What is the mechanism of grpc server side pushing?

Question

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?

Argyres

Answer 1

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.

Reticulate

Thanks! So what is the point of this PUSH_PROMISE here if we can simply push from server to client, I think it just adds more complexity with the term like 'predict' the requests, right? And for now, as I observe, no browser supports grpc, so I'm little bit confused about the explanation in wiki above. – Argyres

Server push is for a different use-case. It allows a server to pre-fill the client's cache which can remove a RTT for small, important resources. – Reticulate