Walkthrough

Setting up

To get started, you must obtain the .proto file(s) that describe the interface you want to use and add those files to your project.

Add .proto files to your project’s src/main/protobufsrc/main/protosrc/main/proto directory. See the detailed chapters on sbt, Gradle and Maven for information on picking up .proto definitions from dependencies automatically.

Then add the following configuration to your build:

sbt

// in project/plugins.sbt:
addSbtPlugin("org.apache.pekko" % "pekko-grpc-sbt-plugin" % "1.0.2")
// in build.sbt:
enablePlugins(PekkoGrpcPlugin)

Gradle

buildscript {
  dependencies {
// version here is a placeholder,
// it is replaced with a project dependency during integration tests
// by adding --include-build <path> to gradlew
classpath 'org.apache.pekko:pekko-grpc-gradle-plugin:1.0.2'
  }
}
plugins {
  id 'java'
  id 'application'
}
apply plugin: 'org.apache.pekko.grpc.gradle'

Maven

<project>
  <modelVersion>4.0.0</modelVersion>
  <name>Project name</name>
  <groupId>com.example</groupId>
  <artifactId>my-grpc-app</artifactId>
  <version>0.1-SNAPSHOT</version>
  <properties>
  <maven.compiler.source>1.8</maven.compiler.source>
  <maven.compiler.target>1.8</maven.compiler.target>
  <pekko.grpc.version>1.0.2</pekko.grpc.version>
  <grpc.version>1.54.2</grpc.version>
  <project.encoding>UTF-8</project.encoding>
</properties>
  <dependencies>
<dependency>
  <groupId>org.apache.pekko</groupId>
  <artifactId>pekko-grpc-runtime_2.12</artifactId>
  <version>${pekko.grpc.version}</version>
</dependency>
<!-- for loading of cert, issue #89 -->
<dependency>
  <groupId>io.grpc</groupId>
  <artifactId>grpc-testing</artifactId>
  <version>${grpc.version}</version>
</dependency>
  </dependencies>
  <build>
<plugins>
  <plugin>
    <groupId>org.apache.pekko</groupId>
    <artifactId>pekko-grpc-maven-plugin</artifactId>
    <version>${pekko.grpc.version}</version>
    <!-- Hook the generate goal into the lifecycle,
         automatically tied to generate-sources -->
    <executions>
      <execution>
        <goals>
          <goal>generate</goal>
        </goals>
      </execution>
    </executions>
  </plugin>
</plugins>
  </build>
</project>

For a complete overview of the configuration options see the chapter for your build tool, sbt, Gradle or Maven.

Dependencies

The Pekko gRPC plugin makes your code depend on the pekko-grpc-runtime library.

The table below shows direct dependencies of it and the second tab shows all libraries it depends on transitively. Be aware that the io.grpc.grpc-api library depends on Guava.

Generating Service Stubs

To use a service, such as the Hello World service described in the server documentation, you only need the protobuf definition (the .proto files) of the service. No additional dependencies to the server project are needed.

For example, this is the definition of a Hello World service:

sourcesyntax = "proto3";

option java_multiple_files = true;
option java_package = "example.myapp.helloworld.grpc";
option java_outer_classname = "HelloWorldProto";

package helloworld;

////////////////////////////////////// The greeting service definition.
service GreeterService {
    //////////////////////
    // Sends a greeting //
    ////////*****/////////
    //      HELLO       //
    ////////*****/////////
    rpc SayHello (HelloRequest) returns (HelloReply) {}

    // Comment spanning
    // on several lines
    rpc ItKeepsTalking (stream HelloRequest) returns (HelloReply) {}

    /*
     * C style comments
     */
    rpc ItKeepsReplying (HelloRequest) returns (stream HelloReply) {}

    /* C style comments
     * on several lines
     * with non-empty heading/trailing line */
    rpc StreamHellos (stream HelloRequest) returns (stream 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;
}

From this definition, Pekko gRPC generates interfaces that look like this:

