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grpchttp2server-push

What is the mechanism of grpc server side pushing?

While I'm writing a service with grpc, I'm trying to compare http/2 with websocket by server side pushing mechanism.

I know for websocket, the client will send a request with Upgrade: WebSocket and Connection: Upgrade headers to server and establish the long-lived connection. Then server will send the data freely after the connection is established.

But for grpc, as it is routed upon http/2, from the wiki page, https://en.wikipedia.org/wiki/HTTP/2_Server_Push, it says the server would need to predict the potential requests the client would send, and send a PUSH_PROMISE frame as early as possible.

Here are my two questions:

  1. Does it mean that the server would also need to receive a corresponding response(request) from client in response to this PUSH_PROMISE header to decide if client wants to receive or decline the certain push?

  2. In Grpc, if I have a sever side streaming, say send a message every 1 second from server. Does it mean the server need to send a PUSH_PROMISE to client every 1 second or at least before every data frame that server pushes to client?

Solution

  • gRPC does not currently support/use PUSH_PROMISE.

    Streaming RPCs in gRPC use HTTP/2 streams; the entire RPC is contained in a request/response in HTTP. The main difference is that HTTP/2 implementations generally allow such streams to be streaming and bidirectional (the client can send more in the request after reading part of the response), while in HTTP/1 that was hit-or-miss.

    In gRPC the client will always initiate the RPC. But for server-streaming the server can then reply with multiple messages over time via the stream. This would be similar to the scenario you described with websockets.