# 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 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 several parts:

- Vert.x gRPC Server

- Vert.x gRPC Client

- Vert.x gRPC/IO Server

- Vert.x gRPC/IO Client

- Vert.x gRPC/IO Context Storage

## Vert.x gRPC Protoc Plugin

The easiest way to start using Vert.x 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>${project.version}</version>
           <mainClass>io.vertx.grpc.plugin.VertxGrpcGenerator</mainClass>
         </protocPlugin>
       </protocPlugins>
     </configuration>
     <executions>
       <execution>
         <id>compile</id>
         <configuration>
           <outputDirectory>${project.basedir}/src/main/java</outputDirectory>
           <clearOutputDirectory>false</clearOutputDirectory>
         </configuration>
         <goals>
           <goal>compile</goal>
           <goal>compile-custom</goal>
         </goals>
       </execution>
     </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.

### Generated RPC files

For each service definition, the plugin creates two Java RPC files.

For the `Greeter` service:

- `examples/GreeterGrpcClient.java`

- `examples/GreeterGrpcService.java`

Besides the usual client/server generated code, these files contains service method constants:

    public class GreeterGrpcClient {
     public static final ServiceMethod<examples.HelloReply, examples.HelloRequest> SayHello = ServiceMethod.client(
       ServiceName.create("helloworld", "Greeter"),
       "SayHello",
       GrpcMessageEncoder.encoder(),
       GrpcMessageDecoder.decoder(examples.HelloReply.parser())
     );
     // ...
    }

    public class GreeterGrpcServer {
     public static final ServiceMethod<examples.HelloRequest, examples.HelloReply> SayHello = ServiceMethod.server(
       ServiceName.create("helloworld", "Greeter"),
       "SayHello",
       GrpcMessageEncoder.encoder(),
       GrpcMessageDecoder.decoder(examples.HelloRequest.parser())
     );
     // ...
    }

For each service method, a public static final `ServiceMethod` is generated, these constants provide everything Vert.x gRPC needs to know to interact with gRPC.

- the service name: `/helloworld.Greeter`

- the service method name: `SayHello`

- the message decoder

- the message encoder

They can be used to bind services or interact with a remote server.

## Vert.x gRPC Server

Vert.x gRPC Server is a 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 generated stub approach with the Vert.x gRPC Generator.

### 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>4.5.15</version>
    </dependency>

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

    dependencies {
     compile 'io.vertx:vertx-grpc-server:4.5.15'
    }

### Creating a gRPC server

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 used within a Vert.x Web router:
    route.consumes("application/grpc").handler(rc -> grpcServer.handle(rc.request()));

### Server request/response API

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

#### Request/response

Each service method is processed by a handler, the handler is bound using a `ServiceMethod`.

    ServiceMethod<HelloRequest, HelloReply> serviceMethod = VertxGreeterGrpcServer.SayHello;

    server.callHandler(serviceMethod, request -> {

      request.handler(hello -> {

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

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

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

`ServiceMethod` constants are generated by the Vert.x gRPC protoc plugin.

#### Streaming request

You can set handlers to process request events

    server.callHandler(VertxStreamingGrpcServer.Sink, 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(VertxStreamingGrpcServer.Source, 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(VertxStreamingGrpcServer.Pipe, request -> {

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

Note | The gRPC-Web protocol does not support bidirectional streaming.

### 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());

Note | Compression is not supported over the gRPC-Web protocol.

### Decompression

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

Note | Decompression is not supported over the gRPC-Web protocol.

### 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

### Server stub API

In addition to the request/response API, the Vert.x gRPC protoc plugin idiomatic service stubs.

Each service comes in two flavors, you can override the method you like depending on the style.

#### Unary services

Unary services can return a Vert.x `Future`:

    VertxGreeterGrpcServer.GreeterApi stub = new VertxGreeterGrpcServer.GreeterApi() {
      @Override
      public Future<HelloReply> sayHello(HelloRequest request) {
        return Future.succeededFuture(HelloReply.newBuilder().setMessage("Hello " + request.getName()).build());
      }
    };

or process a Vert.x `Promise`

    VertxGreeterGrpcServer.GreeterApi stub = new VertxGreeterGrpcServer.GreeterApi() {
      @Override
      public void sayHello(HelloRequest request, Promise<HelloReply> response) {
        response.complete(HelloReply.newBuilder().setMessage("Hello " + request.getName()).build());
      }
    };

In both case you need to bind the stub to an existing `GrpcServer`:

    stub.bindAll(server);

#### Streaming requests

Streaming requests are implemented with a `ReadStream`:

    VertxStreamingGrpcServer.StreamingApi stub = new VertxStreamingGrpcServer.StreamingApi() {
      @Override
      public void sink(ReadStream<Item> stream, Promise<Empty> response) {
        stream.handler(item -> {
          System.out.println("Process item " + item.getValue());
        });
        // Send response
        stream.endHandler(v -> response.complete(Empty.getDefaultInstance()));
      }
    };

#### Streaming responses

Streaming responses are implemented with Vert.x streams and comes in two flavors.

