grpc c++ 異步完成隊列事件

Question

我試圖了解 grpc c++ 異步 model 流程。 這篇文章（ 鏈接）已經解釋了我的許多疑惑。 這是grpc_asycn_server的代碼。 為了了解 CompletionQueue 何時收到請求，我添加了一些打印語句，如下所示：

首先在 HandleRpcs() function 中。

void HandleRpcs() {
    // Spawn a new CallData instance to serve new clients.
    new CallData(&service_, cq_.get());
    void* tag;  // uniquely identifies a request.
    bool ok;
    int i = 0;
    while (true) {
      std::cout << i << std::endl; ///////////////////////////////
      // Block waiting to read the next event from the completion queue. The
      // event is uniquely identified by its tag, which in this case is the
      // memory address of a CallData instance.
      // The return value of Next should always be checked. This return value
      // tells us whether there is any kind of event or cq_ is shutting down.
      GPR_ASSERT(cq_->Next(&tag, &ok));
      GPR_ASSERT(ok);
      static_cast<CallData*>(tag)->Proceed();
      i++;
    }
  }

在proceed() function中：

void Proceed() {
  if (status_ == CREATE) {
    // Make this instance progress to the PROCESS state.
    status_ = PROCESS;

    // As part of the initial CREATE state, we *request* that the system
    // start processing SayHello requests. In this request, "this" acts are
    // the tag uniquely identifying the request (so that different CallData
    // instances can serve different requests concurrently), in this case
    // the memory address of this CallData instance.
    std::cout<<"RequestSayHello called"<<std::endl; ////////////////////////////
    service_->RequestSayHello(&ctx_, &request_, &responder_, cq_, cq_,
                              this);
  } else if (status_ == PROCESS) {
    // Spawn a new CallData instance to serve new clients while we process
    // the one for this CallData. The instance will deallocate itself as
    // part of its FINISH state.
    new CallData(service_, cq_);

    // The actual processing.
    std::string prefix("Hello ");
    reply_.set_message(prefix + request_.name());

    // And we are done! Let the gRPC runtime know we've finished, using the
    // memory address of this instance as the uniquely identifying tag for
    // the event.
    status_ = FINISH;
    responder_.Finish(reply_, Status::OK, this);
  } else {
    std::cout<<"deallocated"<<std::endl; ////////////////////////////
    GPR_ASSERT(status_ == FINISH);
    // Once in the FINISH state, deallocate ourselves (CallData).
    delete this;
  }
}

一旦我運行服務器和一個客戶端（ 客戶端），服務器就會打印以下內容：

RequestSayHello called
i = 0
RequestSayHello called
i = 1
deallocated
i = 2

由於創建了新的CallData實例，調用的第二個RequestSayHello called是有意義的。 我的問題是如何proceed() function 第二次執行並deallocated被打印？

Answer 1

完成隊列 ( cq_ ) 結構處理多種類型的事件，包括請求和響應事件。 對proceed()的第一次調用進入PROCESS機器的處理階段，用於CallData object。

在此階段：
1、新建CallData object； 如您所述，這會將請求事件插入cq_
2. responder_ _被調用，回復為object； 這會將響應事件插入cq_

在收到來自cq_的響應事件后，在第一個CallData object 上再次調用proceed() ，它現在位於FINISH state 中，因此執行清理並打印deallocated 。