Installation

You can install this package in the usual way using pip:

pip install flask-socketio

Requirements

Flask-SocketIO is compatible with Python 3.6+. The asynchronous services that this package relies on can be selected among three choices:

The extension automatically detects which asynchronous framework to use based on what is installed. Preference is given to eventlet, followed by gevent. For WebSocket support in gevent, uWSGI is preferred, followed by gevent-websocket. If neither eventlet nor gevent are installed, then the Flask development server is used.

If using multiple processes, a message queue service must be configured to allow the servers to coordinate operations such as broadcasting. The supported queues are Redis, RabbitMQ, Kafka, and any other message queues supported by the Kombu package.

On the client-side, the official Socket.IO Javascript client library can be used to establish a connection to the server. There are also official clients written in Swift, Java and C++. Unofficial clients may also work, as long as they implement the Socket.IO protocol. The python-socketio package (which provides the Socket.IO server implementation used by Flask-SocketIO) includes a Python client.

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.

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.

Initialization

The following code example shows how to add Flask-SocketIO to a Flask application:

from flask import Flask, render_template
from flask_socketio import SocketIO
app = Flask(__name__)
app.config['SECRET_KEY'] = 'secret!'
socketio = SocketIO(app)
if __name__ == '__main__':
    socketio.run(app)

The init_app() style of initialization is also supported. To start the web server simply execute your script. Note the way the web server is started. The socketio.run() function encapsulates the start up of the web server and replaces the app.run() standard Flask development server start up. When the application is in debug mode the Werkzeug development server is still used and configured properly inside socketio.run(). In production mode the eventlet web server is used if available, else the gevent web server is used. If eventlet and gevent are not installed, the Werkzeug development web server is used.

The flask run command introduced in Flask 0.11 can be used to start a Flask-SocketIO development server based on Werkzeug, but this method of starting the Flask-SocketIO server is not recommended due to lack of WebSocket support. Previous versions of this package included a customized version of the flask run command that allowed the use of WebSocket on eventlet and gevent production servers, but this functionality has been discontinued in favor of the socketio.run(app) startup method shown above which is more robust.

The application must serve a page to the client that loads the Socket.IO library and establishes a connection:

<script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/4.0.1/socket.io.js" integrity="sha512-q/dWJ3kcmjBLU4Qc47E4A9kTB4m3wuTY7vkFJDTZKjTs8jhyGQnaUrxa0Ytd0ssMZhbNua9hE+E7Qv1j+DyZwA==" crossorigin="anonymous"></script>
<script type="text/javascript" charset="utf-8">
    var socket = io();
    socket.on('connect', function() {
        socket.emit('my event', {data: 'I\'m connected!'});
    });
</script>

Receiving Messages

When using SocketIO, messages are received by both parties as events. On the client side Javascript callbacks are used. With Flask-SocketIO the server needs to register handlers for these events, similarly to how routes are handled by view functions.

The following example creates a server-side event handler for an unnamed event:

@socketio.on('message')
def handle_message(data):
    print('received message: ' + data)

The above example uses string messages. Another type of unnamed events use JSON data:

@socketio.on('json')
def handle_json(json):
    print('received json: ' + str(json))

The most flexible type of event uses custom event names. The message data for these events can be string, bytes, int, or JSON:

@socketio.on('my event')
def handle_my_custom_event(json):
    print('received json: ' + str(json))

Custom named events can also support multiple arguments:

@socketio.on('my_event')
def handle_my_custom_event(arg1, arg2, arg3):
    print('received args: ' + arg1 + arg2 + arg3)

When the name of the event is a valid Python identifier that does not collide with other defined symbols, the @socketio.event decorator provides a more compact syntax that takes the event name from the decorated function:

@socketio.event
def my_custom_event(arg1, arg2, arg3):
    print('received args: ' + arg1 + arg2 + arg3)

Named events are the most flexible, as they eliminate the need to include additional metadata to describe the message type. The names message, json, connect and disconnect are reserved and cannot be used for named events.

Flask-SocketIO also supports SocketIO namespaces, which allow the client to multiplex several independent connections on the same physical socket:

@socketio.on('my event', namespace='/test')
def handle_my_custom_namespace_event(json):
    print('received json: ' + str(json))

When a namespace is not specified a default global namespace with the name '/' is used.

For cases when a decorator syntax isn't convenient, the on_event method can be used:

def my_function_handler(data):
    pass
socketio.on_event('my event', my_function_handler, namespace='/test')

Clients may request an acknowledgement callback that confirms receipt of a message they sent. Any values returned from the handler function will be passed to the client as arguments in the callback function:

@socketio.on('my event')
def handle_my_custom_event(json):
    print('received json: ' + str(json))
    return 'one', 2

In the above example, the client callback function will be invoked with two arguments, 'one' and 2. If a handler function does not return any values, the client callback function will be invoked without arguments.

