miguelgrinberg.python-socketio/docs/index.rst

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socketio documentation
======================

:ref:`genindex` | :ref:`modindex` | :ref:`search`

This project implements an Socket.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
  `socket.io-client <https://github.com/Automattic/socket.io-client>`_ library,
  versions 1.3.5 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 <http://eventlet.net/>`_ or
  `gevent <http://gevent.org/>`_. For development and testing, any WSGI
  complaint multi-threaded server can be used.
- Includes a WSGI middleware that integrates Socket.IO traffic with standard
  WSGI applications.
- Broadcasting of messages to all connected clients, or to subsets of them
  assigned to "rooms".
- Uses an event-based architecture implemented with decorators that hides the
  details of the protocol.
- Support for HTTP long-polling and WebSocket transports.
- Support for XHR2 and XHR browsers.
- Support for text and binary messages.
- Support for gzip and deflate HTTP compression.
- Configurable CORS responses to avoid cross-origin problems with browsers.

What is Socket.IO?
------------------

Socket.IO is a 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.

Getting Started
---------------

The following is a basic example of a Socket.IO server that uses Flask to
deploy the client code to the browser::

    import socketio
    import eventlet
    from flask import Flask, render_template

    sio = socketio.Server()
    app = Flask(__name__)

    @app.route('/')
    def index():
        """Serve the client-side application."""
        return render_template('index.html')

    @sio.on('connect')
    def connect(sid, environ):
        print('connect ', sid)

    @sio.on('my message')
    def message(sid, data):
        print('message ', data)

    @sio.on('disconnect')
    def disconnect(sid):
        print('disconnect ', sid)

    if __name__ == '__main__':
        # wrap Flask application with socketio's middleware
        app = socketio.Middleware(sio, app)

        # deploy as an eventlet WSGI server
        eventlet.wsgi.server(eventlet.listen(('', 8000)), app)

The client-side application must include the
`socket.io-client <https://github.com/Automattic/socket.io-client>`_ library
(versions 1.3.5 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 an event to the server the appropriate event handler is
invoked with the ``sid`` and the message. The application can define as many
events as needed and associate them with event handlers. An event is defined
simply by a name.

When a connection with a client is broken, the ``disconnect`` event is called,
allowing the application to perform cleanup.

Rooms
-----

Because Socket.IO is a bidirectional protocol, the server can send messages to
any connected client at any time. To make it easy to address groups of clients,
the application can put clients into rooms, and then address messages to all
the clients in a room.

When clients first connect, they are assigned to their own rooms, named with
the session ID (the ``sid`` argument passed to all event handlers). The
application is free to create additional rooms and manage which clients are in
them using the :func:`socketio.Server.enter_room` and
:func:`socketio.Server.leave_room` methods. Clients can be in as many rooms as
needed and can be moved between rooms as often as necessary. The individual
rooms assigned to clients when they connect are not special in any way, the
application is free to add or remove clients from them, though once it does
that it will lose the ability to address individual clients.

::

    @sio.on('enter room')
    def enter_room(sid, data):
        sio.enter_room(sid, data['room'])

    @sio.on('enter room')
    def leave_room(sid, data):
        sio.leave_room(sid, data['room'])

The :func:`socketio.Server.emit` method takes an event name, a message payload
of type ``str``, ``bytes``, ``list`` or ``dict``, and the recipient room. To
address an individual client, the ``sid`` of that client should be given as
room (assuming the application did not alter these initial rooms). To address
all connected clients, the ``room`` argument should be omitted.

::

    @sio.on('my message')
    def message(sid, data):
        print('message ', data)
        sio.emit('my reply', data, room='my room')

Often when broadcasting a message to group of users in a room, it is desirable
that the sender does not receive its own message. The
:func:`socketio.Server.emit` method provides an optional ``skip_sid`` argument
to specify a client that should be skipped during the broadcast.

::

    @sio.on('my message')
    def message(sid, data):
        print('message ', data)
        sio.emit('my reply', data, room='my room', skip_sid=sid)

Responses
---------

When a client sends an event to the server, it can optionally provide a
callback function, to be invoked with a response provided by the server. The
server can provide a response simply by returning it from the corresponding
event handler.

::

    @sio.on('my event', namespace='/chat')
    def my_event_handler(sid, data):
        # handle the message
        return "OK", 123

The event handler can return a single value, or a tuple with several values.
The callback function on the client side will be invoked with these returned
values as arguments.

Callbacks
---------

The server can also request a response to an event sent to a client. The
:func:`socketio.Server.emit` method has an optional ``callback`` argument that
can be set to a callable. When this argument is given, the callable will be
invoked with the arguments returned by the client as a response.

Using callback functions when broadcasting to multiple clients is not
recommended, as the callback function will be invoked once for each client
that received the message.

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 is handled independently from the others, with separate event
handlers and rooms. 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 available in all the
methods in the :class:`socketio.Server` class.

When the ``namespace`` argument is omitted, set to ``None`` or to ``'/'``, the
default namespace, representing the physical connection, is used.

Deployment
----------

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

Eventlet
~~~~~~~~

`Eventlet <http://eventlet.net/>`_ 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 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.Middleware``::

    app = socketio.Middleware(sio)
    import eventlet
    eventlet.wsgi.server(eventlet.listen(('', 8000)), app)

An alternative to running the eventlet WSGI server as above is to use
`gunicorn <gunicorn.org>`_, 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-socketio does not require monkey patching, other
libraries such as database drivers are likely to require it.

Gevent
~~~~~~

`Gevent <http://gevent.org>`_ is another asynchronous framework based on
coroutines, very similar to eventlet. Only the long-polling transport is
currently available when using gevent.

Instances of class ``socketio.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::

    eio = socketio.Server(async_mode='gevent')

A server configured for gevent is deployed as a regular WSGI application,
using the provided ``socketio.Middleware``::

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

An alternative to running the eventlet WSGI server as above is to use
`gunicorn <gunicorn.org>`_, 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

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 eventlet 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-socketio does not require monkey patching, other
libraries such as database drivers are likely to require it.

Standard Threading Library
~~~~~~~~~~~~~~~~~~~~~~~~~~

While not comparable to eventlet and gevent in terms of performance,
the Socket.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 <http://werkzeug.pocoo.org>`_. Only the
long-polling transport is currently available when using standard threads.

Instances of class ``socketio.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::

    sio = socketio.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
Socket.IO application combined with a Flask web application, using Flask's
development web server based on Werkzeug::

    sio = socketio.Server(async_mode='threading')
    app = Flask(__name__)
    app.wsgi_app = socketio.Middleware(sio, app.wsgi_app)

    # ... Socket.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.

Multi-process deployments
~~~~~~~~~~~~~~~~~~~~~~~~~

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

- Each Socket.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*.

A limitation in the current release of the Socket.IO server is that because
the clients are randomly assigned to different server processes, any form of
broadcasting is not supported. A storage backend that enables multiple
processes to share information about clients is currently in development to
address this important limitation.

API Reference
-------------

.. toctree::
   :maxdepth: 2

.. module:: socketio

.. autoclass:: Middleware
   :members:

.. autoclass:: Server
   :members: