This guide gets you started with gRPC in Dart with a simple working example.

Before you begin

Dart gRPC is currently in beta. Please help us out by filing issues if you encounter any.

Prerequisites

Dart SDK

gRPC requires Dart SDK version 1.24.3 or higher. Dart gRPC supports Flutter and Server platforms.

For installation instructions, follow this guide: Install Dart

Install Protocol Buffers v3

While not mandatory to use gRPC, gRPC applications usually leverage Protocol Buffers v3 for service definitions and data serialization, and our example code uses Protocol Buffers as well as gRPC.

The simplest way to install the protoc compiler is to download pre-compiled binaries for your operating system (protoc-<version>-<os>.zip) from here: https://github.com/google/protobuf/releases

  • Unzip this file.
  • Update the environment variable PATH to include the path to the protoc binary file.

Next, install the protoc plugin for Dart

$ pub global activate protoc_plugin

The compiler plugin, protoc-gen-dart, is installed in $HOME/.pub-cache/bin. It must be in your $PATH for the protocol compiler, protoc, to find it.

$ export PATH=$PATH:$HOME/.pub-cache/bin

Download the example

You’ll need a local copy of the example code to work through this quickstart. Download the example code from our Github repository (the following command clones the entire repository, but you just need the examples for this quickstart and other tutorials):

$ # Clone the repository at the latest release to get the example code:
$ git clone https://github.com/grpc/grpc-dart
$ # Navigate to the "Hello World" Dart example:
$ cd grpc-dart/example/helloworld

Run a gRPC application

From the example/helloworld directory:

  1. Download package dependencies

    $ pub get
    
  2. Run the server

    $ dart bin/server.dart
    
  3. In another terminal, run the client

    $ dart bin/client.dart
    

Congratulations! You’ve just run a client-server application with gRPC.

Update a gRPC service

Now let’s look at how to update the application with an extra method on the server for the client to call. Our gRPC service is defined using protocol buffers; you can find out lots more about how to define a service in a .proto file in gRPC Basics: Dart. For now all you need to know is that both the server and the client “stub” have a SayHello RPC method that takes a HelloRequest parameter from the client and returns a HelloReply from the server, and that this method is defined like this:

// 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;
}

Let’s update this so that the Greeter service has two methods. Edit protos/helloworld.proto and update it with a new SayHelloAgain method, with the same request and response types:

// The greeting service definition.
service Greeter {
  // Sends a greeting
  rpc SayHello (HelloRequest) returns (HelloReply) {}
  // Sends another greeting
  rpc SayHelloAgain (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;
}

(Don’t forget to save the file!)

Generate gRPC code

Next we need to update the gRPC code used by our application to use the new service definition.

From the example/helloworld directory, run:

$ protoc --dart_out=grpc:lib/src/generated -Iprotos protos/helloworld.proto

This regenerates the files in lib/src/generated which contain our generated request and response classes, and client and server classes.

Update and run the application

We now have new generated server and client code, but we still need to implement and call the new method in the human-written parts of our example application.

Update the server

In the same directory, open bin/server.dart. Implement the new method like this:

class GreeterService extends GreeterServiceBase {
  @override
  Future<HelloReply> sayHello(ServiceCall call, HelloRequest request) async {
    return new HelloReply()..message = 'Hello, ${request.name}!';
  }

  @override
  Future<HelloReply> sayHelloAgain(
      ServiceCall call, HelloRequest request) async {
    return new HelloReply()..message = 'Hello again, ${request.name}!';
  }
}
...

Update the client

In the same directory, open bin/client.dart. Call the new method like this:

Future<Null> main(List<String> args) async {
  final channel = new ClientChannel('localhost',
      port: 50051,
      options: const ChannelOptions(
          credentials: const ChannelCredentials.insecure()));
  final stub = new GreeterClient(channel);

  final name = args.isNotEmpty ? args[0] : 'world';

  try {
    var response = await stub.sayHello(new HelloRequest()..name = name);
    print('Greeter client received: ${response.message}');
    response = await stub.sayHelloAgain(new HelloRequest()..name = name);
    print('Greeter client received: ${response.message}');
  } catch (e) {
    print('Caught error: $e');
  }
  await channel.shutdown();
}

Run!

Just like we did before, from the example/helloworld directory:

  1. Run the server

    $ dart bin/server.dart
    
  2. In another terminal, run the client

    $ dart bin/client.dart
    

What’s next