Sending Messages

SocketIO event handlers defined as shown in the previous section can send reply messages to the connected client using the send() and emit() functions.

The following examples bounce received events back to the client that sent them:

from flask_socketio import send, emit
@socketio.on('message')
def handle_message(message):
    send(message)
@socketio.on('json')
def handle_json(json):
    send(json, json=True)
@socketio.on('my event')
def handle_my_custom_event(json):
    emit('my response', json)

Note how send() and emit() are used for unnamed and named events respectively.

When working with namespaces, send() and emit() use the namespace of the incoming message by default. A different namespace can be specified with the optional namespace argument:

@socketio.on('message')
def handle_message(message):
    send(message, namespace='/chat')
@socketio.on('my event')
def handle_my_custom_event(json):
    emit('my response', json, namespace='/chat')

To send an event with multiple arguments, send a tuple:

@socketio.on('my event')
def handle_my_custom_event(json):
    emit('my response', ('foo', 'bar', json), namespace='/chat')

SocketIO supports acknowledgment callbacks that confirm that a message was received by the client:

def ack():
    print('message was received!')
@socketio.on('my event')
def handle_my_custom_event(json):
    emit('my response', json, callback=ack)

When using callbacks, the Javascript client receives a callback function to invoke upon receipt of the message. After the client application invokes the callback function the server invokes the corresponding server-side callback. If the client-side callback is invoked with arguments, these are provided as arguments to the server-side callback as well.

Broadcasting

Another very useful feature of SocketIO is the broadcasting of messages. Flask-SocketIO supports this feature with the broadcast=True optional argument to send() and emit():

@socketio.on('my event')
def handle_my_custom_event(data):
    emit('my response', data, broadcast=True)

When a message is sent with the broadcast option enabled, all clients connected to the namespace receive it, including the sender. When namespaces are not used, the clients connected to the global namespace receive the message. Note that callbacks are not invoked for broadcast messages.

In all the examples shown until this point the server responds to an event sent by the client. But for some applications, the server needs to be the originator of a message. This can be useful to send notifications to clients of events that originated in the server, for example in a background thread. The socketio.send() and socketio.emit() methods can be used to broadcast to all connected clients:

def some_function():
    socketio.emit('some event', {'data': 42})

Note that socketio.send() and socketio.emit() are not the same functions as the context-aware send() and emit(). Also note that in the above usage there is no client context, so broadcast=True is assumed and does not need to be specified.

Rooms

For many applications it is necessary to group users into subsets that can be addressed together. The best example is a chat application with multiple rooms, where users receive messages from the room or rooms they are in, but not from other rooms where other users are. Flask-SocketIO supports this concept of rooms through the join_room() and leave_room() functions:

from flask_socketio import join_room, leave_room
@socketio.on('join')
def on_join(data):
    username = data['username']
    room = data['room']
    join_room(room)
    send(username + ' has entered the room.', to=room)
@socketio.on('leave')
def on_leave(data):
    username = data['username']
    room = data['room']
    leave_room(room)
    send(username + ' has left the room.', to=room)

The send() and emit() functions accept an optional to argument that cause the message to be sent to all the clients that are in the given room.

All clients are assigned a room when they connect, named with the session ID of the connection, which can be obtained from request.sid. A given client can join any rooms, which can be given any names. When a client disconnects it is removed from all the rooms it was in. The context-free socketio.send() and socketio.emit() functions also accept a to argument to broadcast to all clients in a room.

Since all clients are assigned a personal room, to address a message to a single client, the session ID of the client can be used as the to argument.

Connection Events

Flask-SocketIO also dispatches connection and disconnection events. The following example shows how to register handlers for them:

@socketio.on('connect')
def test_connect(auth):
    emit('my response', {'data': 'Connected'})
@socketio.on('disconnect')
def test_disconnect():
    print('Client disconnected')

The auth argument in the connection handler is optional. The client can use it to pass authentication data such as tokens in dictionary format. If the client does not provide authentication details, then this argument is set to None. If the server defines a connection event handler without this argument, then any authentication data passed by the client is discarded.

The connection event handler can return False to reject the connection, or it can also raise ConnectionRefusedError. This is so that the client can be authenticated at this point. When using the exception, any arguments passed to the exception are returned to the client in the error packet. Examples:

from flask_socketio import ConnectionRefusedError
@socketio.on('connect')
def connect():
    if not self.authenticate(request.args):
        raise ConnectionRefusedError('unauthorized!')

Note that connection and disconnection events are sent individually on each namespace used.

