gRPC - C++ Async HelloWorld Client Example doesn't do anything asynchronously

Question

I am trying to learn how to use gRPC asynchronously in C++. Going over the client example at https://github.com/grpc/grpc/blob/v1.33.1/examples/cpp/helloworld/greeter_async_client.cc

Unless I am misunderstanding, I don't see anything asynchronous being demonstrated. There is one and only one RPC call, and it blocks on the main thread until the server processes it and the result is sent back.

What I need to do is create a client that can make one RPC call, and then start another while waiting for the result of the first to come back from the server.

I've got no idea how to go about that.

Does anyone have a working example, or can anyone describe how to actually use gRPC asynchronously?

Their example code:

/*
 *
 * Copyright 2015 gRPC authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

#include <iostream>
#include <memory>
#include <string>

#include <grpcpp/grpcpp.h>
#include <grpc/support/log.h>

#ifdef BAZEL_BUILD
#include "examples/protos/helloworld.grpc.pb.h"
#else
#include "helloworld.grpc.pb.h"
#endif

using grpc::Channel;
using grpc::ClientAsyncResponseReader;
using grpc::ClientContext;
using grpc::CompletionQueue;
using grpc::Status;
using helloworld::HelloRequest;
using helloworld::HelloReply;
using helloworld::Greeter;

class GreeterClient {
 public:
  explicit GreeterClient(std::shared_ptr<Channel> channel)
      : stub_(Greeter::NewStub(channel)) {}

  // Assembles the client's payload, sends it and presents the response back
  // from the server.
  std::string SayHello(const std::string& user) {
    // Data we are sending to the server.
    HelloRequest request;
    request.set_name(user);

    // Container for the data we expect from the server.
    HelloReply reply;

    // Context for the client. It could be used to convey extra information to
    // the server and/or tweak certain RPC behaviors.
    ClientContext context;

    // The producer-consumer queue we use to communicate asynchronously with the
    // gRPC runtime.
    CompletionQueue cq;

    // Storage for the status of the RPC upon completion.
    Status status;

    // 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.
    std::unique_ptr<ClientAsyncResponseReader<HelloReply> > rpc(
    stub_->PrepareAsyncSayHello(&context, request, &cq));

    // StartCall initiates the RPC call
    rpc->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 integer 1.
    rpc->Finish(&reply, &status, (void*)1);
    void* got_tag;
    bool ok = false;
    // Block until the next result is available in the completion queue "cq".
    // The return value of Next should always be checked. This return value
    // tells us whether there is any kind of event or the cq_ is shutting down.
    GPR_ASSERT(cq.Next(&got_tag, &ok));

    // Verify that the result from "cq" corresponds, by its tag, our previous
    // request.
    GPR_ASSERT(got_tag == (void*)1);
    // ... and that the request was completed successfully. Note that "ok"
    // corresponds solely to the request for updates introduced by Finish().
    GPR_ASSERT(ok);

    // Act upon the status of the actual RPC.
    if (status.ok()) {
      return reply.message();
    } else {
      return "RPC failed";
    }
  }

 private:
  // Out of the passed in Channel comes the stub, stored here, our view of the
  // server's exposed services.
  std::unique_ptr<Greeter::Stub> stub_;
};

int main(int argc, char** argv) {
  // Instantiate the client. It requires a channel, out of which the actual RPCs
  // are created. This channel models a connection to an endpoint (in this case,
  // localhost at port 50051). We indicate that the channel isn't authenticated
  // (use of InsecureChannelCredentials()).
  GreeterClient greeter(grpc::CreateChannel(
      "localhost:50051", grpc::InsecureChannelCredentials()));
  std::string user("world");
  std::string reply = greeter.SayHello(user);  // The actual RPC call!
  std::cout << "Greeter received: " << reply << std::endl;

  return 0;
}

Answer 1

You are right, this is a really bad example, it blocks and not async at all.

better look at this example: grpc/greeter_async_client2.

Here you can see in the main that they send the rpc messages in a loop in async non-blocking way:

Client Async send function:

void SayHello(const std::string& user) {
    // Data we are sending to the server.
    HelloRequest request;
    request.set_name(user);

    // Call object to store rpc data
    AsyncClientCall* call = new AsyncClientCall;

    call->response_reader =
        stub_->PrepareAsyncSayHello(&call->context, request, &cq_);

    // StartCall initiates the RPC call
    call->response_reader->StartCall();

    call->response_reader->Finish(&call->reply, &call->status, (void*)call);
}

Client Async receive function:

// Loop while listening for completed responses.
// Prints out the response from the server.
void AsyncCompleteRpc() {
    void* got_tag;
    bool ok = false;

    // Block until the next result is available in the completion queue "cq".
    while (cq_.Next(&got_tag, &ok)) {
        // The tag in this example is the memory location of the call object
        AsyncClientCall* call = static_cast<AsyncClientCall*>(got_tag);

        if (call->status.ok())
            std::cout << "Greeter received: " << call->reply.message() << std::endl;
        else
            std::cout << "RPC failed" << std::endl;

        // Once we're complete, deallocate the call object.
        delete call;
    }
}

Main function:

int main(int argc, char** argv) {
    GreeterClient greeter(grpc::CreateChannel(
            "localhost:50051", grpc::InsecureChannelCredentials()));

    // Spawn reader thread that loops indefinitely
    std::thread thread_ = std::thread(&GreeterClient::AsyncCompleteRpc, &greeter);

    for (int i = 0; i < 100; i++) {
        std::string user("world " + std::to_string(i));
        greeter.SayHello(user);  // The actual RPC call!
    }

    std::cout << "Press control-c to quit" << std::endl << std::endl;
    thread_.join();  //blocks forever

    return 0;
}

addition

As @nmgeek noted, there is a potential memory leak in this solution, please see memory-leak-in-grpc-async-client.