Scala

source
// Generated by Pekko gRPC. DO NOT EDIT.
package example.myapp.helloworld.grpc

import org.apache.pekko
import pekko.annotation.ApiMayChange

import pekko.grpc.PekkoGrpcGenerated

/**
 * #services
 * //////////////////////////////////// The greeting service definition.
 */
@PekkoGrpcGenerated
trait GreeterService {
  
  /**
   * ////////////////////
   * Sends a greeting //
   * //////&#42;****&#47;////////
   *      HELLO       //
   * //////&#42;****&#47;////////
   */
  def sayHello(in: example.myapp.helloworld.grpc.HelloRequest): scala.concurrent.Future[example.myapp.helloworld.grpc.HelloReply]
  
  /**
   * Comment spanning
   * on several lines
   */
  def itKeepsTalking(in: org.apache.pekko.stream.scaladsl.Source[example.myapp.helloworld.grpc.HelloRequest, org.apache.pekko.NotUsed]): scala.concurrent.Future[example.myapp.helloworld.grpc.HelloReply]
  
  /**
   *  C style comments
   */
  def itKeepsReplying(in: example.myapp.helloworld.grpc.HelloRequest): org.apache.pekko.stream.scaladsl.Source[example.myapp.helloworld.grpc.HelloReply, org.apache.pekko.NotUsed]
  
  /**
   * C style comments
   * on several lines
   * with non-empty heading/trailing line    */
  def streamHellos(in: org.apache.pekko.stream.scaladsl.Source[example.myapp.helloworld.grpc.HelloRequest, org.apache.pekko.NotUsed]): org.apache.pekko.stream.scaladsl.Source[example.myapp.helloworld.grpc.HelloReply, org.apache.pekko.NotUsed]
  
}



@PekkoGrpcGenerated
object GreeterService extends pekko.grpc.ServiceDescription {
  val name = "helloworld.GreeterService"

  val descriptor: com.google.protobuf.Descriptors.FileDescriptor =
    example.myapp.helloworld.grpc.HelloworldProto.javaDescriptor;

  object Serializers {
    import pekko.grpc.scaladsl.ScalapbProtobufSerializer
    
    val HelloRequestSerializer = new ScalapbProtobufSerializer(example.myapp.helloworld.grpc.HelloRequest.messageCompanion)
    
    val HelloReplySerializer = new ScalapbProtobufSerializer(example.myapp.helloworld.grpc.HelloReply.messageCompanion)
    
  }

  @ApiMayChange
  @PekkoGrpcGenerated
  object MethodDescriptors {
    import pekko.grpc.internal.Marshaller
    import io.grpc.MethodDescriptor
    import Serializers._

    
    val sayHelloDescriptor: MethodDescriptor[example.myapp.helloworld.grpc.HelloRequest, example.myapp.helloworld.grpc.HelloReply] =
      MethodDescriptor.newBuilder()
        .setType(
   MethodDescriptor.MethodType.UNARY 
  
  
  
)
        .setFullMethodName(MethodDescriptor.generateFullMethodName("helloworld.GreeterService", "SayHello"))
        .setRequestMarshaller(new Marshaller(HelloRequestSerializer))
        .setResponseMarshaller(new Marshaller(HelloReplySerializer))
        .setSampledToLocalTracing(true)
        .build()
    
    val itKeepsTalkingDescriptor: MethodDescriptor[example.myapp.helloworld.grpc.HelloRequest, example.myapp.helloworld.grpc.HelloReply] =
      MethodDescriptor.newBuilder()
        .setType(
  
  MethodDescriptor.MethodType.CLIENT_STREAMING 
  
  
)
        .setFullMethodName(MethodDescriptor.generateFullMethodName("helloworld.GreeterService", "ItKeepsTalking"))
        .setRequestMarshaller(new Marshaller(HelloRequestSerializer))
        .setResponseMarshaller(new Marshaller(HelloReplySerializer))
        .setSampledToLocalTracing(true)
        .build()
    
    val itKeepsReplyingDescriptor: MethodDescriptor[example.myapp.helloworld.grpc.HelloRequest, example.myapp.helloworld.grpc.HelloReply] =
      MethodDescriptor.newBuilder()
        .setType(
  
