= Vert.x gRPC

The best description of gRPC can be seen at wikipedia.

[quote, wikipedia, 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.
____

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/.

In addition Vert.x gRPC supports

* gRPC service scaling with Verticles
* non-blocking native transports

== gRPC types

With gRPC you benefit from HTTP/2 which means that you will have asynchronous streaming support which means that your
Remote Procedure Calls can have the following characteristics:

* Client streams request objects while Server replies with a single response object
* Client streams request objects while Server replies with a stream of response objects
* Client sends a single request object while Server replies with a single response object
* Client sends a single request object while Server replies with a stream of response objects

While to the untrained eye this might not look to different from other HTTP based RPC approaches you should be aware
that with HTTP/2 your requests do not need to complete before the responses start to arrive. This means that your
communication channel is full duplex. Being full duplex allows you to reduce the response latency and make more
response application.

== A simple Hello World

In order to start with your first hello world example, one needs to define the protocol. gRPC requires you to define
this protocol using the `protobuffer` format.

[source,proto]
----
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 https://github.com/google/protobuf/tree/master/java#installation---without-maven[like]
or you can integrate it in your build.

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

[source,xml]
----
 <plugin>
  <groupId>org.xolstice.maven.plugins</groupId>
  <artifactId>protobuf-maven-plugin</artifactId>
  <version>0.6.1</version>
  <configuration>
    <protocArtifact>com.google.protobuf:protoc:3.2.0:exe:${os.detected.classifier}</protocArtifact>
    <pluginId>grpc-java</pluginId>
    <pluginArtifact>io.grpc:protoc-gen-grpc-java:${vertx.grpc.version}:exe:${os.detected.classifier}</pluginArtifact>
    <protocPlugins>
      <protocPlugin>
        <id>vertx-grpc-protoc-plugin</id>
        <groupId>io.vertx</groupId>
        <artifactId>vertx-grpc-protoc-plugin</artifactId>
        <version>${stack.version}</version>
        <mainClass>io.vertx.grpc.protoc.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`:

[source,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:

[source,groovy]
----
...
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-plugin:${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.

=== gRPC Server

Now you should have your RPC base code setup it is time to implement your server. As you should recall from above we
described that our server should implement a `sayHello` method that receives a `HelloRequest` objects and returns a
`HelloReply` object. So you can implement it as:

[source,$lang]
----
{@link examples.Examples#simpleServer}
----

Once you're happy with it you can then make your service available on a server. Vert.x makes the creation of a server
quite simple all you need to add is:

[source,$lang]
----
{@link examples.Examples#startServer}
----

==== Using Vert.x future and streams

The previous example was using a gRPC server processing asynchronously using gRPC asynchronous constructs such
as `io.grpc.stub.StreamObserver`. This code is generated by the protoc compiler.

The plugin configuration above configures the following plugin:

```xml
<protocPlugin>
  <id>vertx-grpc-protoc-plugin</id>
  <groupId>io.vertx</groupId>
  <artifactId>vertx-grpc-protoc-plugin</artifactId>
  <version>${stack.version}</version>
  <mainClass>io.vertx.grpc.protoc.plugin.VertxGrpcGenerator</mainClass>
</protocPlugin>
```

This generates a service version that uses Vert.x asynchronous constructs such as `Future` or `ReadStream` or `WriteStream`
which can be more convenient in the Vert.x ecosystem.

[source,$lang]
----
{@link examples.Examples#vertxSimpleServer}
----

==== Server gzip compression

You can enable gzip compression to tell the server to send compressed responses (compressed requests
are automatically handled by the server).

[source,$lang]
----
{@link examples.Examples#vertxServerWithCompression}
----

The `withCompression` configuration is generated by the Vert.x gRPC protoc plugin. You can also enable compression
on default services by casting the `ResponseObserver` to `ServerCallStreamObserver` and call `setCompression` before
sending the response.

[source,$lang]
----
{@link examples.Examples#serverWithCompression}
----

NOTE: you can use other compressors as long as the server support them and they are registered against the compressor
registry when building the `ManagedChannel`

==== SSL configuration

The previous example was simple but your RPC is not secure. In order to make it secure we should enable SSL/TLS:

[source,$lang]
----
{@link examples.Examples#sslServer}
----

Congratulations you just completed your first gRPC server.

