Is Flask-SocketIO's emit function thread safe?

Question

I have a Flask-SocketIO application. Can I safely call socketio.emit() from different threads? Is socketio.emit() atomic like the normal socket.send()?

Answer 1

The socketio.emit() function is thread safe, or I should say that it is intended to be thread-safe, as there is currently one open issue related to this. Note that 'thread' in this context means a supported threading model. Most people use Flask-SocketIO in conjunction with eventlet or gevent in production, so in those contexts thread means "green" thread.

The open issue is related to using a message queue, which is necessary when you have multiple servers. In that set up, the accesses to the queue are not thread safe at this time. This is a bug that needs to be fixed, but as a workaround, you can create a different socketio object per thread.

On second question regarding if socketio.emit() is atomic, the answer is no. This is not a simple socket write operation. The payload needs to be formatted in certain way to comply with the Socket.IO protocol, then depending on the selected transport (long-polling or websocket) the write happens in a completely different way.