  
   MethodDescriptor.MethodType.SERVER_STREAMING 
  
)
        .setFullMethodName(MethodDescriptor.generateFullMethodName("helloworld.GreeterService", "ItKeepsReplying"))
        .setRequestMarshaller(new Marshaller(HelloRequestSerializer))
        .setResponseMarshaller(new Marshaller(HelloReplySerializer))
        .setSampledToLocalTracing(true)
        .build()
    
    val streamHellosDescriptor: MethodDescriptor[example.myapp.helloworld.grpc.HelloRequest, example.myapp.helloworld.grpc.HelloReply] =
      MethodDescriptor.newBuilder()
        .setType(
  
  
  
  MethodDescriptor.MethodType.BIDI_STREAMING 
)
        .setFullMethodName(MethodDescriptor.generateFullMethodName("helloworld.GreeterService", "StreamHellos"))
        .setRequestMarshaller(new Marshaller(HelloRequestSerializer))
        .setResponseMarshaller(new Marshaller(HelloReplySerializer))
        .setSampledToLocalTracing(true)
        .build()
    
  }
}

Java

source
// Generated by Pekko gRPC. DO NOT EDIT.
package example.myapp.helloworld.grpc;

import org.apache.pekko.grpc.ProtobufSerializer;
import org.apache.pekko.grpc.javadsl.GoogleProtobufSerializer;

import org.apache.pekko.grpc.PekkoGrpcGenerated;

/**
 * //////////////////////////////////// The greeting service definition.
 */
public interface GreeterService {
  
  /**
   * ////////////////////
   * Sends a greeting //
   * //////&#42;****&#47;////////
   *      HELLO       //
   * //////&#42;****&#47;////////
   */
  java.util.concurrent.CompletionStage<example.myapp.helloworld.grpc.HelloReply> sayHello(example.myapp.helloworld.grpc.HelloRequest in);
  
  /**
   * Comment spanning
   * on several lines
   */
  java.util.concurrent.CompletionStage<example.myapp.helloworld.grpc.HelloReply> itKeepsTalking(org.apache.pekko.stream.javadsl.Source<example.myapp.helloworld.grpc.HelloRequest, org.apache.pekko.NotUsed> in);
  
  /**
   *  C style comments
   */
  org.apache.pekko.stream.javadsl.Source<example.myapp.helloworld.grpc.HelloReply, org.apache.pekko.NotUsed> itKeepsReplying(example.myapp.helloworld.grpc.HelloRequest in);
  
  /**
   * C style comments
   * on several lines
   * with non-empty heading/trailing line    */
  org.apache.pekko.stream.javadsl.Source<example.myapp.helloworld.grpc.HelloReply, org.apache.pekko.NotUsed> streamHellos(org.apache.pekko.stream.javadsl.Source<example.myapp.helloworld.grpc.HelloRequest, org.apache.pekko.NotUsed> in);
  

  static String name = "helloworld.GreeterService";
  static org.apache.pekko.grpc.ServiceDescription description = new org.apache.pekko.grpc.internal.ServiceDescriptionImpl(name, HelloWorldProto.getDescriptor());

  @PekkoGrpcGenerated
  public static class Serializers {
    
      public static ProtobufSerializer<example.myapp.helloworld.grpc.HelloRequest> HelloRequestSerializer = new GoogleProtobufSerializer<>(example.myapp.helloworld.grpc.HelloRequest.parser());
    
      public static ProtobufSerializer<example.myapp.helloworld.grpc.HelloReply> HelloReplySerializer = new GoogleProtobufSerializer<>(example.myapp.helloworld.grpc.HelloReply.parser());
    
  }
}

and model case classes for HelloRequest and HelloResponse.

