError
exception can be raised,
and all of its arguments will be sent to the client with the rejection
message:
@sio.event
def connect(sid, environ, auth):
raise ConnectionRefusedError('authentication failed')
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 any_event(event, sid, data):
pass
Asyncio servers can also use a coroutine:
@sio.on('*')
async def any_event(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.
Note that the connect
and disconnect
events have to be defined
explicitly and are not invoked on a catch-all event handler.
Emitting Events to Clients¶
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'})
The first argument is the event name, followed by an optional data payload of
type str
, bytes
, list
, dict
or tuple
. When sending a
list
, dict
or tuple
, the elements are also constrained to the same
data types. When a tuple
is sent, the elements of the tuple will be passed
as multiple arguments to the client-side event handler function.
The above example will send the event to all the clients are connected.
Sometimes the server may want to send an event just to one particular client.
This can be achieved by adding a to
argument to the emit call, with the
sid
of the client:
sio.emit('my event', {'data': 'foobar'}, to=user_sid)
The to
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 to
is omitted, the event is broadcasted to all
connected clients.
Acknowledging Events¶
When a client sends an event to the server, it can optionally request to receive acknowledgment from the server. The sending of acknowledgements is automatically managed by the Socket.IO server, but the event handler function can provide a list of values that are to be passed on to the client with the acknowledgement simply by returning them:
@sio.event
def my_event(sid, data):
# handle the message
return "OK", 123 # <-- client will have these as acknowledgement
Requesting Client Acknowledgements¶
Similar to how clients can request acknowledgements from the server, when the server is emitting to a single client it can also ask the client to acknowledge the event, and optionally return one or more values as a response.
The Socket.IO server supports two ways of working with client acknowledgements.
The most convenient method is to replace socketio.Server.emit()
with
socketio.Server.call()
. The call()
method will emit the event, and
then wait until the client sends an acknowledgement, returning any values
provided by the client:
response = sio.call('my event', {'data': 'foobar'}, to=user_sid)
A much more primitive acknowledgement solution uses callback functions. 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:
def my_callback():
print("callback invoked!")
sio.emit('my event', {'data': 'foobar'}, to=user_sid, callback=my_callback)
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 previous examples, the to
argument of the socketio.SocketIO.emit()
method was used to designate a specific client as the recipient of the event.
The to
argument can also be given the name of a room, and then all the
clients that are in that room will receive the event.
The application can create as many rooms as needed 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 when 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)
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 works independently from the others, with separate session
IDs (sid
s), event handlers and rooms. Namespaces can be defined directly
in the event handler functions, or they can also be created as classes.
Decorator-Based Namespaces¶
Decorator-based namespaces are regular event handlers that include the
namespace
argument in their decorator:
@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.
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. This argument must also be specified when
emitting events under a namespace. Most methods in the socketio.Server
class have the optional namespace
argument.
Class-Based Namespaces¶
As an alternative to the decorator-based namespaces, the event handlers that
belong to a namespace can be created as methods in 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.
Catch-All Namespaces¶
Similarily to catch-all event handlers, a “catch-all” namespace can be used
when defining event handlers for any connected namespaces that do not have an
explicitly defined event handler. As with catch-all events, '*'
is used in
place of a namespace:
@sio.on('my_event', namespace='*')
def my_event_any_namespace(namespace, sid, data):
pass
For these events, the namespace is passed as first argument, followed by the regular arguments of the event.
A catch-all class-based namespace handler can be defined by passing '*'
as
the namespace during registration:
sio.register_namespace(MyCustomNamespace('*'))
A “catch-all” handler for all events on all namespaces can be defined as follows:
@sio.on('*', namespace='*')
def any_event_any_namespace(event, namespace, sid, data):
pass
Event handlers with catch-all events and namespaces receive the event name and the namespace as first and second arguments.
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
async def connect(sid, environ):
username = authenticate_user(environ)
async with sio.session(sid) as session:
session['username'] = username
@sio.event
async 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.
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.
