The Engine.IO Server

This package contains two Engine.IO servers:

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

Installation

To install the Python Engine.IO server use the following command:

pip install "python-engineio"

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

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

import engineio

# create a Engine.IO server
eio = engineio.Server()

# wrap with a WSGI application
app = engineio.WSGIApp(eio)

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

# create a Engine.IO server
eio = engineio.AsyncServer()

# wrap with ASGI application
app = engineio.ASGIApp(eio)

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

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

Serving Static Files

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

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

static_files = {
    '/': 'latency.html',
    '/static/engine.io.js': 'static/engine.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 engineio.WSGIApp or engineio.ASGIApp classes:

# for standard WSGI applications
eio = engineio.Server()
app = engineio.WSGIApp(eio, static_files=static_files)

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

The routing precedence in these two classes is as follows:

  • First, the path is checked against the Engine.IO path.
  • Next, the path is checked against the static file configuration, if present.
  • If the path did not match the Engine.IO path 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

To responds to events triggered by the connection or the client, event Handler functions must be defined using the on decorator:

@eio.on('connect')
def on_connect(sid):
    print('A client connected!')

@eio.on('message')
def on_message(sid, data):
    print('I received a message!')

@eio.on('disconnect')
def on_disconnect(sid):
    print('Client disconnected!')

For the asyncio server, event handlers can be regular functions as above, or can also be coroutines:

@eio.on('message')
async def on_message(sid, data):
    print('I received a message!')

The argument given to the on decorator is the event name. The events that are supported are connect, message and disconnect. Note that the disconnect handler is invoked for client initiated disconnects, server initiated disconnects, or accidental disconnects, for example due to networking failures.

The sid argument passed into all the event handlers is a connection identifier for the client. All the events from a client will use the same sid value.

The connect handler is the place where the server can perform authentication. The value returned by this handler is used to determine if the connection is accepted or rejected. When the handler does not return any value (which is the same as returning None) or when it returns True the connection is accepted. If the handler returns False or any JSON compatible data type (string, integer, list or dictionary) the connection is rejected. A rejected connection triggers a response with a 401 status code.

The data argument passed to the 'message' event handler contains application-specific data provided by the client with the event.

Sending Messages

The server can send a message to any client using the send() method:

eio.send(sid, {'foo': 'bar'})

Or in the case of asyncio, as a coroutine:

await eio.send(sid, {'foo': 'bar'})

The first argument provided to the method is the connection identifier for the recipient client. The second argument is the data that is passed on to the server. The data can be of type str, bytes, dict or list. The data included inside dictionaries and lists is also constrained to these types.

The send() method can be invoked inside an event handler as a response to a client event, or in any other part of the application, including in background tasks.

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:

@eio.on('connect')
def on_connect(sid, environ):
    username = authenticate_user(environ)
    eio.save_session(sid, {'username': username})

@eio.on('message')
def on_message(sid, data):
    session = eio.get_session(sid)
    print('message from ', session['username'])

For the asyncio server, these methods are coroutines:

@eio.on('connect')
async def on_connect(sid, environ):
    username = authenticate_user(environ)
    await eio.save_session(sid, {'username': username})

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

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

@eio.on('connect')
def on_connect(sid, environ):
    username = authenticate_user(environ)
    with eio.session(sid) as session:
        session['username'] = username

@eio.on('message')
def on_message(sid, data):
    with eio.session(sid) as session:
        print('message from ', session['username'])

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

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

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

Note: the contents of the user session are destroyed when the client disconnects.

Disconnecting a Client

At any time the server can disconnect a client from the server by invoking the disconnect() method and passing the sid value assigned to the client:

eio.disconnect(sid)

For the asyncio client this is a coroutine:

await eio.disconnect(sid)

Managing Background Tasks

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

eio.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

eio.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, but the background function is.