Class-Based Namespaces

As an alternative to the decorator-based event handlers described above, the event handlers that belong to a namespace can be created as methods of a class. The flask_socketio.Namespace is provided as a base class to create class-based namespaces:

from flask_socketio import Namespace, emit
class MyCustomNamespace(Namespace):
    def on_connect(self):
        pass
    def on_disconnect(self):
        pass
    def on_my_event(self, data):
        emit('my_response', data)
socketio.on_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 flask_socketio.SocketIO class that default to the proper namespace when the namespace argument is not given.

If an event has a handler in a class-based namespace, and also a decorator-based function handler, only the decorated function handler is invoked.

Error Handling

Flask-SocketIO can also deal with exceptions:

@socketio.on_error()        # Handles the default namespace
def error_handler(e):
    pass
@socketio.on_error('/chat') # handles the '/chat' namespace
def error_handler_chat(e):
    pass
@socketio.on_error_default  # handles all namespaces without an explicit error handler
def default_error_handler(e):
    pass

Error handler functions take the exception object as an argument.

The message and data arguments of the current request can also be inspected with the request.event variable, which is useful for error logging and debugging outside the event handler:

from flask import request
@socketio.on("my error event")
def on_my_event(data):
    raise RuntimeError()
@socketio.on_error_default
def default_error_handler(e):
    print(request.event["message"]) # "my error event"
    print(request.event["args"])    # (data,)

Debugging and Troubleshooting

To help you debug issues, the server can be configured to output logs to the terminal:

socketio = SocketIO(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.

Access to Flask's Context Globals

Handlers for SocketIO events are different than handlers for routes and that introduces a lot of confusion around what can and cannot be done in a SocketIO handler. The main difference is that all the SocketIO events generated for a client occur in the context of a single long running request.

In spite of the differences, Flask-SocketIO attempts to make working with SocketIO event handlers easier by making the environment similar to that of a regular HTTP request. The following list describes what works and what doesn't:

Authentication

A common need of applications is to validate the identity of their users. The traditional mechanisms based on web forms and HTTP requests cannot be used in a SocketIO connection, since there is no place to send HTTP requests and responses. If necessary, an application can implement a customized login form that sends credentials to the server as a SocketIO message when the submit button is pressed by the user.

However, in most cases it is more convenient to perform the traditional authentication process before the SocketIO connection is established. The user's identity can then be recorded in the user session or in a cookie, and later when the SocketIO connection is established that information will be accessible to SocketIO event handlers.

Recent revisions of the Socket.IO protocol include the ability to pass a dictionary with authentication information during the connection. This is an ideal place for the client to include a token or other authentication details. If the client uses this capability, the server will provide this dictionary as an argument to the connect event handler, as shown above.

Using Flask-Login with Flask-SocketIO

Flask-SocketIO can access login information maintained by Flask-Login. After a regular Flask-Login authentication is performed and the login_user() function is called to record the user in the user session, any SocketIO connections will have access to the current_user context variable:

@socketio.on('connect')
def connect_handler():
    if current_user.is_authenticated:
        emit('my response',
             {'message': '{0} has joined'.format(current_user.name)},
             broadcast=True)
    else:
        return False  # not allowed here

Note that the login_required decorator cannot be used with SocketIO event handlers, but a custom decorator that disconnects non-authenticated users can be created as follows:

import functools
from flask import request
from flask_login import current_user
from flask_socketio import disconnect, emit
def authenticated_only(f):
    @functools.wraps(f)
    def wrapped(*args, **kwargs):
        if not current_user.is_authenticated:
            disconnect()
        else:
            return f(*args, **kwargs)
    return wrapped
@socketio.on('my event')
@authenticated_only
def handle_my_custom_event(data):
    emit('my response', {'message': '{0} has joined'.format(current_user.name)},
         broadcast=True)

Embedded Server

The simplest deployment strategy is to start the web server by calling socketio.run(app) as shown in examples above. This will look through the packages that are installed for the best available web server start the application on it. The current web server choices that are evaluated are eventlet, gevent and the Flask development server.

If eventlet or gevent are available, socketio.run(app) starts a production-ready server using one of these frameworks. If neither of these are installed, then the Flask development web server is used, and in this case the server is not intended to be used in a production deployment.

Unfortunately this option is not available when using gevent with uWSGI. See the uWSGI section below for information on this option.

Gunicorn Web Server

An alternative to socketio.run(app) is to use gunicorn as web server, using the eventlet or gevent workers. For this option, eventlet or gevent need to be installed, in addition to gunicorn. The command line that starts the eventlet server via gunicorn is:

gunicorn --worker-class eventlet -w 1 module:app

If you prefer to use gevent, the command to start the server is:

gunicorn -k gevent -w 1 module:app

When using gunicorn with the gevent worker and the WebSocket support provided by gevent-websocket, the command that starts the server must be changed to select a custom gevent web server that supports the WebSocket protocol. The modified command is:

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

A third option with Gunicorn is to use the threaded worker, along with the simple-websocket package for WebSocket support. This is a particularly good solution for applications that are CPU heavy or are otherwise incompatible with eventlet and gevent use of green threads. The command to start a threaded web server is:

gunicorn -w 1 --threads 100 module:app

In all these commands, module is the Python module or package that defines the application instance, and app is the application instance itself.

