GRPC待续

异步grpc

有三个参与者：client、server、grpc runtime

client和server通信：server把response放在completion queue中，client从中取出。

基本工作流：

• 在 RPC 调用上绑定一个 CompletionQueue
• 做一些事情如读取或者写入，以唯一的 void * 标签表示
• 调用 CompletionQueue::Next 去等待操作结束。如果标签出现，表示对应的操作已经完成。

异步grpc client

先看同步grpc client：发送请求后，client阻塞等待server返回结果；

异步client：发送完grpc请求，不等待server的response，继续执行，server执行操作后将返回结果放在一个completion queue中，client根据标识从completion queue取到返回的结果。

• 构造rqc request

        // 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;
  

• 把rpc request绑定到一个completion queue

        // stub_->AsyncSayHello() performs the RPC call, returning an instance we
    // store in "rpc". Because we are using the asynchronous API, we need to
    // hold on to the "rpc" instance in order to get updates on the ongoing RPC.
    std::unique_ptr<ClientAsyncResponseReader<HelloReply> > rpc(
        stub_->AsyncSayHello(&context, request, &cq));
  

• 用一个唯一的tag，寻求reply和最终状态

        // 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.
        // 准备一个reply收server response，status表示rpc请求的执行状态，tag告诉server rpc请求的标识
    rpc->Finish(&reply, &status, (void*)1);
  

• 真正发现rpc请求执行完。
  等待完成队列返回下一个标签。当标签被传入对应的 Finish() 调用时，回答和状态就可以被返回了。

        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.
        // 一直等待返回一个result
    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().
        // ok只是说response已经更新到reply中
    GPR_ASSERT(ok);
  
    // Act upon the status of the actual RPC.
        // status反应的是rpc实际的执行结果
    if (status.ok()) {
      return reply.message();
    } else {
      return "RPC failed";
    }
  

异步grpc server

server发起一个带有标识tag的rpc请求，等待completion queue返回这个tag。