The service interface is the same for the client and the server side. On the server side, the service implements the interface, on the client side the Pekko gRPC infrastructure implements a stub that will connect to the remote service when called.

There are 4 different types of calls:

unary call - single request that returns a FutureCompletionStage with a single response, see sayHello in above example
client streaming call - Source (stream) of requests from the client that returns a FutureCompletionStage with a single response, see itKeepsTalking in above example
server streaming call - single request that returns a Source (stream) of responses, see itKeepsReplying in above example
client and server streaming call - Source (stream) of requests from the client that returns a Source (stream) of responses, see streamHellos in above example

Writing a Client Program

Let’s use these 4 calls from a client. Start by generating code from the .proto definition with:

sbt

compile

Gradle

./gradlew build

Maven

mvn pekko-grpc:generate

A main program that calls the server with the GreeterService looks like this:

Scala

sourcepackage example.myapp.helloworld

import org.apache.pekko
import pekko.{ Done, NotUsed }
import pekko.actor.ActorSystem
import pekko.grpc.GrpcClientSettings
import pekko.stream.scaladsl.Source
import example.myapp.helloworld.grpc._

import scala.concurrent.{ ExecutionContext, Future }
import scala.concurrent.duration._
import scala.util.{ Failure, Success }

object GreeterClient {
  def main(args: Array[String]): Unit = {
    // Boot pekko
    implicit val sys: ActorSystem = ActorSystem("HelloWorldClient")
    implicit val ec: ExecutionContext = sys.dispatcher

    // Configure the client by code:
    val clientSettings = GrpcClientSettings.connectToServiceAt("127.0.0.1", 8080).withTls(false)

    // Or via application.conf:
    // val clientSettings = GrpcClientSettings.fromConfig(GreeterService.name)

    // Create a client-side stub for the service
    val client: GreeterService = GreeterServiceClient(clientSettings)

    // Run examples for each of the exposed service methods.
    runSingleRequestReplyExample()
    runStreamingRequestExample()
    runStreamingReplyExample()
    runStreamingRequestReplyExample()

    sys.scheduler.scheduleWithFixedDelay(1.second, 1.second) { () => runSingleRequestReplyExample() }

    def runSingleRequestReplyExample(): Unit = {
      sys.log.info("Performing request")
      val reply = client.sayHello(HelloRequest("Alice"))
      reply.onComplete {
        case Success(msg) =>
          println(s"got single reply: $msg")
        case Failure(e) =>
          println(s"Error sayHello: $e")
      }
    }

    def runStreamingRequestExample(): Unit = {
      val requests = List("Alice", "Bob", "Peter").map(HelloRequest(_))
      val reply = client.itKeepsTalking(Source(requests))
      reply.onComplete {
        case Success(msg) =>
          println(s"got single reply for streaming requests: $msg")
        case Failure(e) =>
          println(s"Error streamingRequest: $e")
      }
    }

    def runStreamingReplyExample(): Unit = {
      val responseStream = client.itKeepsReplying(HelloRequest("Alice"))
      val done: Future[Done] =
        responseStream.runForeach(reply => println(s"got streaming reply: ${reply.message}"))

      done.onComplete {
        case Success(_) =>
          println("streamingReply done")
        case Failure(e) =>
          println(s"Error streamingReply: $e")
      }
    }

    def runStreamingRequestReplyExample(): Unit = {
      val requestStream: Source[HelloRequest, NotUsed] =
        Source
          .tick(100.millis, 1.second, "tick")
          .zipWithIndex
          .map { case (_, i) => i }
          .map(i => HelloRequest(s"Alice-$i"))
          .take(10)
          .mapMaterializedValue(_ => NotUsed)

      val responseStream: Source[HelloReply, NotUsed] = client.streamHellos(requestStream)
      val done: Future[Done] =
        responseStream.runForeach(reply => println(s"got streaming reply: ${reply.message}"))

      done.onComplete {
        case Success(_) =>
          println("streamingRequestReply done")
        case Failure(e) =>
          println(s"Error streamingRequestReply: $e")
      }
    }
  }
}

