grpc Stream creation

[Darshan J Gowda]

Hi everyone,
I'm trying to understand how many streams are getting created in one grpc connection (Bidirectional streaming) whenever I call onNext() using a StreamObserver object. Below is my code context

StreamObserver<Response> responseStreamObserver1 = getResponseObserver();  
StreamObserver<Response> responseStreamObserver2 = getResponseObserver();

StreamObserver<Request> streamClientSender1 = stub.sendRequest(responseStreamObserver1);
StreamObserver<Mbus> streamClientSender2 = stub.sendRequest(responseStreamObserver2);

My question is,
1. when the program calls streamClientSender1.onNext(request), request will be sent in a stream, if I call it again will it be in the same stream as long as the age of the stream is within max connection age ?
2. does streamClientSender1.onNext() and streamClientSender2.onNext() use different streams to send request, since they are two different streamObserver objects. 
3. Is there a way to get StreamID of streams created and print it as a verification step so that the above two questions can be answered ?

Thanks and regards,
Darshan

[Sergii Tkachenko]

Hey Darshan,

Right now I'm not entirely sure I'll be able to answer your question with absolute confidence.

I'd like to say "yes, stream is created on first headers sent, and stream id not going to change until RPC dies", but there might be some nuances depending on the transport used, f.e. Netty's StreamBufferingEncoder. I'll speak with my TL to clarify this.

I don't think there's a clear way to get the stream ID, except I've noticed it's tracked via PerfMark when it's enabled:

https://github.com/grpc/grpc-java/blob/68eb639b1cd38c161a3113bd50e731c04ac51fef/core/src/main/java/io/grpc/internal/ServerImpl.java#L464-L465 

I'll follow up on your questions, but could you please clarify:

1. Your code snippet: are you talking about the client side, or the server side?

2. Is there anything specific you're trying to achieve?

Best Regards,

Sergii

[Sergii Tkachenko]

Ok, I might've went a bit to much into the weeds of HTTP streams with my answer.

> 1. when the program calls streamClientSender1.onNext(request), request will be sent in a stream, if I call it again will it be in the same stream as long as the age of the stream is within max connection age ?

First, to clarify - the stream is not created whenever onNext is called. It's created on the first stub.sendRequest call. The stream will persist until the RPC is complete.

> 2. does streamClientSender1.onNext() and streamClientSender2.onNext() use different streams to send request, since they are two different streamObserver objects. 

Yes, two different streams - just with the same correction that stub.sendRequest is what initiates a stream.

Messages is sent in order and tied to a stream.

> 3. Is there a way to get StreamID of streams created and print it as a verification step so that the above two questions can be answered ?

Not really, stream ids are kept internal. If it's just something you're curious about, you can always use the debugger. But you shouldn't do anything stream-id-related in the code, see https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md#stream-identification:

> All GRPC calls need to specify an internal ID. We will use HTTP2 stream-ids as call identifiers in this scheme. NOTE: These ids are contextual to an open HTTP2 session and will not be unique within a given process that is handling more than one HTTP2 session nor can they be used as GUIDs.

Sergii

[Darshan J Gowda]

Thanks Sergii, for the response. This clarifies my doubt. 

Best regards,
Darshan