The Engine.IO Server
********************

This package contains two Engine.IO servers:

* The "engineio.Server()" class creates a server compatible with the
  standard Python library.

* The "engineio.AsyncServer()" class creates a server compatible with
  the "asyncio" package.

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, reason):
       print('Client disconnected! reason:', reason)

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".

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.

The "disconnect" handler is invoked for client initiated disconnects,
server initiated disconnects, or accidental disconnects, for example
due to networking failures. The second argument passed to this handler
provides the disconnect reason. Example:

   @eio.on('disconnect')
   def on_disconnect(sid, reason):
       if reason == eio.reason.CLIENT_DISCONNECT:
           print('the client went away')
       elif reason == eio.reason.SERVER_DISCONNECT:
           print('the client was kicked out')
       else:
           print(f'disconnect reason: {reason}')


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 controls logging behavior:

* "True": Enables log output to "stderr" at the "INFO" level.

* "False": Enables log output to "stderr" at the "ERROR" level. This
  is the default.

* A "logging.Logger" instance: Uses the provided logger without
  additional configuration.

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 and websocket transports.

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:

   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()


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

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.