The sleep() method is a second convenience function that is provided for the benefit of applications working with background tasks of their own:

eio.sleep(2)

Or for asyncio:

await eio.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 server can be configured to output logs to the terminal:

import engineio

# standard Python
eio = engineio.Server(logger=True)

# asyncio
eio = engineio.AsyncServer(logger=True)

The logger argument 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 Engine.IO servers.

Uvicorn, Daphne, and other ASGI servers

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

eio = engineio.AsyncServer(async_mode='asgi')
app = engineio.ASGIApp(eio)

If desired, the engineio.ASGIApp class can forward any traffic that is not Engine.IO to another ASGI application, making it possible to deploy a standard ASGI web application and the Engine.IO server as a bundle:

eio = engineio.AsyncServer(async_mode='asgi')
app = engineio.ASGIApp(eio, other_app)

The ASGIApp instance is a fully complaint ASGI instance that can be deployed with an ASGI compatible web server.

Aiohttp

aiohttp provides a framework with support for HTTP and WebSocket, based on asyncio.

Instances of class engineio.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:

eio = engineio.AsyncServer(async_mode='aiohttp')

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

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

The aiohttp application can define regular routes that will coexist with the Engine.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. Only Tornado version 5 and newer are supported, thanks to its tight integration with asyncio.

Instances of class engineio.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:

eio = engineio.AsyncServer(async_mode='tornado')

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

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

The tornado application can define other routes that will coexist with the Engine.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.

Instances of class engineio.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:

eio = engineio.AsyncServer(async_mode='sanic')

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

app = Sanic()
eio.attach(app)

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

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:

eio = engineio.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 Engine.IO server deployed with eventlet has access to the long-polling and WebSocket transports.

Instances of class engineio.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:

eio = engineio.Server(async_mode='eventlet')

A server configured for eventlet is deployed as a regular WSGI application using the provided engineio.WSGIApp:

app = engineio.WSGIApp(eio)
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 1 option is required. Note that a single eventlet worker can handle a large number of concurrent clients.

Another limitation when using gunicorn is that the WebSocket transport is not available, because this transport it requires extensions to the WSGI standard.

Note: Eventlet provides a monkey_patch() function that replaces all the blocking functions in the standard library with equivalent asynchronous versions. While python-engineio 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 Engine.IO server deployed with gevent has access to the long-polling transport. If project gevent-websocket is installed, the WebSocket transport is also available. Note that when using the uWSGI server, the native WebSocket implementation of uWSGI can be used instead of gevent-websocket (see next section for details on this).

Instances of class engineio.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:

# gevent alone or with gevent-websocket
eio = engineio.Server(async_mode='gevent')

A server configured for gevent is deployed as a regular WSGI application using the provided engineio.WSGIApp:

from gevent import pywsgi
app = engineio.WSGIApp(eio)
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 = engineio.WSGIApp(eio)
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 1 option is required. Note that a single gevent worker can handle a large number of concurrent clients.

Note: Gevent provides a monkey_patch() function that replaces all the blocking functions in the standard library with equivalent asynchronous versions. While python-engineio 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 Engine.IO server can take advantage of uWSGI’s native WebSocket support.

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

# gevent with uWSGI
eio = engineio.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 Engine.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 engineio.Server will automatically use the threading mode if neither eventlet nor gevent are not installed. To request the threading mode explicitly, the async_mode option can be given in the constructor:

eio = engineio.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 Engine.IO application combined with a Flask web application, using Flask’s development web server based on Werkzeug:

eio = engineio.Server(async_mode='threading')
app = Flask(__name__)
app.wsgi_app = engineio.WSGIApp(eio, app.wsgi_app)

# ... Engine.IO and Flask handler functions ...

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

The example that follows shows how to start an Engine.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

Engine.IO is a stateful protocol, which makes horizontal scaling more difficult. To deploy a cluster of Engine.IO processes hosted on one or multiple servers the following conditions must be met:

  • Each Engine.IO server 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 process. Load balancers call this sticky sessions, or session affinity.

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.