# Vert.x gRPC

The best description of gRPC can be seen at wikipedia.

> gRPC is an open source remote procedure call (RPC) system initially developed at Google. It uses HTTP/2 for transport, Protocol Buffers as the interface description language, and provides features such as authentication, bidirectional streaming and flow control, blocking or nonblocking bindings, and cancellation and timeouts. It generates cross-platform client and server bindings for many languages.

— wikipedia
wikipedia

Vert.x gRPC is a module that will align the programming style of Google gRPC with Vert.x style. As a user of this module you will be more familiar with the code style using Vert.x Streams and Futures while benefiting from all the benefits of gRPC.

For more information related to gRPC please consult the official documentation site http://www.grpc.io/.

Warning | Since Vert.x 4.3, this module is the new support for gRPC in the Vert.x stack, the previous implementation based on gRPC Netty is still available and has been renamed Vert.x gRPC Netty, it can be found at https://vertx.io/docs/vertx-grpc-netty/java/ . This module has Tech Preview status, this means the API can change between versions.

Vert.x gRPC is split into three parts:

- Vert.x gRPC Server

- Vert.x gRPC Client

- Vert.x gRPC Context Storage

## Vert.x gRPC Server

Vert.x gRPC Server is a new gRPC server powered by Vert.x HTTP server superseding the integrated Netty based gRPC client.

This server provides a gRPC request/response oriented API as well as a the generated stub approach with a service bridge.

### Using Vert.x gRPC Server

To use Vert.x gRPC Server, add the following dependency to the dependencies section of your build descriptor:

- Maven (in your `pom.xml`):

    <dependency>
     <groupId>io.vertx</groupId>
     <artifactId>vertx-grpc-server</artifactId>
     <version>4.4.3</version>
    </dependency>

- Gradle (in your `build.gradle` file):

    dependencies {
     compile 'io.vertx:vertx-grpc-server:4.4.3'
    }

### gRPC request/response server API

The gRPC request/response server API provides an alternative way to interact with a client without the need of a generated stub.

A `GrpcServer` is a `Handler<HttpServerRequest>` and can be used as an HTTP server request handler.

    GrpcServer grpcServer = GrpcServer.server(vertx);

    HttpServer server = vertx.createHttpServer(options);

    server
      .requestHandler(grpcServer)
      .listen();

Tip | a `GrpcServer` can be mounted in a Vert.x Web router

#### Request/response

Each service method is processed by a handler

    server.callHandler(GreeterGrpc.getSayHelloMethod(), request -> {

      request.handler(hello -> {

        GrpcServerResponse<HelloRequest, HelloReply> response = request.response();

        HelloReply reply = HelloReply.newBuilder().setMessage("Hello " + hello.getName()).build();

        response.end(reply);
      });
    });

#### Streaming request

You can set handlers to process request events

    server.callHandler(StreamingGrpc.getSinkMethod(), request -> {
      request.handler(item -> {
        // Process item
      });
      request.endHandler(v ->{
        // No more items
        // Send the response
        request.response().end(Empty.getDefaultInstance());
      });
      request.exceptionHandler(err -> {
        // Something wrong happened
      });
    });

#### Streaming response

A streaming response involves calling `write` for each element of the stream and using `end` to end the stream

    server.callHandler(StreamingGrpc.getSourceMethod(), request -> {
      GrpcServerResponse<Empty, Item> response = request.response();
      request.handler(empty -> {
        for (int i = 0;i < 10;i++) {
          response.write(Item.newBuilder().setValue("1").build());
        }
        response.end();
      });
    });

#### Bidi request/response

A bidi request/response is simply the combination of a streaming request and a streaming response

    server.callHandler(StreamingGrpc.getPipeMethod(), request -> {

      request.handler(item -> request.response().write(item));
      request.endHandler(v -> request.response().end());
    });

### Flow control

Request and response are back pressured Vert.x streams.

You can pause/resume/fetch a request

    request.pause();

    performAsyncOperation().onComplete(ar -> {
      // And then resume
      request.resume();
    });

You can check the writability of a response and set a drain handler

    if (response.writeQueueFull()) {
      response.drainHandler(v -> {
        // Writable again
      });
    } else {
      response.write(item);
    }

### Compression

You can compress response messages by setting the response encoding prior before sending any message

    response.encoding("gzip");

    // Write items after encoding has been defined
    response.write(Item.newBuilder().setValue("item-1").build());
    response.write(Item.newBuilder().setValue("item-2").build());
    response.write(Item.newBuilder().setValue("item-3").build());

### Decompression

Decompression is done transparently by the server when the client send encoded requests.

