C++ Async Example ( how to make client unblock from Next )

[Nid Raj]

Using:

protobuf-3.0.0

grpc release 0.12

Looking at the helloworld greeter async example.  I'm able to build and run the client and server example.

My doubt is around unblocking the client side.   If I modify the server and put a sleep (of 10sec) at the processing

side of the RPC ( in the status_ == PROCESS ) the client side blocks on the Next api call.  I'm need to make

client side unblock (ie just make the RPC call and ignore the reply / ie. the block on Next ).

Few things I tried:

1. Removed the Finish, Next (attempt to ignore the reply )

    

I observe an error on the client side: 1 metadata elements were leaked, and the server never receives anything.

2. Attempted to move the Next into a thread.

At the time of connection, I spawned off a thread that sits in a loop on the Next.  cq is global

    void *got_tag;

    bool ok = false;

    while (1) {

       cq.Next(&got_tag,&ok);

    }

Observation is same as #1  ( server doesn't get anything and client gives leaked error ),  I also tried spawning thread

after the RPC call (ie where Next was originally getting called ) same issue.

Wondering if I'm missing something ? My requirements is make the async RPC call and have client side completely unblocked

to continue processing other things / requests.

thanks.

[micha...@gmail.com]

I'm actually in the same boat.  If anyone finds a solution for this, please post back.

Thanks in advance.

[Yang Gao]

I think you can use your option 2 and call Next in a separate thread. In the main thread you can send the rpc, call Finish with a tag. When you get the tag back from Next, you can just do clean-up. The cq can be shared by multiple rpcs.

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[micha...@gmail.com]

I've played around with that option, but haven't gotten it to work correctly.  It works fine when I call it from the main thread, but as soon as I fork a thread, ok is coming back as false:

void
ReadFromQueue()
{
    void *tag;
    bool ok = false;
    while (true)
    {
        cq_.Next(&tag, &ok);
        if (ok)   <--------------- Coming back as false when on a separate thread
        {
            // do something here
        }

    }
}

Does anybody have any working sample code I can try?

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[Vijay Pai]

 test/cpp/end2end/thread_stress_test.cc has examples of async clients and servers using different threads to do the sending and the Next; hope that helps.

[micha...@gmail.com]

Thanks for the pointer.  I was finally able to get this working...

For the next guy that runs into this, I've submitted a PR to hopefully help out:  https://github.com/grpc/grpc/pull/6430

Also if anyone is interested, I've created a PR to update the example async server code to be multi-threaded:  https://github.com/grpc/grpc/pull/6398

Thanks again!