#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:
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 find out more in the proto3 language guide and 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.
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.
The example code for our tutorial is in examples/cpp/route_guide. To download the example, clone this repository by running the following command:
$ git clone https://github.com/grpc/grpc.git
Then change your current directory to examples/cpp/route_guide:
$ cd examples/cpp/route_guide
You also should have the relevant tools installed to generate the server and client interface code - if you don't already, follow the setup instructions in gRPC in 3 minutes.
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 examples/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:
// Obtains the feature at a given position.
rpc GetFeature(Point) returns (Feature) {}
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) {}
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) {}
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;
}
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.grpc.pb.cc route_guide.pb.cc
which actually runs:
$ protoc -I ../../protos --grpc_out=. --plugin=protoc-gen-grpc=`which grpc_cpp_plugin` ../../protos/route_guide.proto $ protoc -I ../../protos --cpp_out=. ../../protos/route_guide.proto
Running this command generates the following files in your current directory:
route_guide.pb.h, the header which declares your generated message classesroute_guide.pb.cc, which contains the implementation of your message classesroute_guide.grpc.pb.h, the header which declares your generated service classesroute_guide.grpc.pb.cc, which contains the implementation of your service classesThese contain:
RouteGuide that containsRouteGuide service.RouteGuide service.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:
You can find our example RouteGuide server in route_guide/route_guide_server.cc. Let's take a closer look at how it works.
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(¬e)) { 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.
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.AddListeningPort(server_address, grpc::InsecureServerCredentials()); builder.RegisterService(&service); std::unique_ptr<Server> server(builder.BuildAndStart()); std::cout << "Server listening on " << server_address << std::endl; server->Wait(); }
As you can see, we build and start our server using a ServerBuilder. To do this, we:
RouteGuideImpl.ServerBuilder class.AddListeningPort() method.BuildAndStart() on the builder to create and start an RPC server for our service.Wait() on the server to do a blocking wait until process is killed or Shutdown() is called.In this section, we'll look at creating a C++ client for our RouteGuide service. You can see our complete example client code in route_guide/route_guide_client.cc.
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 without SSL:
grpc::CreateChannel("localhost:50051", grpc::InsecureChannelCredentials());
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<Channel> channel, const std::string& db) : stub_(RouteGuide::NewStub(channel)) { ... }
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.
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;
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().longitude()/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.
Build client and server:
$ make
Run the server, which will listen on port 50051:
$ ./route_guide_server
Run the client (in a different terminal):
$ ./route_guide_client