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This project implements an Engine.IO server that can run standalone or integrated with a Python WSGI application. The following are some of its features:

• Fully compatible with the Javascript engine.io-client library, versions 1.5.0 and up.
• Compatible with Python 2.7 and Python 3.3+.
• Supports large number of clients even on modest hardware when used with an asynchronous server based on eventlet or gevent. For development and testing, any WSGI compliant multi-threaded server can be used.
• Includes a WSGI middleware that integrates Engine.IO traffic with standard WSGI 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.

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 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!');

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)

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).

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

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)

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 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()

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.

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

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.

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 process must be able to handle multiple requests, either by using eventlet, gevent, or standard threads. Worker processes that 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.

.. toctree::
   :maxdepth: 2

.. module:: engineio

.. autoclass:: Middleware
   :members:

.. autoclass:: Server
   :members: