PYTHON-ENGINEIO(1)

NAME
python-engineio - python-engineio Documentation

This project implements an Engine.IO server that can run standalone or integrated with a variety of Python web frameworks.

GETTING STARTED

  What is Engine.IO?

Engine.IO is a lightweight transport protocol that enables real-time bidirectional event-based communication between clients (typically web browsers) and a server. The official implementations of the client and server components are written in JavaScript.

The Engine.IO protocol is extremely simple. The example that follows shows the client-side Javascript code required to setup an Engine.IO connection to a server:

var socket = eio('http://chat.example.com');
socket.on('open', function() { alert('connected'); });
socket.on('message', function(data) { alert(data); });
socket.on('close', function() { alert('disconnected'); });
socket.send('Hello from the client!');

  Features

•   Fully compatible with the Javascript engine.io-client library, and with other Engine.IO clients.

•   Compatible with Python 2.7 and Python 3.3+.

•   Supports large number of clients even on modest hardware due to being asynchronous.

•   Compatible with aiohttp, sanic, tornado, eventlet, gevent, or any WSGI or ASGI compatible server.

•   Includes WSGI and ASGI middlewares that integrate Engine.IO traffic with other web applications.

•   Uses an event-based architecture implemented with decorators that hides the details of the protocol.

•   Implements HTTP long-polling and WebSocket transports.

•   Supports XHR2 and XHR browsers as clients.

•   Supports text and binary messages.

•   Supports gzip and deflate HTTP compression.

•   Configurable CORS responses to avoid cross-origin problems with browsers.

  Examples

The following application is a basic example that uses the Eventlet asynchronous server and includes a small Flask application that serves the HTML/Javascript to the client:

import engineio
import eventlet
from flask import Flask, render_template
eio = engineio.Server()
app = Flask(__name__)
@app.route('/')
def index():
    """Serve the client-side application."""
    return render_template('index.html')
@eio.on('connect')
def connect(sid, environ):
    print("connect ", sid)
@eio.on('message')
def message(sid, data):
    print("message ", data)
    eio.send(sid, 'reply')
@eio.on('disconnect')
def disconnect(sid):
    print('disconnect ', sid)
if __name__ == '__main__':
    # wrap Flask application with engineio's middleware
    app = engineio.Middleware(eio, app)
    # deploy as an eventlet WSGI server
    eventlet.wsgi.server(eventlet.listen(('', 8000)), app)

Below is a similar application, coded for asyncio (Python 3.5+ only) with the aiohttp framework:

from aiohttp import web
import engineio
eio = engineio.AsyncServer()
app = web.Application()
# attach the Engine.IO server to the application
eio.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')
@eio.on('connect')
def connect(sid, environ):
    print("connect ", sid)
@eio.on('message')
async def message(sid, data):
    print("message ", data)
    await eio.send(sid, 'reply')
@eio.on('disconnect')
def disconnect(sid):
    print('disconnect ', sid)
app.router.add_static('/static', 'static')
app.router.add_get('/', index)
if __name__ == '__main__':
    # run the aiohttp application
    web.run_app(app)

The client-side application must include the engine.io-client library (version 1.5.0 or newer recommended).

Each time a client connects to the server the connect event handler is invoked with the sid (session ID) assigned to the connection and the WSGI environment dictionary. The server can inspect authentication or other headers to decide if the client is allowed to connect. To reject a client the handler must return False.

When the client sends a message to the server the message event handler is invoked with the sid and the message.

Finally, when the connection is broken, the disconnect event is called, allowing the application to perform cleanup.

Because Engine.IO is a bidirectional protocol, the server can send messages to any connected client at any time. The engineio.Server.send() method takes the client's sid and the message payload, which can be of type str, bytes, list or dict (the last two are JSON encoded).

DEPLOYMENT

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

  aiohttp

aiohttp provides 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 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. 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 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

Sanic is a very efficient asynchronous web server for Python 3.5 and newer.

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()

  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)

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

  Eventlet

Eventlet is a high performance concurrent networking library for Python 2 and 3 that uses coroutines, enabling code to be written in the same style used with the blocking standard library functions. An 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.Middleware:

app = engineio.Middleware(eio)
import eventlet
eventlet.wsgi.server(eventlet.listen(('', 8000)), app)

  Using Gunicorn with Eventlet

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.Middleware:

from gevent import pywsgi
app = engineio.Middleware(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.Middleware(eio)
pywsgi.WSGIServer(('', 8000), app,
                  handler_class=WebSocketHandler).serve_forever()

  Using Gunicorn with Gevent

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. Only the long-polling transport is currently available when using standard threads.

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.Middleware(eio, app.wsgi_app)
# ... Engine.IO and Flask handler functions ...
if __name__ == '__main__':
    app.run(threaded=True)

When using the threading mode, it is important to ensure that the WSGI server can handle multiple concurrent requests using threads, since a client can have up to two outstanding requests at any given time. The Werkzeug server is single-threaded by default, so the threaded=True option is required.

Note that servers that use worker processes instead of threads, such as gunicorn, do not support an Engine.IO server configured in threading mode.

  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.

API REFERENCE

  Server class

  AsyncServer class

  WSGIApp class

  ASGIApp class

  Middleware class (deprecated)

•   genindex

•   modindex

•   search

AUTHOR
Miguel Grinberg

COPYRIGHT
2018, Miguel Grinberg