Getting started

Welcome to the developer documentation for gRPC, a language-neutral, platform-neutral remote procedure call (RPC) system developed at Google.

This document introduces you to gRPC with a quick overview and a simple Hello World example. You'll find more tutorials and reference docs in this repository - more documentation is coming soon!

Quick start

You can find quick start guides for each language, including installation instructions, examples, and tutorials here:

• C++
• Java
• Go
• Ruby
• Node.js
• Android Java
• Python

What's in this repository?

The grpc-common repository contains documentation, resources, and examples for all gRPC users. You can find examples and instructions specific to your favourite language in the relevant subdirectory.

You can find out about the gRPC source code repositories in grpc. Each repository provides instructions for building the appropriate libraries for your language.

What is gRPC?

In gRPC a client application can directly call methods on a server application on a different machine as if it was a local object, making it easier for you to create distributed applications and services. As in many RPC systems, gRPC is based around the idea of defining a service, specifying the methods that can be called remotely with their parameters and return types. On the server side, the server implements this interface and runs a gRPC server to handle client calls. On the client side, the client has a stub that provides exactly the same methods as the server.

gRPC clients and servers can run and talk to each other in a variety of environments - from servers inside Google to your own desktop - and can be written in any of gRPC's supported languages. So, for example, you can easily create a gRPC server in Java with clients in Go, Python, or Ruby. In addition, the latest Google APIs will have gRPC versions of their interfaces, letting you easily build Google functionality into your applications.

Working with protocol buffers

By default gRPC uses protocol buffers, Google’s mature open source mechanism for serializing structured data (although it can be used with other data formats such as JSON). As you'll see in our example below, you define gRPC services using proto files, with method parameters and return types specified as protocol buffer message types. You can find out lots more about protocol buffers in the Protocol Buffers documentation.

Protocol buffer versions

While protocol buffers have been available for open source users for some time, our examples use a new flavour of protocol buffers called proto3, which has a slightly simplified syntax, some useful new features, and supports lots more languages. This is currently available as an alpha release in Java, C++, Java_nano (Android Java), Python, and Ruby from the protocol buffers Github repo, as well as a Go language generator from the golang/protobuf Github repo, with more languages in development. Full documentation for proto3 is currently in development, but you can see the major differences from the current default version in the release notes.

In general, while you can use proto2 (the current default protocol buffers version), we recommend that you use proto3 with gRPC as it lets you use the full range of gRPC-supported languages, as well as avoiding compatibility issues with proto2 clients talking to proto3 servers and vice versa.

Hello gRPC!

Now that you know a bit more about gRPC, the easiest way to see how it works is to look at a simple example. Our Hello World walks you through the construction of a simple gRPC client-server application, showing you how to:

• Create a protocol buffers schema that defines a simple RPC service with a single Hello World method.
• Create a Java server that implements this interface.
• Create a Java client that accesses the Java server.
• Create a Go client that accesses the same Java server.

The complete code for the example is available in the grpc-common GitHub repository. We use the Git versioning system for source code management: however, you don't need to know anything about Git to follow along other than how to install and run a few git commands.

This is an introductory example rather than a comprehensive tutorial, so don't worry if you're not a Go or Java developer - the concepts are similar for all languages, and you can find more implementations of our Hello World example in other languages (and full tutorials where available) in the language-specific folders in this repository. Complete tutorials and reference documentation for all gRPC languages are coming soon.

Setup

This section explains how to set up your local machine to work with the example code. If you just want to read the example, you can go straight to the next step.

Install Git

You can download and install Git from http://git-scm.com/download. Once installed you should have access to the git command line tool. The main commands that you will need to use are:

• git clone ... : clone a remote repository onto your local machine
• git checkout ... : check out a particular branch or a tagged version of the code to hack on

Install gRPC

To build and install gRPC plugins and related tools:

• For Java, see the Java quick start.
• For Go, see the Go quick start.

Get the source code

The example code for our Java example lives in the grpc-java GitHub repository. Clone this repository to your local machine by running the following command:

git clone https://github.com/google/grpc-java.git

Change your current directory to grpc-java/examples

cd grpc-java/examples

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 = "io.grpc.examples";

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 - right now we're going to generate Java code, though you can generate gRPC code in any gRPC-supported language (as you'll see later in this example). 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.

(If you didn't install the gRPC plugins and protoc on your system and are just reading along with the example, you can skip this step and move onto the next one where we examine the generated code.)

For simplicity, we've provided a Gradle build file with our Java examples that runs protoc for you with the appropriate plugin, input, and output:

../gradlew build

This generates the following classes from our .proto, which contain all the generated code we need to create our example:

• Helloworld.java, which has all the protocol buffer code to populate, serialize, and retrieve our HelloRequest and HelloReply message types

• GreeterGrpc.java, which contains (along with some other useful code):

  • an interface for Greeter servers to implement

  public static interface Greeter {
    public void sayHello(io.grpc.examples.Helloworld.HelloRequest request,
        io.grpc.stub.StreamObserver<io.grpc.examples.Helloworld.HelloReply> responseObserver);
  }

  • stub classes that clients can use to talk to a Greeter server. As you can see, they also implement the Greeter interface.

  public static class GreeterStub extends
      io.grpc.stub.AbstractStub<GreeterStub, GreeterServiceDescriptor>
      implements Greeter {
   ...
  }

Writing a server

Now let's write some code! First we'll create a server application to implement our service. Note that we're not going to go into a lot of detail about how to create a server in this section. More detailed information will be in the tutorial for your chosen language: check if there's one available yet in the relevant quick start.

