grpc/cpp/cpptutorial.md

#gRPC Basics: C++

This tutorial provides a basic C++ programmer's introduction to working with gRPC. By walking through this example you'll learn how to:

• Define a service in a .proto file.
• Generate server and client code using the protocol buffer compiler.
• Use the C++ gRPC API to write a simple client and server for your service.

It assumes that you have read the Getting started guide and are familiar with [protocol buffers] (https://developers.google.com/protocol-buffers/docs/overview). Note that the example in this tutorial uses the proto3 version of the protocol buffers language, which is currently in alpha release: you can see the release notes for the new version in the protocol buffers Github repository.

This isn't a comprehensive guide to using gRPC in C++: more reference documentation is coming soon.

Why use gRPC?

Our example is a simple route mapping application that lets clients get information about features on their route, create a summary of their route, and exchange route information such as traffic updates with the server and other clients.

With gRPC we can define our service once in a .proto file and implement clients and servers in any of gRPC's supported languages, which in turn can be run in environments ranging from servers inside Google to your own tablet - all the complexity of communication between different languages and environments is handled for you by gRPC. We also get all the advantages of working with protocol buffers, including efficient serialization, a simple IDL, and easy interface updating.

[possibly insert more advantages here]

Example code and setup

The example code for our tutorial is in grpc/grpc-common/cpp/route_guide. To download the example, clone the grpc-common repository by running the following command:

$ git clone https://github.com/google/grpc-common.git

Then change your current directory to grpc-common/cpp/route_guide:

$ cd grpc-common/cpp/route_guide

Although we've provided the complete example so you don't need to generate the gRPC code yourself, if you want to try generating your own server and client interface code you can follow the setup instructions in the C++ quick start guide.

Defining the service

Our first step (as you'll know from Getting started) is to define the gRPC service and the method request and response types using [protocol buffers] (https://developers.google.com/protocol-buffers/docs/overview). You can see the complete .proto file in grpc-common/protos/route_guide.proto.

To define a service, you specify a named service in your .proto file:

service RouteGuide {
   ...
}

Then you define rpc methods inside your service definition, specifying their request and response types. gRPC lets you define four kinds of service method, all of which are used in the RouteGuide service:

• A simple RPC where the client sends a request to the server using the stub and waits for a response to come back, just like a normal function call.

 // Obtains the feature at a given position.
   rpc GetFeature(Point) returns (Feature) {}

• A server-side streaming RPC where the client sends a request to the server and gets a stream to read a sequence of messages back. The client reads from the returned stream until there are no more messages. As you can see in our example, you specify a server-side streaming method by placing the stream keyword before the response type.

  // Obtains the Features available within the given Rectangle.  Results are
  // streamed rather than returned at once (e.g. in a response message with a
  // repeated field), as the rectangle may cover a large area and contain a
  // huge number of features.
  rpc ListFeatures(Rectangle) returns (stream Feature) {}

• A client-side streaming RPC where the client writes a sequence of messages and sends them to the server, again using a provided stream. Once the client has finished writing the messages, it waits for the server to read them all and return its response. You specify a server-side streaming method by placing the stream keyword before the request type.

  // Accepts a stream of Points on a route being traversed, returning a
  // RouteSummary when traversal is completed.
  rpc RecordRoute(stream Point) returns (RouteSummary) {}

• A bidirectional streaming RPC where both sides send a sequence of messages using a read-write stream. The two streams operate independently, so clients and servers can read and write in whatever order they like: for example, the server could wait to receive all the client messages before writing its responses, or it could alternately read a message then write a message, or some other combination of reads and writes. The order of messages in each stream is preserved. You specify this type of method by placing the stream keyword before both the request and the response.

