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 -b $(curl -L https://grpc.io/release) https://github.com/grpc/grpc

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", InsecureChannelCredentials());
```
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.

Initiate the rpc and create a handle for the rpc. Bind the rpc to a CompletionQueue.

CompletionQueue cq;
auto rpc = stub->AsyncSayHello(&context, request, &cq);

Ask for reply and final status, with a unique tag

Status status;
rpc->Finish(&reply, &status, (void*)1);

Wait for the completion queue to return the next tag. The reply and status are ready once the tag passed into the corresponding Finish() call is returned.
```
void* got_tag;
bool ok = false;
cq.Next(&got_tag, &ok);
if (ok && got_tag == (void*)1) {
  // check reply and status
}
```

For a working example, refer to greeter_async_client.cc.

Async server

The server implementation requests a rpc call with a tag and then wait for the completion queue to return the tag. The basic flow is

Build a server exporting the async service

helloworld::Greeter::AsyncService service;
ServerBuilder builder;
builder.AddListeningPort("0.0.0.0:50051", InsecureServerCredentials());
builder.RegisterService(&service);
auto cq = builder.AddCompletionQueue();
auto server = builder.BuildAndStart();

Request one rpc

ServerContext context;
HelloRequest request;
ServerAsyncResponseWriter<HelloReply> responder;
service.RequestSayHello(&context, &request, &responder, &cq, &cq, (void*)1);

Wait for the completion queue to return the tag. The context, request and responder are ready once the tag is retrieved.

HelloReply reply;
Status status;
void* got_tag;
bool ok = false;
cq.Next(&got_tag, &ok);
if (ok && got_tag == (void*)1) {
  // set reply and status
  responder.Finish(reply, status, (void*)2);
}

Wait for the completion queue to return the tag. The rpc is finished when the tag is back.

void* got_tag;
bool ok = false;
cq.Next(&got_tag, &ok);
if (ok && got_tag == (void*)2) {
  // clean up
}

To handle multiple rpcs, the async server creates an object CallData to maintain the state of each rpc and use the address of it as the unique tag. For simplicity the server only uses one completion queue for all events, and runs a main loop in HandleRpcs to query the queue.

For a working example, refer to greeter_async_server.cc.