WebSockets on Azure Functions

We've received feedback from customers that Azure Functions should support WebSockets to enable realtime scenarios. See here for some discussion:
https://github.com/Azure/azure-functions-host/issues/1139

There are some challenges here because WebSocket is really a stateful protocol (you have a long lived connection between a given client and a given server) while Azure Functions is designed to be stateless. For example, we will often deprovision a Function App from one VM and start it on a different VM (perhaps in a different scale unit) based on capacity constraints. This is safe to do for us today because of our stateless design - but if we did it when there were WebSockets connections to that VM, we'd be terminating those connections.

There are also open questions from a billing perspective. Today the consumption plan for Azure Functions bills per execution, but if a WebSocket server is listening on an idle connection, this consumes resources but where are the "executions"?

In summary, there are open issues around design, billing, and technical implementation. I think there are reasonable solutions to these issues and we just need to start working on them once this feature is high enough on our list of priorities.