#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 for the C++ gRPC libraries in grpc/grpc/INSTALL.

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 followed the installation instructions first):

$ make route_guide.pb.cc

which actually runs:

[actual command]

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 both a remote interface type (or stub) for clients to call and an abstract interface for servers to implement, both with the methods defined in the RouteGuide service.

Creating the server

First let's look at how we create a RouteGuide server.

There are two parts to making our RouteGuide service work:

• 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

Creating the client