### Stub API

The Vert.x gRPC Server can bridge a gRPC service to use with a generated server stub in a more traditional fashion

    GrpcServer grpcServer = GrpcServer.server(vertx);

    GreeterGrpc.GreeterImplBase service = new GreeterGrpc.GreeterImplBase() {
      @Override
      public void sayHello(HelloRequest request, StreamObserver<HelloReply> responseObserver) {
        responseObserver.onNext(HelloReply.newBuilder().setMessage("Hello " + request.getName()).build());
        responseObserver.onCompleted();
      }
    };

    // Bind the service bridge in the gRPC server
    GrpcServiceBridge serverStub = GrpcServiceBridge.bridge(service);
    serverStub.bind(grpcServer);

    // Start the HTTP/2 server
    vertx.createHttpServer(options)
      .requestHandler(grpcServer)
      .listen();

### Message level API

The server provides a message level API to interact directly with protobuf encoded gRPC messages.

Tip | the server message level API can be used with the client message level API to write a gRPC reverse proxy

Such API is useful when you are not interested in the content of the messages, and instead you want to forward them to another service, e.g. you are writing a proxy.

    ServiceName greeterServiceName = ServiceName.create("helloworld", "Greeter");

    server.callHandler(request -> {

      if (request.serviceName().equals(greeterServiceName) && request.methodName().equals("SayHello")) {

        request.handler(protoHello -> {
          // Handle protobuf encoded hello
          performAsyncOperation(protoHello)
            .onSuccess(protoReply -> {
              // Reply with protobuf encoded reply
              request.response().end(protoReply);
            }).onFailure(err -> {
              request.response()
                .status(GrpcStatus.ABORTED)
                .end();
            });
        });
      } else {
        request.response()
          .status(GrpcStatus.NOT_FOUND)
          .end();
      }
    });

You can also set a `messageHandler` to handle `GrpcMessage`, such messages preserve the client encoding, which is useful the service you are forwarding to can handle compressed messages directly, in this case the message does not need to be decompressed and compressed again.

    ServiceName greeterServiceName = ServiceName.create("helloworld", "Greeter");

    server.callHandler(request -> {

      if (request.serviceName().equals(greeterServiceName) && request.methodName().equals("SayHello")) {

        request.messageHandler(helloMessage -> {

          // Can be identity or gzip
          String helloEncoding = helloMessage.encoding();

          // Handle hello message
          handleGrpcMessage(helloMessage)
            .onSuccess(replyMessage -> {
              // Reply with reply message

              // Can be identity or gzip
              String replyEncoding = replyMessage.encoding();

              // Send the reply
              request.response().endMessage(replyMessage);
            }).onFailure(err -> {
              request.response()
                .status(GrpcStatus.ABORTED)
                .end();
            });
        });
      } else {
        request.response()
          .status(GrpcStatus.NOT_FOUND)
          .end();
      }
    });

The `writeMessage` and `endMessage` will handle the message encoding:

- when the message uses the response encoding, the message is sent as is

- when the message uses a different encoding, it will be encoded, e.g. compressed or uncompressed

## Vert.x gRPC Client

Vert.x gRPC Client is a new gRPC client powered by Vert.x HTTP client superseding the integrated Netty based gRPC client.

This client provides a gRPC request/response oriented API as well as a the generated stub approach with a gRPC Channel

### Using Vert.x gRPC Client

To use Vert.x gRPC Client, add the following dependency to the dependencies section of your build descriptor:

- Maven (in your `pom.xml`):

    <dependency>
     <groupId>io.vertx</groupId>
     <artifactId>vertx-grpc-client</artifactId>
     <version>4.4.3</version>
    </dependency>

- Gradle (in your `build.gradle` file):

    dependencies {
     compile 'io.vertx:vertx-grpc-client:4.4.3'
    }

### gRPC request/response client API

The gRPC request/response client API provides an alternative way to interact with a server without the need of a generated stub.

You can easily create the gRPC client

    GrpcClient client = GrpcClient.client(vertx);

#### Request/response

Any interaction with a gRPC server involves creating a request to the remote gRPC service

    SocketAddress server = SocketAddress.inetSocketAddress(443, "example.com");
    MethodDescriptor<HelloRequest, HelloReply> sayHelloMethod = GreeterGrpc.getSayHelloMethod();
    Future<GrpcClientRequest<HelloRequest, HelloReply>> fut = client.request(server, sayHelloMethod);
    fut.onSuccess(request -> {
      // The end method calls the service
      request.end(HelloRequest.newBuilder().setName("Bob").build());
    });

The `response` holds the response and the `last` holds the result

    request.response().onSuccess(response -> {
      Future<HelloReply> fut = response.last();
      fut.onSuccess(reply -> {
        System.out.println("Received " + reply.getMessage());
      });
    });