You can return a Vert.x `ReadStream` and let the service send it for you:

    VertxStreamingGrpcServer.StreamingApi stub = new VertxStreamingGrpcServer.StreamingApi() {
      @Override
      public ReadStream<Item> source(Empty request) {
        return streamOfItems();
      }
    };

or you can process a `WriteStream`:

    VertxStreamingGrpcServer.StreamingApi stub = new VertxStreamingGrpcServer.StreamingApi() {
      @Override
      public void source(Empty request, WriteStream<Item> response) {
        response.write(Item.newBuilder().setValue("value-1").build());
        response.end(Item.newBuilder().setValue("value-2").build());
      }
    };

## Vert.x gRPC Client

Vert.x gRPC Client is a gRPC client powered by Vert.x HTTP client.

This client provides a gRPC request/response oriented API as well as a 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>4.5.15</version>
    </dependency>

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

    dependencies {
     compile 'io.vertx:vertx-grpc-client:4.5.15'
    }

### Creating a gRPC client

You can easily create the gRPC client

    GrpcClient client = GrpcClient.client(vertx);

### Client request/response API

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

#### Request/response

Interacting with a gRPC server involves creating a request to the remote gRPC service

    SocketAddress server = SocketAddress.inetSocketAddress(443, "example.com");
    ServiceMethod<HelloReply, HelloRequest> sayHelloMethod = VertxGreeterGrpcClient.SayHello;
    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());
    });

`ServiceMethod` constants are generated by the Vert.x gRPC protoc plugin.

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, VertxGreeterGrpcClient.SayHello).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

Streaming requests involve calling `write` for each element of the stream and using `end` to end the stream

    client
      .request(server, VertxStreamingGrpcClient.Sink)
      .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 of a streaming response

    client
      .request(server, VertxStreamingGrpcClient.Source)
      .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 achieved transparently by the client when the server sends encoded responses.

### 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

### Client stub API

In addition to the request/response API, the Vert.x gRPC protoc plugin idiomatic service clients.

A client wraps a `GrpcClient` and provides Vert.x idiomatic API to interact with the service:

    VertxGreeterGrpcClient client = new VertxGreeterGrpcClient(grpcClient, SocketAddress.inetSocketAddress(port, host));

#### Unary services

Unary services returns a Vert.x `Future`

    Future<HelloReply> response = client.sayHello(HelloRequest.newBuilder().setName("John").build());

    response.onSuccess(result -> System.out.println("Service responded: " + response.result().getMessage()));

    response.onFailure(err -> System.out.println("Service failure: " + response.cause().getMessage()));

#### Streaming requests

Streaming requests use a lambda passed a Vert.x `WriteStream` of messages sent to the service

    Future<Empty> response = client.sink(stream -> {
      stream.write(Item.newBuilder().setValue("Value 1").build());
      stream.write(Item.newBuilder().setValue("Value 2").build());
      stream.end(Item.newBuilder().setValue("Value 3").build());
    });

#### Streaming responses

Streaming responses get a Vert.x `ReadStream` of messages sent by the service

    Future<ReadStream<Item>> response = client.source(Empty.getDefaultInstance());

    response.onSuccess(stream -> stream
      .handler(item -> System.out.println("Item " + item.getValue()))
      .exceptionHandler(err -> System.out.println("Stream failed " + err.getMessage()))
      .endHandler(v -> System.out.println("Stream ended")));

    response.onFailure(err -> System.out.println("Service failure: " + err.getMessage()));

## Vert.x gRPC/IO Server

Vert.x gRPC/IO Server extends the Vert.x gRPC server with grpc-java integration.

This server provides compatibility with the grpc-java generated stub approach with a service bridge.

### Using Vert.x gRPC/IO Server

To use Vert.x gRPC/IO 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-grpcio-server</artifactId>
     <version>4.5.15</version>
    </dependency>

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

    dependencies {
     compile 'io.vertx:vertx-grpcio-server:4.5.15'
    }

### Service bridge

The Vert.x gRPC Server can bridge a gRPC service to use with grpc-java generated server classes.

    GrpcIoServer grpcServer = GrpcIoServer.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
    GrpcIoServiceBridge serverStub = GrpcIoServiceBridge.bridge(service);
    serverStub.bind(grpcServer);

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

The bridge supports deadline automatic cancellation: when a gRPC request carrying a timeout is received, a deadline is associated with the `io.grpc.Context` an can be obtained from the current context. This deadline automatically cancels the request in progress when its associated timeout fires.

## Vert.x gRPC/IO Client

Vert.x gRPC/IO Client extends the Vert.x gRPC client with grpc-java integration.

This client provides a generated stub approach with a gRPC Channel

### Using Vert.x gRPC/IO Client

To use Vert.x gRPC/IO 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-grpcio-client</artifactId>
     <version>4.5.15</version>
    </dependency>

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

    dependencies {
     compile 'io.vertx:vertx-grpcio-client:4.5.15'
    }

### gRPC channel

The Vert.x gRPC/IO Client provides a gRPC channel to use with grpc-java generated client classes.

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

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

    StreamObserver<HelloReply> observer = 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
      }
    };

    greeter.sayHello(HelloRequest.newBuilder().setName("Bob").build(), observer);

## 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>4.5.15</version>
    </dependency>

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

    dependencies {
     compile 'io.vertx:vertx-grpc-context-storage:4.5.15'
    }