Can HTTP/2 client terminate the stream by sending a HEADER frame in bidirectional streaming RPC?

Question

Suppose we have a bidirectional streaming RPC where the client is sending several request messages (ie multiple DATA frames) and the server is answering back with several response messages (ie multiple DATA frames).

As I understand it, when the RPC is complete, the server will normally send a HEADER frame with the status header as well as possibly some trailer headers like grpc-status and grpc-message to mark the completion of the request/response exchange.

My question is, suppose the server sends a bad response message, is it possible for the client to send the HEADER frame with the grpc-status and grpc-message headers to convey information about the error.

The reason why I'm asking is because in the c++ server code (generated from protobuf defininition), I'm struggling to find a way to get a hold of this last HEADER frame sent by the client to verify the values of the grpc-status and grpc-message headers.

Additionally, after going through the unit tests in the grpc project, it seems like only the server returns the status for the RPC, which further raises doubts.

I was however able to send the HEADER frame out from the client, but based on the above, I'm not certain whether this is the correct behavior even though I was able to do it.

I would appreciate it if someone can clarify this for me as I'm fairly new to HTTP/2 and gRPC.

Answer 1

Additionally, after going through the unit tests in the grpc project, it seems like only the server returns the status for the RPC, which further raises doubts.

Correct! In gRPC, the server is responsible for terminating the RPC with a status and optional trailing metadata. The client never sends a status to the server. The client can indicate it is done sending on the stream without a status (which internally happens by sending an empty data frame with the END_STREAM flag set, but users shouldn't need to be concerned with this detail). The client only sends HEADER frames at the start of the RPC.