# 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](../../../INSTALL.md). ### 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: ```sh $ git clone https://github.com/grpc/grpc.git ``` Change your current directory to examples/cpp/helloworld ```sh $ 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](#protocolbuffers) above, gRPC does this using [protocol buffers](https://developers.google.com/protocol-buffers/docs/overview). 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](../../protos/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. ```protobuf 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: ```sh $ make helloworld.grpc.pb.cc helloworld.pb.cc ``` Which internally invokes the proto-compiler as: ```sh $ 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 ```cpp 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: ```cpp auto stub = helloworld::Greeter::NewStub(channel); ``` - Make a unary rpc, with `ClientContext` and request/response proto messages. ```cpp ClientContext context; HelloRequest request; request.set_name("hello"); HelloReply reply; Status status = stub->SayHello(&context, request, &reply); ``` - Check returned status and response. ```cpp if (status.ok()) { // check reply.message() } else { // rpc failed. } ``` For a working example, refer to [greeter_client.cc](greeter_client.cc). ### Writing a server - Implement the service interface ```cpp 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 ```cpp GreeterServiceImpl service; ServerBuilder builder; builder.AddListeningPort("0.0.0.0:50051", grpc::InsecureServerCredentials()); builder.RegisterService(&service); std::unique_ptr server(builder.BuildAndStart()); ``` For a working example, refer to [greeter_server.cc](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`. ```cpp CompletionQueue cq; auto rpc = stub->AsyncSayHello(&context, request, &cq); ``` - Ask for reply and final status, with a unique tag ```cpp 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. ```cpp 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](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 ```cpp helloworld::Greeter::AsyncService service; ServerBuilder builder; builder.AddListeningPort("0.0.0.0:50051", InsecureServerCredentials()); builder.RegisterAsyncService(&service); auto cq = builder.AddCompletionQueue(); auto server = builder.BuildAndStart(); ``` - Request one rpc ```cpp ServerContext context; HelloRequest request; ServerAsyncResponseWriter 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. ```cpp 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. ```cpp 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](greeter_async_server.cc).