As i understand correctly, you need to just stream data from client, and have no response from server?
If so, you can use requestChannel and on responder side you can do several things depending on what you are trying to achieve:
1. Return empty flow, than this part of stream will be finished, and you will be able to stream from client. In this case, flow returned to requester will also be empty
2. Return flow, which will produce some result, f.e. after you will end streaming, which contains some useful data
3. You can do even more, but it fully depends on what you want to archive:)