grpc C++ helloworld 实例分析

proto文件：

syntax = "proto3";

option java_multiple_files = true;
option java_package = "io.grpc.examples.helloworld";
option java_outer_classname = "HelloWorldProto";
option objc_class_prefix = "HLW";

package helloworld;

// The greeting service definition.
service Greeter {
  // Sends a greeting
  rpc SayHello (HelloRequest) returns (HelloReply) {}
}

// The request message containing the user's name.
message HelloRequest {
  string name = 1;
}

// The response message containing the greetings
message HelloReply {
  string message = 1;
}

服务端代码：

include 
#include 
#include 

#include 
#include 
#include 

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

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

// Logic and data behind the server's behavior.
class GreeterServiceImpl final : public Greeter::Service {
  Status SayHello(ServerContext* context, const HelloRequest* request,
                  HelloReply* reply) override {
    std::string prefix("Hello ");
    reply->set_message(prefix + request->name());
    return Status::OK;
  }
};

void RunServer() {
  std::string server_address("0.0.0.0:50051");
  GreeterServiceImpl service;

  grpc::EnableDefaultHealthCheckService(true);
  grpc::reflection::InitProtoReflectionServerBuilderPlugin();
  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 *synchronous* service.
  builder.RegisterService(&service);
  // Finally assemble the server.
  std::unique_ptr server(builder.BuildAndStart());
  std::cout << "Server listening on " << server_address << std::endl;

  // Wait for the server to shutdown. Note that some other thread must be
  // responsible for shutting down the server for this call to ever return.
  server->Wait();
}

int main(int argc, char** argv) {
  RunServer();

  return 0;
}

客户端代码：

#include 
#include 
#include 

#include 

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

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

class GreeterClient {
 public:
  GreeterClient(std::shared_ptr 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 actual RPC.
    Status status = stub_->SayHello(&context, request, &reply);

    // Act upon its status.
    if (status.ok()) {
      return reply.message();
    } else {
      std::cout << status.error_code() << ": " << status.error_message()
                << std::endl;
      return "RPC failed";
    }
  }

 private:
  std::unique_ptr 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 specified by
  // the argument "--target=" which is the only expected argument.
  // We indicate that the channel isn't authenticated (use of
  // InsecureChannelCredentials()).
  std::string target_str;
  std::string arg_str("--target");
  if (argc > 1) {
    std::string arg_val = argv[1];
    size_t start_pos = arg_val.find(arg_str);
    if (start_pos != std::string::npos) {
      start_pos += arg_str.size();
      if (arg_val[start_pos] == '=') {
        target_str = arg_val.substr(start_pos + 1);
      } else {
        std::cout << "The only correct argument syntax is --target=" << std::endl;
        return 0;
      }
    } else {
      std::cout << "The only acceptable argument is --target=" << std::endl;
      return 0;
    }
  } else {
    target_str = "localhost:50051";
  }
  GreeterClient greeter(grpc::CreateChannel(
      target_str, grpc::InsecureChannelCredentials()));
  std::string user("world");
  std::string reply = greeter.SayHello(user);
  std::cout << "Greeter received: " << reply << std::endl;

  return 0;
}

执行makefile会生成文件 helloworld.pb.cc, helloworld.pb.h, helloworld.grpc.pb.cc, helloworld.grpc.pb.h。

根据proto中的service定义：

service Greeter {
  // Sends a greeting
  rpc SayHello (HelloRequest) returns (HelloReply) {}
}

Greeter服务器自动生成了一个嵌套类Service：

class Service : public ::grpc::Service {
public:
  Service();
  virtual ~Service();
  // Sends a greeting
  virtual ::grpc::Status SayHello(::grpc::ServerContext* context, const ::helloworld::HelloRequest* request, ::helloworld::HelloReply* response);
};

实现了我们的接口函数SayHello的虚函数，用于继承。在服务部分源文件中,我们构造了Service的子类对象GreeterServiceImpl用于实现我们特定的方法。

SayHello接口的参数如下
grpc::ServerContext context：为每个grpc添加上下文。
helloworld::HelloRequest request：请求时传入的参数。
helloworld::HelloReply* response：回复时传出的结果。

最后的接口函数实现如下：

class GreeterServiceImpl final : public Greeter::Service {
  Status SayHello(ServerContext* context, const HelloRequest* request,
                  HelloReply* reply) override {
    std::string prefix("Hello ");
    reply->set_message(prefix + request->name());
    return Status::OK;
  }
};