Our server application has two classes:

• a main server class that hosts the service implementation and allows access over the network: HelloWorldServer.java.

• a simple service implementation class GreeterImpl.java.

Service implementation

GreeterImpl.java actually implements our GreetingService's required behaviour.

As you can see, the class GreeterImpl implements the interface GreeterGrpc.Greeter that we generated from our proto IDL by implementing the method sayHello:

    @Override
    public void sayHello(HelloRequest req, StreamObserver<HelloReply> responseObserver) {
      HelloReply reply = HelloReply.newBuilder().setMessage("Hello " + req.getName()).build();
      responseObserver.onValue(reply);
      responseObserver.onCompleted();
    }

• hello takes two parameters:
  • HelloRequest: the request
  • StreamObserver<HelloReply>: a response observer, which is a special interface for the server to call with its response

To return our response to the client and complete the call:

1. We construct and populate a HelloReply response object with our exciting message, as specified in our interface definition.
2. We return the HelloReply to the client and then specify that we've finished dealing with the RPC.

Server implementation

HelloWorldServer.java shows the other main feature required to provide a gRPC service; making the service implementation available from the network.

  /* The port on which the server should run */
  private int port = 50051;
  private ServerImpl server;

  private void start() throws Exception {
    server = NettyServerBuilder.forPort(port)
        .addService(GreeterGrpc.bindService(new GreeterImpl()))
        .build().start();
    logger.info("Server started, listening on " + port);
    Runtime.getRuntime().addShutdownHook(new Thread() {
      @Override
      public void run() {
        // Use stderr here since the logger may has been reset by its JVM shutdown hook.
        System.err.println("*** shutting down gRPC server since JVM is shutting down");
        HelloWorldServer.this.stop();
        System.err.println("*** server shut down");
      }
    });
  }

Here we create an appropriate gRPC server, binding the GreeterService implementation that we created to a port. Then we start the server running: the server is now ready to receive requests from Greeter service clients on our specified port. We'll cover how all this works in a bit more detail in our language-specific documentation.

Writing a client

Client-side gRPC is pretty simple. In this step, we'll use the generated code to write a simple client that can access the Greeter server we created in the previous section. You can see the complete client code in HelloWorldClient.java.

Again, we're not going to go into much detail about how to implement a client; we'll leave that for the tutorial.

Connecting to the service

First let's look at how we connect to the Greetings server. First we need to create a gRPC channel, specifying the hostname and port of the server we want to connect to. Then we use the channel to construct the stub instance.

  private final ChannelImpl channel;
  private final GreeterGrpc.GreeterBlockingStub blockingStub;

  public HelloWorldClient(String host, int port) {
    channel =
        NettyChannelBuilder.forAddress(host, port).negotiationType(NegotiationType.PLAINTEXT)
            .build();
    blockingStub = GreeterGrpc.newBlockingStub(channel);
  }

In this case, we create a blocking stub. This means that the RPC call waits for the server to respond, and will either return a response or raise an exception. gRPC Java has other kinds of stubs that make non-blocking calls to the server, where the response is returned asynchronously.

Calling an RPC

Now we can contact the service and obtain a greeting:

1. We construct and fill in a HelloRequest to send to the service.
2. We call the stub's hello() RPC with our request and get a HelloReply back, from which we can get our greeting.

    HelloRequest req = HelloRequest.newBuilder().setName(name).build();
    HelloReply reply = blockingStub.sayHello(req);

Try it out!

Our Gradle build file simplifies building and running the examples.

You can build and run the server from the grpc-java root folder with:

$ ./gradlew :grpc-examples:helloWorldServer

and in another terminal window confirm that it receives a message.

$  ./gradlew :grpc-examples:helloWorldClient

Adding another client

Finally, let's look at one of gRPC's most useful features - interoperability between code in different languages. So far, we've just looked at Java code generated from and implementing our Greeter service definition. However, as you'll see if you look at the language-specific subdirectories in this repository, we've also generated and implemented Greeter in some of gRPC's other supported languages. Each service and client uses interface code generated from the same proto that we used for the Java example.

So, for example, if we visit the go example directory and look at the greeter_client, we can see that like the Java client, it connects to a Greeter service at localhost:50051 and uses a stub to call the SayHello method with a HelloRequest:

const (
	address = "localhost:50051"
	defaultName = "world"
)

func main() {
	// Set up a connection to the server.
	conn, err := grpc.Dial(address)
	if err != nil {
		log.Fatalf("did not connect: %v", err)
	}
	defer conn.Close()
	c := pb.NewGreeterClient(conn)

	// Contact the server and print out its response.
	name := defaultName
	if len(os.Args) > 1 {
		name = os.Args[1]
	}
	r, err := c.SayHello(context.Background(), &pb.HelloRequest{Name:
	name})
	if err != nil {
		log.Fatalf("could not greet: %v", err)
	}
	log.Printf("Greeting: %s", r.Message)
}

If we run the Java server from earlier in another terminal window, we can run the Go client and connect to it just like the Java client, even though it's written in a different language.

$ greeter_client