== 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 client 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`):

[source,xml,subs="+attributes"]
----
<dependency>
 <groupId>io.vertx</groupId>
 <artifactId>vertx-grpc-client</artifactId>
 <version>4.3.1</version>
</dependency>
----

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

[source,groovy,subs="+attributes"]
----
dependencies {
 compile 'io.vertx:vertx-grpc-client:4.3.1'
}
----

=== 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

[source,java]
----
GrpcClient client = GrpcClient.client(vertx);
----

==== Request/response

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

[source,java]
----
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 ``link:../../apidocs/io/vertx/grpc/client/GrpcClientRequest.html#response--[response]`` holds the response and the
``link:../../apidocs/io/vertx/grpc/common/GrpcReadStream.html#last--[last]`` holds the result

[source,java]
----
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

[source,java]
----
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 ``link:../../apidocs/io/vertx/core/streams/WriteStream.html#write-java.lang.Object-[write]`` for each element of the stream
and using ``link:../../apidocs/io/vertx/core/streams/WriteStream.html#end--[end]`` to end the stream

[source,java]
----
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

[source,java]
----
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

[source,java]
----
if (request.writeQueueFull()) {
  request.drainHandler(v -> {
    // Writable again
  });
} else {
  request.write(item);
}
----

You can pause/resume/fetch a response

[source,java]
----
response.pause();

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

=== Cancellation

You can call ``link:../../apidocs/io/vertx/grpc/common/GrpcWriteStream.html#cancel--[cancel]`` to cancel a request

[source,java]
----
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
    });
  });
----

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

[source,java]
----
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

[source,java]
----
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.

[source,java]
----
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 ``link:../../apidocs/io/vertx/grpc/common/GrpcMessage.html[GrpcMessage]``, such messages preserve the server encoding.

[source,java]
----
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 ``link:../../apidocs/io/vertx/grpc/common/GrpcWriteStream.html#writeMessage-io.vertx.grpc.common.GrpcMessage-[writeMessage]`` and ``link:../../apidocs/io/vertx/grpc/common/GrpcWriteStream.html#endMessage-io.vertx.grpc.common.GrpcMessage-[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