I was hoping for a good answer, rather than "this is hard and it will just get worse." We have an old app that my team is modernizing with this exact situation. Uses websockets now, but that's an historical thing and all the web apps were non-secure so it worked okay. Now everyone wants SSL turned on, and this puts the websocket method in jeopardy. Somebody before me decided we should switch all the inter-app communication to an external XMPP server. Ugh.