C++ "ok" semantics for streaming responses in asynchronous API

[ma...@unoperate.com]

Hi,

I'm trying to find what the proper reaction to ok == false should be in asynchronous API with streaming responses. I can't infer it from the documentation.

The example in https://grpc.io/docs/tutorials/async/helloasync-cpp.html simply does GRP_ASSERT(ok), so I can't infer this from it either.

This is my understanding of the state machine allowing to properly read a whole stream of responses (please correct me if I'm wrong).

The stream can be in one of the following states:

• NOT_CREATED (before we touch anything)
• CREATING (after calling Stub::AsyncFoo(), before CompletionQueue::AsyncNext() reports completion)
• PROCESSING (after calling ClientAsyncReaderInterface<>::Read())
• FINISHING (after calling calling ClientAsyncReaderInterface<>::Finish(), before CompletionQueue::AsyncNext() reports completion)
• FINISHED (the stream is dead, all resources are freed)

These are the transitions between the states:

State NOT_CREATED:

• Calling Stub::AsyncFoo() transitions to CREATING

State CREATING:

• if CompletionQueue::AsyncNext() sets ok to true, call ClientAsyncReaderInterface<>::Read() and transition to PROCESSING; (we just got a response, BTW)
• if CompletionQueue::AsyncNext() sets ok to false, call ClientAsyncReaderInterface<>::Finish() transition to FINISHING

State PROCESSING:

• if CompletionQueue::AsyncNext() sets ok to true, call ClientAsyncReaderInterface<>::Read() and transition to PROCESSING
• if CompletionQueue::AsyncNext() sets ok to false, call ClientAsyncReaderInterface<>::Finish() and transition to FINISHING

State FINISHING:

• when CompletionQueue::AsyncNext() notifies (it will always set `ok == true`) transition to FINISHED; (here we learn if the stream was just closed or if there was an error)

State FINISHED:

• nothing else can happen

I have a couple of questions:

• Is this a understanding correct?
• If I wanted to finish the stream in the client, I presume, I can call Finish() in CREATING and PROCESSING states, right?
• Is this the same state machine on the server side or if streaming in the other direction?

The reason I'm asking is that I'm getting a grpc assertion when calling Finish() after having received ok == false in state PROCESSING:

grpc/src/core/lib/surface/call.cc:1853: grpc_cq_begin_op(call->cq, notify_tag)

I'd be grateful for clarification, because I'm currently don't know if I'm misusing the library or I have a bug.

Thanks in advance

[Vijay Pai]

Hi there,

The detailed comment on "ok" is at https://github.com/grpc/grpc/blob/master/include/grpcpp/impl/codegen/completion_queue.h#L128-L170 . Does this answer the questions that you have about its semantics?

Thanks,

Vijay

[Marek Dopiera]

Hi Vijay,

thanks for your quick response.

It answers part of my question. I still can't find anywhere how should I react to "ok" being false. More specifically, how should I react to `ok` being false after RPC invocation or after client Read. Is the stream then dead and resources are released or should I still call Finish? If it is the former, how should I learn the status in such a case?

Thanks

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Marek Dopiera

ma...@unoperate.com

[Vijay Pai]

Hi there,

You should call Finish. It's part of the lifecycle of an async RPC and that's what ultimately allows the RPC to free its resources (and, as you noted, check its status).

Best regards,

Vijay