Monitoring and Administration¶
The Socket.IO server can be configured to accept connections from the official Socket.IO Admin UI. This tool provides real-time information about currently connected clients, rooms in use and events being emitted. It also allows an administrator to manually emit events, change room assignments and disconnect clients. The hosted version of this tool is available at https://admin.socket.io.
Given that enabling this feature can affect the performance of the server, it
is disabled by default. To enable it, call the
instrument()
method. For example:
import os
import socketio
sio = socketio.Server(cors_allowed_origins=[
'http://localhost:5000',
'https://admin.socket.io',
])
sio.instrument(auth={
'username': 'admin',
'password': os.environ['ADMIN_PASSWORD'],
})
This configures the server to accept connections from the hosted Admin UI
client. Administrators can then open https://admin.socket.io in their web
browsers and log in with username admin
and the password given by the
ADMIN_PASSWORD
environment variable. To ensure the Admin UI front end is
allowed to connect, CORS is also configured.
Consult the reference documentation to learn about additional configuration options that are available.
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.
Concurrency and Web Server Integration¶
The Socket.IO server can be configured with different concurrency models
depending on the needs of the application and the web server that is used. The
concurrency model is given by the async_mode
argument in the server. For
example:
sio = socketio.Server(async_mode='threading')
The following sub-sections describe the available concurrency options for synchronous and asynchronous servers.
Standard Modes¶
threading
: the server will use Python threads for concurrency and will run on any multi-threaded WSGI server. This is the default mode when no other concurrency libraries are installed.gevent
: the server will use greenlets through the gevent library for concurrency. A web server that is compatible withgevent
is required.gevent_uwsgi
: a variation of thegevent
mode that is designed to work with the uWSGI web server.eventlet
: the server will use greenlets through the eventlet library for concurrency. A web server that is compatible witheventlet
is required. Use ofeventlet
is not recommended due to this project being in maintenance mode.
Asyncio Modes¶
The asynchronous options are all based on the asyncio package of the Python standard library, with minor variations depending on the web server platform that is used.
asgi
: use of any ASGI web server is required.aiohttp
: use of the aiohttp web framework and server is required.tornado
: use of the Tornado web framework and server is required.sanic
: use of the Sanic web framework and server is required. When using Sanic, it is recommended to use theasgi
mode instead.
Deployment Strategies¶
The following sections describe a variety of deployment strategies for Socket.IO servers.
Gunicorn¶
The simplest deployment strategy for the Socket.IO server is to use the popular
Gunicorn web server in multi-threaded mode. The
Socket.IO server must be wrapped by the socketio.WSGIApp
class, so
that it is compatible with the WSGI protocol:
sio = socketio.Server(async_mode='threading')
app = socketio.WSGIApp(sio)
If desired, the socketio.WSGIApp
class can forward any traffic that is not
Socket.IO to another WSGI application, making it possible to deploy a standard
WSGI web application built with frameworks such as Flask or Django and the
Socket.IO server as a bundle:
sio = socketio.Server(async_mode='threading')
app = socketio.WSGIApp(sio, other_wsgi_app)
The example that follows shows how to start a Socket.IO application using Gunicorn’s threaded worker class:
$ gunicorn --workers 1 --threads 100 --bind 127.0.0.1:5000 module:app
With the above configuration the server will be able to handle close to 100 concurrent clients.
It is also possible to use more than one worker process, but this has two additional requirements:
The clients must connect directly over WebSocket. The long-polling transport is incompatible with the way Gunicorn load balances requests among workers. To disable long-polling in the server, add
transports=['websocket']
in the server constructor. Clients will have a similar option to initiate the connection with WebSocket.The
socketio.Server()
instances in each worker must be configured with a message queue to allow the workers to communicate with each other. See the Using a Message Queue section for more information.
When using multiple workers, the approximate number of connections the server will be able to accept can be calculated as the number of workers multiplied by the number of threads per worker.
Note that Gunicorn can also be used alongside uvicorn
, gevent
and
eventlet
. These options are discussed under the appropriate sections below.