通过reply->set_message()设置回复消息。

在看下RunServer()函数：

void RunServer() {
  std::string server_address("0.0.0.0:50051");
  GreeterServiceImpl service;

  grpc::EnableDefaultHealthCheckService(true);
  grpc::reflection::InitProtoReflectionServerBuilderPlugin();
  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 *synchronous* service.
  builder.RegisterService(&service);
  // Finally assemble the server.
  std::unique_ptr server(builder.BuildAndStart());
  std::cout << "Server listening on " << server_address << std::endl;

  // Wait for the server to shutdown. Note that some other thread must be
  // responsible for shutting down the server for this call to ever return.
  server->Wait();
}

ServerBuilder用于创建和启动一个grpc服务实例，首先调用：

builder.AddListeningPort(server_address, grpc::InsecureServerCredentials());

监听指定的ip和端口号，接着注册grpc服务实例：

builder.RegisterService(&service);

最后组装并启动该服务：

std::unique_ptr server(builder.BuildAndStart());

此时，返回一个运行的服务器，已经准备好了去处理函数调用，智能指针 std::unique_ptr 会自动清理内存。

接下来看客户端的代码，首先看class Stub内部类：

  class Stub final : public StubInterface {
   public:
    Stub(const std::shared_ptr< ::grpc::ChannelInterface>& channel);
    ::grpc::Status SayHello(::grpc::ClientContext* context, const ::helloworld::HelloRequest& request, ::helloworld::HelloReply* response) override;
    std::unique_ptr< ::grpc::ClientAsyncResponseReader< ::helloworld::HelloReply>> AsyncSayHello(::grpc::ClientContext* context, const ::helloworld::HelloRequest& request, ::grpc::CompletionQueue* cq) {
      return std::unique_ptr< ::grpc::ClientAsyncResponseReader< ::helloworld::HelloReply>>(AsyncSayHelloRaw(context, request, cq));
    }
    std::unique_ptr< ::grpc::ClientAsyncResponseReader< ::helloworld::HelloReply>> PrepareAsyncSayHello(::grpc::ClientContext* context, const ::helloworld::HelloRequest& request, ::grpc::CompletionQueue* cq) {
      return std::unique_ptr< ::grpc::ClientAsyncResponseReader< ::helloworld::HelloReply>>(PrepareAsyncSayHelloRaw(context, request, cq));
    }
    class experimental_async final :
      public StubInterface::experimental_async_interface {
     public:
      void SayHello(::grpc::ClientContext* context, const ::helloworld::HelloRequest* request, ::helloworld::HelloReply* response, std::function) override;
      #ifdef GRPC_CALLBACK_API_NONEXPERIMENTAL
      void SayHello(::grpc::ClientContext* context, const ::helloworld::HelloRequest* request, ::helloworld::HelloReply* response, ::grpc::ClientUnaryReactor* reactor) override;
      #else
      void SayHello(::grpc::ClientContext* context, const ::helloworld::HelloRequest* request, ::helloworld::HelloReply* response, ::grpc::experimental::ClientUnaryReactor* reactor) override;
      #endif
     private:
      friend class Stub;
      explicit experimental_async(Stub* stub): stub_(stub) { }
      Stub* stub() { return stub_; }
      Stub* stub_;
    };
    class experimental_async_interface* experimental_async() override { return &async_stub_; }

   private:
    std::shared_ptr< ::grpc::ChannelInterface> channel_;
    class experimental_async async_stub_{this};
    ::grpc::ClientAsyncResponseReader< ::helloworld::HelloReply>* AsyncSayHelloRaw(::grpc::ClientContext* context, const ::helloworld::HelloRequest& request, ::grpc::CompletionQueue* cq) override;
    ::grpc::ClientAsyncResponseReader< ::helloworld::HelloReply>* PrepareAsyncSayHelloRaw(::grpc::ClientContext* context, const ::helloworld::HelloRequest& request, ::grpc::CompletionQueue* cq) override;
    const ::grpc::internal::RpcMethod rpcmethod_SayHello_;
  };

这个类继承于类StubInterface，是定义客户端操作的存根(stub)的接口类，stub类实现了给Client 客户端使用的接口，stub_ 的赋值使用 Greeter::NewStub() 类静态方法获取，后续调用 SayHello() 完成对 RPC server 远程服务器的调用。

chnnel通道的远离，每个 Client 都需要 Channel 来实现对远程服务器的调用，使用 grpc::CreateChannel() 函数来创建一个通信管道。