Future composition can combine all the previous steps together in a compact fashion

    client
      .request(server, GreeterGrpc.getSayHelloMethod()).compose(request -> {
        request.end(HelloRequest
          .newBuilder()
          .setName("Bob")
          .build());
        return request.response().compose(response -> response.last());
      }).onSuccess(reply -> {
        System.out.println("Received " + reply.getMessage());
      });

#### Streaming request

A streaming request involves calling `write` for each element of the stream and using `end` to end the stream

    client
      .request(server, StreamingGrpc.getSinkMethod())
      .onSuccess(request -> {
      for (int i = 0;i < 10;i++) {
        request.write(Item.newBuilder().setValue("1").build());
      }
      request.end();
    });

#### Streaming response

You can set handlers to process response events

    client
      .request(server, StreamingGrpc.getSourceMethod())
      .compose(request -> {
        request.end(Empty.getDefaultInstance());
        return request.response();
      })
      .onSuccess(response -> {
        response.handler(item -> {
          // Process item
        });
        response.endHandler(v -> {
          // Done
        });
        response.exceptionHandler(err -> {
          // Something went bad
        });
      });

#### Bidi request/response

A bidi request/response is simply the combination of a streaming request and a streaming response.

### Flow control

Request and response are back pressured Vert.x streams.

You can check the writability of a request and set a drain handler

    if (request.writeQueueFull()) {
      request.drainHandler(v -> {
        // Writable again
      });
    } else {
      request.write(item);
    }

You can pause/resume/fetch a response

    response.pause();

    performAsyncOperation().onComplete(ar -> {
      // And then resume
      response.resume();
    });

### Cancellation

You can call `cancel` to cancel a request

    request.cancel();

Note | cancellation sends an HTTP/2 reset frame to the server

### Compression

You can compress request messages by setting the request encoding prior before sending any message

    request.encoding("gzip");

    // Write items after encoding has been defined
    request.write(Item.newBuilder().setValue("item-1").build());
    request.write(Item.newBuilder().setValue("item-2").build());
    request.write(Item.newBuilder().setValue("item-3").build());

### Decompression

Decompression is done transparently by the client when the server send encoded responses.

### Stub API

The Vert.x gRPC Client provides a gRPC channel to use with a generated client stub in a more traditional fashion

    GrpcClientChannel channel = new GrpcClientChannel(client, SocketAddress.inetSocketAddress(443, "example.com"));

    GreeterGrpc.GreeterStub greeter = GreeterGrpc.newStub(channel);

    greeter.sayHello(HelloRequest.newBuilder().setName("Bob").build(), new StreamObserver<HelloReply>() {
      @Override
      public void onNext(HelloReply value) {
        // Process response
      }
      @Override
      public void onCompleted() {
        // Done
      }
      @Override
      public void onError(Throwable t) {
        // Something went bad
      }
    });

### Message level API

The client provides a message level API to interact directly with protobuf encoded gRPC messages.

Tip | the client message level API can be used with the server message level API to write a gRPC reverse proxy

Such API is useful when you are not interested in the content of the messages, and instead you want to forward them to another service, e.g. you are writing a proxy.

    Future<GrpcClientRequest<Buffer, Buffer>> requestFut = client.request(server);

    requestFut.onSuccess(request -> {

      // Set the service name and the method to call
      request.serviceName(ServiceName.create("helloworld", "Greeter"));
      request.methodName("SayHello");

      // Send the protobuf request
      request.end(protoHello);

      // Handle the response
      Future<GrpcClientResponse<Buffer, Buffer>> responseFut = request.response();
      responseFut.onSuccess(response -> {
        response.handler(protoReply -> {
          // Handle the protobuf reply
        });
      });
    });

You can also set a `messageHandler` to handle `GrpcMessage`, such messages preserve the server encoding.

    Future<GrpcClientRequest<Buffer, Buffer>> requestFut = client.request(server);

    requestFut.onSuccess(request -> {

      // Set the service name and the method to call
      request.serviceName(ServiceName.create("helloworld", "Greeter"));
      request.methodName("SayHello");

      // Send the protobuf request
      request.endMessage(GrpcMessage.message("identity", protoHello));

      // Handle the response
      Future<GrpcClientResponse<Buffer, Buffer>> responseFut = request.response();
      responseFut.onSuccess(response -> {
        response.messageHandler(replyMessage -> {
          System.out.println("Got reply message encoded as " + replyMessage.encoding());
        });
      });
    });

The `writeMessage` and `endMessage` will handle the message encoding:

- when the message uses the response encoding, the message is sent as is

- when the message uses a different encoding, it will be encoded, e.g. compressed or uncompressed

## Vert.x gRPC Protoc Plugin

The easiest way to start using vertx-grpc is to utilize its built-in code generator plugin. To do so, one must define the protocol in the `protobuffer` format as required by gRPC.

    syntax = "proto3";

    option java_multiple_files = true;
    option java_package = "examples";
    option java_outer_classname = "HelloWorldProto";
    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;
    }

This is a very simple example showing the single request, single response mode.