Uvicorn (and other ASGI web servers)¶
When working with an asynchronous Socket.IO server, the easiest deployment strategy is to use an ASGI web server such as Uvicorn.
The socketio.ASGIApp
class is an ASGI compatible application that can
forward Socket.IO traffic to a 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 built with a framework such as FastAPI and the Socket.IO
server as a bundle:
sio = socketio.AsyncServer(async_mode='asgi')
app = socketio.ASGIApp(sio, other_asgi_app)
The following example starts the application with Uvicorn:
uvicorn --port 5000 module:app
Uvicorn can also be used through its Gunicorn worker:
gunicorn --workers 1 --worker-class uvicorn.workers.UvicornWorker --bind 127.0.0.1:5000
See the Gunicorn section above for information on how to use Gunicorn with multiple workers.
Hypercorn, Daphne, and other ASGI servers¶
To use an ASGI web server other than Uvicorn, configure the application for ASGI as shown above for Uvicorn, then follow the documentation of your chosen web server to start the application.
Aiohttp¶
Another option for deploying an asynchronous Socket.IO server is to use the
Aiohttp web framework and server. Instances
of class socketio.AsyncServer
will automatically use Aiohttp
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)
Gevent¶
When a multi-threaded web server is unable to satisfy the concurrency and scalability requirements of the application, an option to try is Gevent. Gevent is a coroutine-based concurrency library based on greenlets, which are significantly lighter than threads.
Instances of class socketio.Server
will automatically use Gevent if the
library is installed. To request gevent to be selected explicitly, the
async_mode
option can be given in the constructor:
sio = socketio.Server(async_mode='gevent')
The Socket.IO server must be wrapped by the socketio.WSGIApp
class, so
that it is compatible with the WSGI protocol:
app = socketio.WSGIApp(sio)
If desired, the socketio.WSGIApp
class can forward any traffic that is not
Socket.IO to another WSGI application, making it possible to deploy a standard
WSGI web application built with frameworks such as Flask or Django and the
Socket.IO server as a bundle:
sio = socketio.Server(async_mode='gevent')
app = socketio.WSGIApp(sio, other_wsgi_app)
A server configured for Gevent is deployed as a regular WSGI application
using the provided socketio.WSGIApp
:
from gevent import pywsgi
pywsgi.WSGIServer(('', 8000), app).serve_forever()
Gevent with Gunicorn¶
An alternative to running the gevent WSGI server as above is to use Gunicorn with its Gevent worker. The command to launch the application under Gunicorn and Gevent is shown below:
$ gunicorn -k gevent -w 1 -b 127.0.0.1:5000 module:app
See the Gunicorn section above for information on how to use Gunicorn with multiple workers.
Gevent provides a monkey_patch()
function that replaces all the blocking
functions in the standard library with equivalent asynchronous versions. While
the Socket.IO server does not require monkey patching, other libraries such as
database or message queue drivers are likely to require it.
Gevent with 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
Tornado¶
Instances of class socketio.AsyncServer
will automatically use
Tornado 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()
Eventlet¶
Note
Eventlet is not in active development anymore, and for that reason the current recommendation is to not use it for new projects.
Eventlet is a high performance concurrent networking library for Python 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
:
import eventlet
app = socketio.WSGIApp(sio)
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
See the Gunicorn section above for information on how to use Gunicorn with multiple workers.
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.
Sanic¶
Note
The Sanic integration has not been updated in a long time. It is currently recommended that a Sanic application is deployed with the ASGI integration.
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:
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.
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.
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
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)
Horizontal Scaling¶
Socket.IO is a stateful protocol, which makes horizontal scaling more
difficult. When deploying a cluster of Socket.IO processes, all processes must
connect to the message queue by passing the client_manager
argument to the
server instance. This enables the workers to communicate and coordinate complex
operations such as broadcasts.
If the long-polling transport is used, then there are two additional requirements that must be met:
Each Socket.IO process must be able to handle multiple requests concurrently. This is needed 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, so that all requests coming from a client are handled by the same node. Load balancers call this sticky sessions, or session affinity.
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.