GRPC/C++ - 如何檢測客戶端在異步服務器中斷開連接

Question

我正在使用此示例的代碼來創建我的 GRPC 異步服務器：

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

#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::Server;
using grpc::ServerAsyncResponseWriter;
using grpc::ServerBuilder;
using grpc::ServerContext;
using grpc::ServerCompletionQueue;
using grpc::Status;
using helloworld::HelloRequest;
using helloworld::HelloReply;
using helloworld::Greeter;

class ServerImpl final {
 public:
  ~ServerImpl() {
    server_->Shutdown();
    // Always shutdown the completion queue after the server.
    cq_->Shutdown();
  }

  // There is no shutdown handling in this code.
  void Run() {
    std::string server_address("0.0.0.0:50051");

    ServerBuilder builder;
    // Listen on the given address without any authentication mechanism.
    builder.AddListeningPort(server_address, grpc::InsecureServerCredentials());
    // Register "service_" as the instance through which we'll communicate with
    // clients. In this case it corresponds to an *asynchronous* service.

    //LINES ADDED BY ME TO IMPLEMENT KEEPALIVE 
    builder.AddListeningPort(server_address, grpc::InsecureServerCredentials());
    builder.AddChannelArgument(GRPC_ARG_KEEPALIVE_TIME_MS, 2000);
    builder.AddChannelArgument(GRPC_ARG_KEEPALIVE_TIMEOUT_MS, 3000);
    builder.AddChannelArgument(GRPC_ARG_KEEPALIVE_PERMIT_WITHOUT_CALLS, 1);
    //END OF LINES ADDED BY ME

    builder.RegisterService(&service_);
    // Get hold of the completion queue used for the asynchronous communication
    // with the gRPC runtime.
    cq_ = builder.AddCompletionQueue();
    // Finally assemble the server.
    server_ = builder.BuildAndStart();
    std::cout << "Server listening on " << server_address << std::endl;

    // Proceed to the server's main loop.
    HandleRpcs();
  }

 private:
  // Class encompasing the state and logic needed to serve a request.
  class CallData {
   public:
    // Take in the "service" instance (in this case representing an asynchronous
    // server) and the completion queue "cq" used for asynchronous communication
    // with the gRPC runtime.
    CallData(Greeter::AsyncService* service, ServerCompletionQueue* cq)
        : service_(service), cq_(cq), responder_(&ctx_), status_(CREATE) {
      // Invoke the serving logic right away.
      Proceed();
    }

    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.
        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 {
        GPR_ASSERT(status_ == FINISH);
        // Once in the FINISH state, deallocate ourselves (CallData).
        delete this;
      }
    }

   private:
    // The means of communication with the gRPC runtime for an asynchronous
    // server.
    Greeter::AsyncService* service_;
    // The producer-consumer queue where for asynchronous server notifications.
    ServerCompletionQueue* cq_;
    // Context for the rpc, allowing to tweak aspects of it such as the use
    // of compression, authentication, as well as to send metadata back to the
    // client.
    ServerContext ctx_;

    // What we get from the client.
    HelloRequest request_;
    // What we send back to the client.
    HelloReply reply_;

    // The means to get back to the client.
    ServerAsyncResponseWriter<HelloReply> responder_;

    // Let's implement a tiny state machine with the following states.
    enum CallStatus { CREATE, PROCESS, FINISH };
    CallStatus status_;  // The current serving state.
  };

  // This can be run in multiple threads if needed.
  void HandleRpcs() {
    // Spawn a new CallData instance to serve new clients.
    new CallData(&service_, cq_.get());
    void* tag;  // uniquely identifies a request.
    bool ok;
    while (true) {
      // 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();
    }
  }

  std::unique_ptr<ServerCompletionQueue> cq_;
  Greeter::AsyncService service_;
  std::unique_ptr<Server> server_;
};

int main(int argc, char** argv) {
  ServerImpl server;
  server.Run();

  return 0;
}

因為我做了一項研究，在那里我發現我必須實現 KeepAlive ( https://grpc.github.io/grpc/cpp/md_doc_keepalive.html ) 我添加了這些行：

builder.AddListeningPort(server_address, grpc::InsecureServerCredentials());
builder.AddChannelArgument(GRPC_ARG_KEEPALIVE_TIME_MS, 2000);
builder.AddChannelArgument(GRPC_ARG_KEEPALIVE_TIMEOUT_MS, 3000);
builder.AddChannelArgument(GRPC_ARG_KEEPALIVE_PERMIT_WITHOUT_CALLS, 1);

到目前為止一切順利，服務器工作正常，通信流暢。 但是，如何檢測客戶端已斷開連接？ 我添加的KeepAlive所謂的KeepAlive方法似乎對我不起作用。

當客戶端因任何原因斷開連接時，我的錯誤在哪里，如何在異步服務器上檢測到？

Answer 1

讓我從一些背景信息開始。

了解 gRPC 很重要的一件事是它使用 HTTP/2 在單個 TCP 連接上多路復用多個流。 每個 gRPC 調用都是一個單獨的流，無論調用是一元的還是流的。 一般而言，任何 gRPC 調用都可以從雙方發送零個或多個消息； 一元調用只是一種特殊情況，從客戶端到服務器只有一條消息，然后從服務器到客戶端只有一條消息。

我們通常使用“斷開連接”一詞來表示 TCP 連接中斷，而不是單個流終止，盡管有時人們會使用相反的含義。 我不確定你在這里指的是哪一個，所以我會回答兩個。

gRPC API 向應用程序公開流生命周期，但不公開 TCP 連接生命周期。 目的是該庫處理管理 TCP 連接的所有細節並將它們隱藏在應用程序之外——我們實際上並沒有公開一種方法來判斷連接何時斷開，您不需要關心，因為庫將自動為您重新連接。 :) 對應用程序可見的唯一情況是，如果在單個 TCP 連接失敗時已經有流在傳輸，則這些流將失敗。

正如我所說，該庫確實向應用程序公開了各個流的生命周期； 流的生命周期基本上就是上面代碼中CallData對象的生命周期。 有兩種方法可以確定流是否已終止。 一種是顯式調用ServerContext::IsCancelled() 。 另一種是在 CQ 上請求一個事件，通過ServerContext::AsyncNotifyWhenDone()異步通知應用取消。

請注意，一般來說，像上面的 HelloWorld 這樣的一元示例並不真正需要擔心檢測流取消，因為從服務器的角度來看，整個流實際上並不會持續很長時間。 在流式調用的情況下，它通常更有用。 但是也有一些例外，例如如果您有一個一元調用，它必須在發送響應之前執行大量昂貴的異步工作。

我希望這些信息有幫助。