Hi
Sorry for the noob question, I've just started grpc. I'm a bit puzzled on
how the backpressure can be done on the client side.
For instance, consider the canonical example `greeter_async_client2.cc`:
for (int i = 0; i < 100; i++) {
std::string user("world " + std::to_string(i));
greeter.SayHello(user); // The actual RPC call!
}
This enqueues 100 requests to be sent (and everything is processed
asynchronously, fine).
Suppose the server/network is slow and the client wants to enqueue faster
than what the server can process, what will happen ?
My understanding of the sample code is that none of the API in `SayHello`
blocks the caller:
// stub_->PrepareAsyncSayHello() creates an RPC object, returning
// an instance to store in "call" but does not actually start the RPC
// Because we are using the asynchronous API, we need to hold on to
// the "call" instance in order to get updates on the ongoing RPC.
call->response_reader =
stub_->PrepareAsyncSayHello(&call->context, request, &cq_);
// StartCall initiates the RPC call
call->response_reader->StartCall();
// Request that, upon completion of the RPC, "reply" be updated
with the
// server's response; "status" with the indication of whether the
operation
// was successful. Tag the request with the memory address of the
call object.
call->response_reader->Finish(&call->reply, &call->status, (void
*)call);
Request will then accumulate somewhere in a hidden queue (bad), or some
request will be dropped (also bad), or one of these API actually blocks
(still bad, for an async API). Maybe some request fails with a specific
error ? (grpc::StatusCode::UNAVAILABLE ?)
I suppose there is a way to notify the client that he should not enqueue
new requests (or that he can now start pushing new requests).
`ChannelInterface::NotifyOnStateChange` looks a good candidate, except that
I could not find a value from `grpc_connectivity_state` that would
represent "*stop pushing new requests, bro, I'm busy*". Could it be
classified as GRPC_CHANNEL_TRANSIENT_FAILURE ?
Thanks for your help
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