Implementation Notes

Access to Flask’s Context Globals

Handlers for SocketIO events are different than handlers for routes and that introduces a lot of confusion around what can and cannot be done in a SocketIO handler. The main difference is that all the SocketIO events generated for a client occur in the context of a single long running request.

In spite of the differences, Flask-SocketIO attempts to make working with SocketIO event handlers easier by making the environment similar to that of a regular HTTP request. The following list describes what works and what doesn’t:

  • An application context is pushed before invoking an event handler making current_app and g available to the handler.
  • A request context is also pushed before invoking a handler, also making request and session available. But note that WebSocket events do not have individual requests associated with them, so the request context that started the connection is pushed for all the events that are dispatched during the life of the connection.
  • The request context global is enhanced with a sid member that is set to a unique session ID for the connection. This value is used as an initial room where the client is added.
  • The request context global is enhanced with namespace and event members that contain the currently handled namespace and event arguments. The event member is a dictionary with message and args keys.
  • The session context global behaves in a different way than in regular requests. A copy of the user session at the time the SocketIO connection is established is made available to handlers invoked in the context of that connection. If a SocketIO handler modifies the session, the modified session will be preserved for future SocketIO handlers, but regular HTTP route handlers will not see these changes. Effectively, when a SocketIO handler modifies the session, a “fork” of the session is created exclusively for these handlers. The technical reason for this limitation is that to save the user session a cookie needs to be sent to the client, and that requires HTTP request and response, which do not exist in a SocketIO connection. When using server-side sessions such as those provided by the Flask-Session or Flask-KVSession extensions, changes made to the session in HTTP route handlers can be seen by SocketIO handlers, as long as the session is not modified in the SocketIO handlers.
  • The before_request and after_request hooks are not invoked for SocketIO event handlers.
  • SocketIO handlers can take custom decorators, but most Flask decorators will not be appropriate to use for a SocketIO handler, given that there is no concept of a Response object during a SocketIO connection.

Authentication

A common need of applications is to validate the identity of their users. The traditional mechanisms based on web forms and HTTP requests cannot be used in a SocketIO connection, since there is no place to send HTTP requests and responses. If necessary, an application can implement a customized login form that sends credentials to the server as a SocketIO message when the submit button is pressed by the user.

However, in most cases it is more convenient to perform the traditional authentication process before the SocketIO connection is established. The user’s identity can then be recorded in the user session or in a cookie, and later when the SocketIO connection is established that information will be accessible to SocketIO event handlers.

Recent revisions of the Socket.IO protocol include the ability to pass a dictionary with authentication information during the connection. This is an ideal place for the client to include a token or other authentication details. If the client uses this capability, the server will provide this dictionary as an argument to the connect event handler, as shown above.

Using Flask-Login with Flask-SocketIO

Flask-SocketIO can access login information maintained by Flask-Login. After a regular Flask-Login authentication is performed and the login_user() function is called to record the user in the user session, any SocketIO connections will have access to the current_user context variable:

@socketio.on('connect')
def connect_handler():
    if current_user.is_authenticated:
        emit('my response',
             {'message': '{0} has joined'.format(current_user.name)},
             broadcast=True)
    else:
        return False  # not allowed here

Note that the login_required decorator cannot be used with SocketIO event handlers, but a custom decorator that disconnects non-authenticated users can be created as follows:

import functools
from flask import request
from flask_login import current_user
from flask_socketio import disconnect, emit

def authenticated_only(f):
    @functools.wraps(f)
    def wrapped(*args, **kwargs):
        if not current_user.is_authenticated:
            disconnect()
        else:
            return f(*args, **kwargs)
    return wrapped

@socketio.on('my event')
@authenticated_only
def handle_my_custom_event(data):
    emit('my response', {'message': '{0} has joined'.format(current_user.name)},
         broadcast=True)