Due to the limited load balancing algorithm used by gunicorn, it is not possible to use more than one worker process when using this web server. For that reason, all the examples above include the -w 1 option.

The workaround to use multiple worker processes with gunicorn is to launch several single-worker instances and put them behind a more capable load balancer such as nginx.

uWSGI Web Server

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

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 example application app.py on port 5000:

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

Using nginx as a WebSocket Reverse Proxy

It is possible to use nginx as a front-end reverse proxy that passes requests to the application. However, only releases of nginx 1.4 and newer support proxying of the WebSocket protocol. Below is a basic nginx configuration that proxies HTTP and WebSocket requests:

server {
    listen 80;
    server_name _;
    location / {
        include proxy_params;
        proxy_pass http://127.0.0.1:5000;
    }
    location /static/ {
        alias <path-to-your-application>/static/;
        expires 30d;
    }
    location /socket.io {
        include proxy_params;
        proxy_http_version 1.1;
        proxy_buffering off;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
        proxy_pass http://127.0.0.1:5000/socket.io;
    }
}

The next example adds the support for load balancing multiple Socket.IO servers:

upstream socketio_nodes {
    ip_hash;
    server 127.0.0.1:5000;
    server 127.0.0.1:5001;
    server 127.0.0.1:5002;
    # to scale the app, just add more nodes here!
}
server {
    listen 80;
    server_name _;
    location / {
        include proxy_params;
        proxy_pass http://127.0.0.1:5000;
    }
    location /static/ {
        alias <path-to-your-application>/static/;
        expires 30d;
    }
    location /socket.io {
        include proxy_params;
        proxy_http_version 1.1;
        proxy_buffering off;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
        proxy_pass http://socketio_nodes/socket.io;
    }
}

While the above examples can work as an initial configuration, be aware that a production install of nginx will need a more complete configuration covering other deployment aspects such as SSL support.

Using Multiple Workers

Flask-SocketIO supports multiple workers behind a load balancer starting with release 2.0. Deploying multiple workers gives applications that use Flask-SocketIO the ability to spread the client connections among multiple processes and hosts, and in this way scale to support very large numbers of concurrent clients.

There are two requirements to use multiple Flask-SocketIO workers:

When working with a message queue, there are additional dependencies that need to be installed:

For eventlet, monkey patching is done with:

import eventlet
eventlet.monkey_patch()

For gevent, you can monkey patch the standard library with:

from gevent import monkey
monkey.patch_all()

In both cases it is recommended that you apply the monkey patching at the top of your main script, even above your imports.

To start multiple Flask-SocketIO servers, you must first ensure you have the message queue service running. To start a Socket.IO server and have it connect to the message queue, add the message_queue argument to the SocketIO constructor:

socketio = SocketIO(app, message_queue='redis://')

The value of the message_queue argument is the connection URL of the queue service that is used. For a redis queue running on the same host as the server, the 'redis://' URL can be used. Likewise, for a default RabbitMQ queue the 'amqp://' URL can be used. For Kafka, use a kafka:// URL. The Kombu package has a documentation section that describes the format of the URLs for all the supported queues.

Emitting from an External Process

For many types of applications, it is necessary to emit events from a process that is not the SocketIO server, for an example a Celery worker. If the SocketIO server or servers are configured to listen on a message queue as shown in the previous section, then any other process can create its own SocketIO instance and use it to emit events in the same way the server does.

For example, for an application that runs on an eventlet web server and uses a Redis message queue, the following Python script broadcasts an event to all clients:

socketio = SocketIO(message_queue='redis://')
socketio.emit('my event', {'data': 'foo'}, namespace='/test')

When using the SocketIO instance in this way, the Flask application instance is not passed to the constructor.

The channel argument to SocketIO can be used to select a specific channel of communication through the message queue. Using a custom channel name is necessary when there are multiple independent SocketIO services sharing the same queue.

Flask-SocketIO does not apply monkey patching when eventlet or gevent are used. But when working with a message queue, it is very likely that the Python package that talks to the message queue service will hang if the Python standard library is not monkey patched.

It is important to note that an external process that wants to connect to a SocketIO server does not need to use eventlet or gevent like the main server. Having a server use a coroutine framework, while an external process is not a problem. For example, Celery workers do not need to be configured to use eventlet or gevent just because the main server does. But if your external process does use a coroutine framework for whatever reason, then monkey patching is likely required, so that the message queue accesses coroutine friendly functions and classes.

Cross-Origin Controls

For security reasons, this server enforces a same-origin policy by default. In practical terms, this means the following:

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.