README.md

gRPC C++ Hello World Tutorial

Install gRPC

Make sure you have installed gRPC on your system. Follow the instructions here: https://github.com/grpc/grpc/blob/master/INSTALL.

Get the tutorial source code

The example code for this and our other examples lives in the examples directory. Clone this repository to your local machine by running the following command:

$ git clone https://github.com/grpc/grpc.git

Change your current directory to examples/cpp/helloworld

$ cd examples/cpp/helloworld/

Defining a service

The first step in creating our example is to define a service: an RPC service specifies the methods that can be called remotely with their parameters and return types. As you saw in the overview above, gRPC does this using protocol buffers. We use the protocol buffers interface definition language (IDL) to define our service methods, and define the parameters and return types as protocol buffer message types. Both the client and the server use interface code generated from the service definition.

Here's our example service definition, defined using protocol buffers IDL in helloworld.proto. The Greeting service has one method, hello, that lets the server receive a single HelloRequest message from the remote client containing the user's name, then send back a greeting in a single HelloReply. This is the simplest type of RPC you can specify in gRPC - we'll look at some other types later in this document.

syntax = "proto3";

option java_package = "ex.grpc";

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;
}

Generating gRPC code

Once we've defined our service, we use the protocol buffer compiler protoc to generate the special client and server code we need to create our application. The generated code contains both stub code for clients to use and an abstract interface for servers to implement, both with the method defined in our Greeting service.

To generate the client and server side interfaces:

$ make helloworld.grpc.pb.cc helloworld.pb.cc

Which internally invokes the proto-compiler as:

$ protoc -I ../../protos/ --grpc_out=. --plugin=protoc-gen-grpc=grpc_cpp_plugin ../../protos/helloworld.proto
$ protoc -I ../../protos/ --cpp_out=. ../../protos/helloworld.proto

Writing a client

• Create a channel. A channel is a logical connection to an endpoint. A gRPC channel can be created with the target address, credentials to use and arguments as follows

  auto channel = CreateChannel("localhost:50051", InsecureCredentials());

• Create a stub. A stub implements the rpc methods of a service and in the generated code, a method is provided to created a stub with a channel:

  auto stub = helloworld::Greeter::NewStub(channel);

• Make a unary rpc, with ClientContext and request/response proto messages.

  ClientContext context;
  HelloRequest request;
  request.set_name("hello");
  HelloReply reply;
  Status status = stub->SayHello(&context, request, &reply);

• Check returned status and response.

  if (status.ok()) {
    // check reply.message()
  } else {
    // rpc failed.
  }

For a working example, refer to greeter_client.cc.

Writing a server

• Implement the service interface

  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;
    }
  };
  

• Build a server exporting the service

  GreeterServiceImpl service;
  ServerBuilder builder;
  builder.AddListeningPort("0.0.0.0:50051", grpc::InsecureServerCredentials());
  builder.RegisterService(&service);
  std::unique_ptr<Server> server(builder.BuildAndStart());

For a working example, refer to greeter_server.cc.

Writing asynchronous client and server

gRPC uses CompletionQueue API for asynchronous operations. The basic work flow is

• bind a CompletionQueue to a rpc call
• do something like a read or write, present with a unique void* tag
• call CompletionQueue::Next to wait for operations to complete. If a tag appears, it indicates that the corresponding operation is complete.

Async client

The channel and stub creation code is the same as the sync client.