  // Accepts a stream of RouteNotes sent while a route is being traversed,
  // while receiving other RouteNotes (e.g. from other users).
  rpc RouteChat(stream RouteNote) returns (stream RouteNote) {}

Our .proto file also contains protocol buffer message type definitions for all the request and response types used in our service methods - for example, here's the Point message type:

// Points are represented as latitude-longitude pairs in the E7 representation
// (degrees multiplied by 10**7 and rounded to the nearest integer).
// Latitudes should be in the range +/- 90 degrees and longitude should be in
// the range +/- 180 degrees (inclusive).
message Point {
  int32 latitude = 1;
  int32 longitude = 2;
}

Generating client and server code

Next we need to generate the gRPC client and server interfaces from our .proto service definition. We do this using the protocol buffer compiler protoc with a special gRPC C++ plugin.

For simplicity, we've provided a makefile that runs protoc for you with the appropriate plugin, input, and output (if you want to run this yourself, make sure you've installed protoc and followed the gRPC code installation instructions first):

$ make route_guide.pb.cc

which actually runs:

$ protoc -I ../../protos --cpp_out=. --grpc_out=. --plugin=protoc-gen-grpc=`which grpc_cpp_plugin` ../../protos/route_guide.proto

Running this command generates the following files:

• route_guide.pb.h, the header which declares your generated classes
• route_guide.pb.cc, which contains the implementation of your classes

These contain:

• All the protocol buffer code to populate, serialize, and retrieve our request and response message types
• A class called RouteGuide that contains
  • a remote interface type (or stub) for clients to call with the methods defined in the RouteGuide service.
  • two abstract interfaces for servers to implement, also with the methods defined in the RouteGuide service.

Creating the server

First let's look at how we create a RouteGuide server. If you're only interested in creating gRPC clients, you can skip this section and go straight to Creating the client (though you might find it interesting anyway!).

There are two parts to making our RouteGuide service do its job:

• Implementing the service interface generated from our service definition: doing the actual "work" of our service.
• Running a gRPC server to listen for requests from clients and return the service responses.

You can find our example RouteGuide server in [grpc-common/cpp/route_guide/route_guide_server.cc]((https://github.com/grpc/grpc-common/blob/master/cpp/route_guide/route_guide_server.cc). Let's take a closer look at how it works.

Implementing RouteGuide

As you can see, our server has a RouteGuideImpl class that implements the generated RouteGuide::Service interface:

class RouteGuideImpl final : public RouteGuide::Service {
...
}

In this case we're implementing the synchronous version of RouteGuide, which provides our default gRPC server behaviour. It's also possible to implement an asynchronous interface, RouteGuide::AsyncService, which allows you to further customize your server's threading behaviour, though we won't look at this in this tutorial.

RouteGuideImpl implements all our service methods. Let's look at the simplest type first, GetFeature, which just gets a Point from the client and returns the corresponding feature information from its database in a Feature.

  Status GetFeature(ServerContext* context, const Point* point,
                    Feature* feature) override {
    feature->set_name(GetFeatureName(*point, feature_list_));
    feature->mutable_location()->CopyFrom(*point);
    return Status::OK;
  }

The method is passed a context object for the RPC, the client's Point protocol buffer request, and a Feature protocol buffer to fill in with the response information. In the method we populate the Feature with the appropriate information, and then return with an OK status to tell gRPC that we've finished dealing with the RPC and that the Feature can be returned to the client.

Now let's look at something a bit more complicated - a streaming RPC. ListFeatures is a server-side streaming RPC, so we need to send back multiple Features to our client.

  Status ListFeatures(ServerContext* context, const Rectangle* rectangle,
                      ServerWriter<Feature>* writer) override {
    auto lo = rectangle->lo();
    auto hi = rectangle->hi();
    long left = std::min(lo.longitude(), hi.longitude());
    long right = std::max(lo.longitude(), hi.longitude());
    long top = std::max(lo.latitude(), hi.latitude());
    long bottom = std::min(lo.latitude(), hi.latitude());
    for (const Feature& f : feature_list_) {
      if (f.location().longitude() >= left &&
          f.location().longitude() <= right &&
          f.location().latitude() >= bottom &&
          f.location().latitude() <= top) {
        writer->Write(f);
      }
    }
    return Status::OK;
  }

As you can see, instead of getting simple request and response objects in our method parameters, this time we get a request object (the Rectangle in which our client wants to find Features) and a special ServerWriter object. In the method, we populate as many Feature objects as we need to return, writing them to the ServerWriter using its Write() method. Finally, as in our simple RPC, we return Status::OK to tell gRPC that we've finished writing responses.

