WebSockets

Open the cats API to socket.io clients: a gateway on the app's own port, message handlers that acknowledge with their return value, a property picked out of the payload, and an event sent back to the sender instead of an acknowledgement.

Make the desk feel live: lifecycle hooks for clients coming and going, the handshake's auth payload, the server behind the gateway, and the three ways to send an event: to one client, to everyone but the sender, to everyone.

Send events to some clients only: a namespace that keeps the staff's channel apart from the public desk, rooms that clients join and leave, and the difference between telling a room and telling the rest of it.

Answer one message more than once: an Observable of events, a handler that waits for a promise, an acknowledgement sent by hand before the work is done, and a gateway the HTTP controllers use to tell every socket what changed.

Refuse a message the socket way: WsException with a message or an object, what every other error becomes on the wire, and a filter that sends the HTTP API's exceptions to a socket with their message instead of hiding them.

Validate what a socket sends: the ValidationPipe on a gateway, refusals turned into WsException so the client learns what was wrong, and a parameter pipe whose ordinary BadRequestException would reach the client only as Internal server error.

Decide who may do what, at two moments: a guard that reads the handshake and refuses one kind of message, and a socket.io middleware, installed by an IoAdapter of your own, that refuses the connection itself before any message is sent.

Run code around every message, and keep state for each connection: an interceptor that audits who sent what through the WebSocket execution context, and a request-scoped provider that Nest 12 creates once per connection, with that connection's socket injected.

Write the tests: end-to-end specs that start the application, connect socket.io clients to it, and check what every client receives, so that a broadcast reaching the wrong client, a refused adoption that goes through, or a departure nobody hears fails a test.