Can gRPC bidi streaming server send a message to a client that it is not currently responding to?

Question

Can gRPC bidi streaming server respond out of sequence to a client ?
All examples on the net show server responding to an incoming request only.
The StreamObserver interface contains implementation for the response responding to a request
In onNext method, can the StreamObserver parameter be cached and reused later to send messages ?
What I need :
I have cached the StreamObserver on the first request for Client1
On a request from Client2, I need to send a message to Client1

Using the cached StreamObserver object throws a CANCELLED error and I notice onComplete is called for the first request from Client1
Is there a way to do this ?
This seems to be a similar ask, but was not supported 2015, and am not sure if this now possible

Answer 1

Once the server receives the StreamObserver for responses to the client it can call the object from any thread at any time (although the object is not thread-safe, so concurrent calls are not permitted). There's no need to couple it with incoming requests in onNext().

The client calling onComplete() is normal and does not end the RPC. If your code turns around and calls onComplete(), however, at that point you can no longer send any more messages on the stream. You will probably want to observe client cancellations, which can be achieved by casting the StreamObserver to ServerCallStreamObserver and calling setOnCancelHandler(Runnable) at the start of the RPC.