If you look at the client-side streaming method RecordRoute you'll see it's quite similar, except this time we get a ServerReader instead of a request object and a single response. We use the ServerReaders Read() method to repeatedly read in our client's requests to a request object (in this case a Point) until there are no more messages: the server needs to check the return value of Read() after each call. If true, the stream is still good and it can continue reading; if false the message stream has ended.

while (stream->Read(&point)) {
  ...//process client input
}

Finally, let's look at our bidirectional streaming RPC RouteChat().

  Status RouteChat(ServerContext* context,
                   ServerReaderWriter<RouteNote, RouteNote>* stream) override {
    std::vector<RouteNote> received_notes;
    RouteNote note;
    while (stream->Read(&note)) {
      for (const RouteNote& n : received_notes) {
        if (n.location().latitude() == note.location().latitude() &&
            n.location().longitude() == note.location().longitude()) {
          stream->Write(n);
        }
      }
      received_notes.push_back(note);
    }

    return Status::OK;
  }

This time we get a ServerReaderWriter that can be used to read and write messages. The syntax for reading and writing here is exactly the same as for our client-streaming and server-streaming methods. Although each side will always get the other's messages in the order they were written, both the client and server can read and write in any order — the streams operate completely independently.

Starting the server

Once we've implemented all our methods, we also need to start up a gRPC server so that clients can actually use our service. The following snippet shows how we do this for our RouteGuide service:

void RunServer(const std::string& db_path) {
  std::string server_address("0.0.0.0:50051");
  RouteGuideImpl service(db_path);

  ServerBuilder builder;
  builder.AddPort(server_address);
  builder.RegisterService(&service);
  std::unique_ptr<Server> server(builder.BuildAndStart());
  std::cout << "Server listening on " << server_address << std::endl;
  while (true) {
    std::this_thread::sleep_for(std::chrono::seconds(5));
  }
}

As you can see, we build and start our server using a ServerBuilder. To do this, we:

1. Create an instance of our service implementation class RouteGuideImpl.
2. Create an instance of the factory ServerBuilder class.
3. Specify the address and port we want to use to listen for client requests using the builder's AddPort() method.
4. Register our service implementation with the builder.
5. Call BuildAndStart() on the builder to create and start an RPC server for our service.

[is there no equivalent of the Stubby4 wait() method, ie do you have to do the while(true) loop to keep the server running?]

Creating the client

In this section, we'll look at creating a C++ client for our RouteGuide service. You can see our complete example client code in grpc-common/cpp/route_guide/route_guide_client.cc.

Creating a stub

To call service methods, we first need to create a stub.

First we need to create a gRPC channel for our stub, specifying the server address and port we want to connect to and any special channel arguments - in our case we'll use the default ChannelArguments:

grpc::CreateChannelDeprecated("localhost:50051", ChannelArguments());

Now we can use the channel to create our stub using the NewStub method provided in the RouteGuide class we generated from our .proto.

 public:
  RouteGuideClient(std::shared_ptr<ChannelInterface> channel,
                   const std::string& db)
      : stub_(RouteGuide::NewStub(channel)) {
    ...
  }

Calling service methods

Now let's look at how we call our service methods. Note that in this tutorial we're calling the blocking/synchronous versions of each method: this means that the RPC call waits for the server to respond, and will either return a response or raise an exception.

Simple RPC

Calling the simple RPC GetFeature is nearly as straightforward as calling a local method.

