Vaadin 14: sessionExpired error when reading from WebSocket

I need to display information coming from a (single) WebSocket which is my data feed. The intent is that the web server listens on the websocket and pushes updates to the Vaadin pages.

My understanding is I need to tell Atmosphere to dispatch to my code – if I simply add a WebSocket servlet mapped to /echo the calling code gets a “sessionExpired” response on connect. Note that the same servlet runs fine alone in a server where there is no Vaadin Flow.

Any hints on getting a WebSocket routine connected to Atmosphere side-by-side with Flow would be appreciated.