### Compile the RPC definition

Using the definition above we need to compile it.

You can compile the proto file using the `protoc` compiler if you like or you can integrate it in your build.

If you’re using Apache Maven you need to add the plugin:

    <plugin>
     <groupId>org.xolstice.maven.plugins</groupId>
     <artifactId>protobuf-maven-plugin</artifactId>
     <version>0.6.1</version>
     <configuration>
       <protocArtifact>com.google.protobuf:protoc:${protoc.version}:exe:${os.detected.classifier}</protocArtifact>
       <pluginId>grpc-java</pluginId>
       <pluginArtifact>io.grpc:protoc-gen-grpc-java:${grpc.version}:exe:${os.detected.classifier}</pluginArtifact>
       <protocPlugins>
         <protocPlugin>
           <id>vertx-grpc-protoc-plugin2</id>
           <groupId>io.vertx</groupId>
           <artifactId>vertx-grpc-protoc-plugin2</artifactId>
           <version>${stack.version}</version>
           <mainClass>io.vertx.grpc.plugin.VertxGrpcGenerator</mainClass>
         </protocPlugin>
       </protocPlugins>
     </configuration>
     <executions>
       <execution>
         <id>compile</id>
         <configuration>
           <outputDirectory>${project.basedir}/src/main/java</outputDirectory>
           <clearOutputDirectory>false</clearOutputDirectory>
         </configuration>
         <goals>
           <goal>compile</goal>
           <goal>compile-custom</goal>
         </goals>
       </execution>
       <execution>
         <id>test-compile</id>
         <goals>
           <goal>test-compile</goal>
           <goal>test-compile-custom</goal>
         </goals>
       </execution>
     </executions>
    </plugin>

The `${os.detected.classifier}` property is used to make the build OS independant, on OSX it is replaced by osx-x86_64 and so on. To use it you need to add the os-maven-plugin[https://github.com/trustin/os-maven-plugin] in the `build` section of your `pom.xml`:

    <build>
     ...
     <extensions>
       <extension>
         <groupId>kr.motd.maven</groupId>
         <artifactId>os-maven-plugin</artifactId>
         <version>1.4.1.Final</version>
       </extension>
     </extensions>
     ...
    </build>

This plugin will compile your proto files under `src/main/proto` and make them available to your project.

If you’re using Gradle you need to add the plugin:

    ...
    apply plugin: 'com.google.protobuf'
    ...
    buildscript {
     ...
     dependencies {
       // ASSUMES GRADLE 2.12 OR HIGHER. Use plugin version 0.7.5 with earlier gradle versions
       classpath 'com.google.protobuf:protobuf-gradle-plugin:0.8.0'
     }
    }
    ...
    protobuf {
     protoc {
       artifact = 'com.google.protobuf:protoc:3.2.0'
     }
     plugins {
       grpc {
         artifact = "io.grpc:protoc-gen-grpc-java:1.25.0"
       }
       vertx {
         artifact = "io.vertx:vertx-grpc-protoc-plugin2:${vertx.grpc.version}"
       }
     }
     generateProtoTasks {
       all()*.plugins {
         grpc
         vertx
       }
     }
    }

This plugin will compile your proto files under `build/generated/source/proto/main` and make them available to your project.

## Vert.x gRPC Context Storage

Vert.x gRPC Context Storage overrides the default `io.grpc.Context.Storage` implementation.

The default implementation always stores the gRPC context in a thread-local variable. This implementation stores the gRPC context the same way as Vert.x core stores request tracing data.

This means, for example, that when you implement a service method, the gRPC context is propagated across Vert.x async API calls:

    Context grpcCtx1 = Context.current();

    vertx.<String>executeBlocking(prom -> {

      // Same as grpcCtx1
      Context grpcCtx2 = Context.current();

      String result = doSomething();
      prom.complete(result);

    }, ar -> {

      // Same as grpcCtx1 and grpcCtx2
      Context grpcCtx3 = Context.current();

    });

Caution | The gRPC context is propagated across Vert.x async API calls only when the current Vert.x `Context` is bound to a Vert.x HTTP server request. It is not propagated if, for example, you invoke a stub on a non-Vert.x thread or from a verticle `start` method.

### Using Vert.x gRPC Context Storage

To use Vert.x gRPC Context Storage, add the following dependency to the dependencies section of your build descriptor:

- Maven (in your `pom.xml`):

    <dependency>
     <groupId>io.vertx</groupId>
     <artifactId>vertx-grpc-context-storage</artifactId>
     <version>4.4.3</version>
    </dependency>

- Gradle (in your `build.gradle` file):

    dependencies {
     compile 'io.vertx:vertx-grpc-context-storage:4.4.3'
    }