  Point point;
  Feature feature;
  point = MakePoint(409146138, -746188906);
  GetOneFeature(point, &feature);

...

  bool GetOneFeature(const Point& point, Feature* feature) {
    ClientContext context;
    Status status = stub_->GetFeature(&context, point, feature);
    ...
  }

As you can see, we create and populate a request protocol buffer object (in our case Point), and create a response protocol buffer object for the server to fill in. We also create a ClientContext object for our call - you can optionally set RPC configuration values on this object, such as deadlines, though for now we'll use the default settings. Note that you cannot reuse this object between calls. Finally, we call the method on the stub, passing it the context, request, and response. If the method returns OK, then we can read the response information from the server from our response object.

      std::cout << "Found feature called " << feature->name()  << " at "
                << feature->location().latitude()/kCoordFactor_ << ", "
                << feature->location().longitude()/kCoordFactor_ << std::endl;

Streaming RPCs

Now let's look at our streaming methods. If you've already read Creating the server some of this may look very familiar - streaming RPCs are implemented in a similar way on both sides. Here's where we call the server-side streaming method ListFeatures, which returns a stream of geographical Features:

    std::unique_ptr<ClientReader<Feature> > reader(
        stub_->ListFeatures(&context, rect));
    while (reader->Read(&feature)) {
      std::cout << "Found feature called "
                << feature.name() << " at "
                << feature.location().latitude()/kCoordFactor_ << ", "
                << feature.location().latitude()/kCoordFactor_ << std::endl;
    }
    Status status = reader->Finish();

Instead of passing the method a context, request, and response, we pass it a context and request and get a ClientReader object back. The client can use the ClientReader to read the server's responses. We use the ClientReaders Read() method to repeatedly read in the server's responses to a response protocol buffer object (in this case a Feature) until there are no more messages: the client needs to check the return value of Read() after each call. If true, the stream is still good and it can continue reading; if false the message stream has ended. Finally, we call Finish() on the stream to complete the call and get our RPC status.

The client-side streaming method RecordRoute is similar, except there we pass the method a context and response object and get back a ClientWriter.

    std::unique_ptr<ClientWriter<Point> > writer(
        stub_->RecordRoute(&context, &stats));
    for (int i = 0; i < kPoints; i++) {
      const Feature& f = feature_list_[feature_distribution(generator)];
      std::cout << "Visiting point "
                << f.location().latitude()/kCoordFactor_ << ", "
                << f.location().longitude()/kCoordFactor_ << std::endl;
      if (!writer->Write(f.location())) {
        // Broken stream.
        break;
      }
      std::this_thread::sleep_for(std::chrono::milliseconds(
          delay_distribution(generator)));
    }
    writer->WritesDone();
    Status status = writer->Finish();
    if (status.IsOk()) {
      std::cout << "Finished trip with " << stats.point_count() << " points\n"
                << "Passed " << stats.feature_count() << " features\n"
                << "Travelled " << stats.distance() << " meters\n"
                << "It took " << stats.elapsed_time() << " seconds"
                << std::endl;
    } else {
      std::cout << "RecordRoute rpc failed." << std::endl;
    }

Once we've finished writing our client's requests to the stream using Write(), we need to call WritesDone() on the stream to let gRPC know that we've finished writing, then Finish() to complete the call and get our RPC status. If the status is OK, our response object that we initially passed to RecordRoute() will be populated with the server's response.

Finally, let's look at our bidirectional streaming RPC RouteChat(). In this case, we just pass a context to the method and get back a ClientReaderWriter, which we can use to both write and read messages.

    std::shared_ptr<ClientReaderWriter<RouteNote, RouteNote> > stream(
        stub_->RouteChat(&context));

The syntax for reading and writing here is exactly the same as for our client-streaming and server-streaming methods. Although each side will always get the other's messages in the order they were written, both the client and server can read and write in any order — the streams operate completely independently.

Try it out!

[need build and run instructions here]