Java

sourcepackage example.myapp.helloworld;

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;
import java.time.Duration;

import io.grpc.StatusRuntimeException;

import org.apache.pekko.Done;
import org.apache.pekko.NotUsed;
import org.apache.pekko.actor.ActorSystem;
import org.apache.pekko.stream.SystemMaterializer;
import org.apache.pekko.stream.Materializer;
import org.apache.pekko.stream.javadsl.Source;
import org.apache.pekko.grpc.GrpcClientSettings;

import example.myapp.helloworld.grpc.*;

class GreeterClient {
  public static void main(String[] args) throws Exception {

    String serverHost = "127.0.0.1";
    int serverPort = 8090;

    ActorSystem system = ActorSystem.create("HelloWorldClient");
    Materializer materializer = SystemMaterializer.get(system).materializer();

    // Configure the client by code:
    GrpcClientSettings settings = GrpcClientSettings.connectToServiceAt("127.0.0.1", 8090, system).withTls(false);

    // Or via application.conf:
    // GrpcClientSettings settings = GrpcClientSettings.fromConfig(GreeterService.name, system);

    GreeterServiceClient client = null;
    try {
      client = GreeterServiceClient.create(settings, system);

      singleRequestReply(client);
      streamingRequest(client);
      streamingReply(client, materializer);
      streamingRequestReply(client, materializer);


    } catch (StatusRuntimeException e) {
      System.out.println("Status: " + e.getStatus());
    } catch (Exception e)  {
      System.out.println(e);
    } finally {
      if (client != null) client.close();
      system.terminate();
    }

  }

  private static void singleRequestReply(GreeterService client) throws Exception {
    HelloRequest request = HelloRequest.newBuilder().setName("Alice").build();
    CompletionStage<HelloReply> reply = client.sayHello(request);
    System.out.println("got single reply: " + reply.toCompletableFuture().get(5, TimeUnit.SECONDS));
  }

  private static void streamingRequest(GreeterService client) throws Exception {
    List<HelloRequest> requests = Arrays.asList("Alice", "Bob", "Peter")
        .stream().map(name -> HelloRequest.newBuilder().setName(name).build())
        .collect(Collectors.toList());
    CompletionStage<HelloReply> reply = client.itKeepsTalking(Source.from(requests));
    System.out.println("got single reply for streaming requests: " +
        reply.toCompletableFuture().get(5, TimeUnit.SECONDS));
  }

  private static void streamingReply(GreeterService client, Materializer mat) throws Exception {
    HelloRequest request = HelloRequest.newBuilder().setName("Alice").build();
    Source<HelloReply, NotUsed> responseStream = client.itKeepsReplying(request);
    CompletionStage<Done> done =
      responseStream.runForeach(reply ->
        System.out.println("got streaming reply: " + reply.getMessage()), mat);

    done.toCompletableFuture().get(60, TimeUnit.SECONDS);
  }

  private static void streamingRequestReply(GreeterService client, Materializer mat) throws Exception {
    Duration interval = Duration.ofSeconds(1);
    Source<HelloRequest, NotUsed> requestStream = Source
      .tick(interval, interval, "tick")
      .zipWithIndex()
      .map(pair -> pair.second())
      .map(i -> HelloRequest.newBuilder().setName("Alice-" + i).build())
      .take(10)
      .mapMaterializedValue(m -> NotUsed.getInstance());

    Source<HelloReply, NotUsed> responseStream = client.streamHellos(requestStream);
    CompletionStage<Done> done =
      responseStream.runForeach(reply ->
        System.out.println("got streaming reply: " + reply.getMessage()), mat);

    done.toCompletableFuture().get(60, TimeUnit.SECONDS);
  }

}

You can run the example with

sbt

runMain io.grpc.examples.helloworld.GreeterClient

Gradle

./gradlew run

Maven

mvn pekko-grpc:generate compile exec:java -Dexec.mainClass=io.grpc.examples.helloworld.GreeterClient

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