I use SignalR in order to expose RabbitMQ messages to browsers. This works fine with one app instance obviously. The question is if it could work with multiple instances too without a backplane. I understand that SignalR client could be disconnected from the pod A and connected back to the pod B but what exactly is the issue here? I am fine to lose some messages during reconnection. Is it the only issue? Is reconnection to the pod B treated as a regular new connection so that the client is just subscribed again as it was subscribed normally without reconnection? Or the system doesn't have input parameters it had during initial subscription and therefore it cannot resubscribe without hints?
As long as all of your SignalR servers are getting the same data from RabbitMQ or getting only the data for the clients connected to them, you don't need a backplane.
You will need a backplane if you have one of the following:
I have a similar setup with a database instead of RabbitMQ and need a backplane to either have only one of the SignalR servers access the database (and have data be sent to all clients) or to share the database load between servers (and have data be sent to all clients). This way, the server getting the data can have it sent to a client connected to a different server.
I am using SignalR for ASP.NET and the servers do not know who is subscribed to the other servers. All messages are sent over the backplane and each server determines if they apply to their connected clients. This works well with broadcasts for example or if the same user has multiple clients to make sure they all get the same data regardless of the server.