IMPORTANT: since gRPC uses HTTP/2 transport, SSL/TLS setup requires the
https://wikipedia.org/wiki/Application-Layer_Protocol_Negotiation[Application-Layer Protocol Negotiation]
in your server

==== Server scaling

When you deploy several instances of the same verticles, the gRPC server will be scaled
on the verticle event-loops.

[source,$lang]
----
{@link examples.Examples#serverScaling}
----

==== BlockingServerInterceptor

gRPC https://grpc.io/grpc-java/javadoc/io/grpc/ServerInterceptor.html[ServerInterceptor] is a mechanism
for intercepting incoming calls before they are sent to the service.
It has synchronous behavior and will be execute on the Vert.x event loop.

[source,$lang]
----
{@link examples.Examples#nonblockingInterceptorUsage}
----

Suppose we have an interceptor that does something blocking the event loop:

[source,$lang]
----
{@link examples.Examples#blockingInterceptor}
----

To avoid the blocking one should wrap the interceptor. Then it will be called on the Vert.x worker thread.

[source,$lang]
----
{@link examples.Examples#blockingInterceptorUsage}
----

==== Context Server Interceptor

An abstract context server interceptor is available to allow intercepting server calls and extract metadata into the
vert.x context. This context does not rely on thread locals so it is safe to use on vert.x APIs. This interceptor should
be the first (or one of the firsts to be added to the interceptors list).

A typical example is the use of a session id. A client can create a client interceptor that sets a session id in all
connections as:

[source,$lang]
----
{@link examples.Examples#clientSessionInterceptorUsage}
----

And then on the server side an interceptor can be added as:

[source,$lang]
----
{@link examples.Examples#serverSessionInterceptorUsage}
----

=== gRPC Client

A server without a client is of no use, so let's create a client. In order to do this some steps overlap with the
server. First we need to have the RPC definition, which should already done otherwise there would be no server and
the same definition should have been compiled.

Note that the compiler will always generate both the base server and a client stub so if you already compiled once
you do not need to re-compile it again.

Every client stub will always require a communication channel to a server so first we need to create a gRPC channel:

[source,$lang]
----
{@link examples.Examples#connectClient}
----

Once the stub is created we can communicate with our server, this time it is easier since the stub already provides
the correct method definition and parameter types:

[source,$lang]
----
{@link examples.Examples#simpleClient}
----

==== Using Vert.x future and streams

The previous example was using a gRPC client processing asynchronously using gRPC asynchronous constructs such
as `io.grpc.stub.StreamObserver`. This code is generated by the protoc compiler.

The plugin configuration above configures the following plugin:

```xml
<protocPlugin>
  <id>vertx-grpc-protoc-plugin</id>
  <groupId>io.vertx</groupId>
  <artifactId>vertx-grpc-protoc-plugin</artifactId>
  <version>${stack.version}</version>
  <mainClass>io.vertx.grpc.protoc.plugin.VertxGrpcGenerator</mainClass>
</protocPlugin>
```

This generates a client version that uses Vert.x asynchronous constructs such as `Future` or `ReadStream` or `WriteStream`
which can be more convenient in the Vert.x ecosystem.

[source,$lang]
----
{@link examples.Examples#vertxSimpleClient}
----

==== Client gzip compression

You can enable gzip compression to tell the client to send compressed messages.

[source,$lang]
----
{@link examples.Examples#clientWithCompression}
----

NOTE: you can use other compressors as long as the server support them and they are registered against the compressor
      registry when building the `ManagedChannel`

==== SSL configuration

If you enabled SSL previously your client will also require SSL, in order to do this we need to configure the channel:

[source,$lang]
----
{@link examples.Examples#sslClient}
----

IMPORTANT: since gRPC uses HTTP/2 transport, SSL/TLS setup requires the
https://wikipedia.org/wiki/Application-Layer_Protocol_Negotiation[Application-Layer Protocol Negotiation]
in your client

== Advanced configuration

Until now all gRPC examples where using sensible defaults but there is more, if you need to have full control over
the server configuration you should refer to the documentation: {@link io.vertx.grpc.VertxServerBuilder}, or if you
need to control your client channel {@link io.vertx.grpc.VertxChannelBuilder}. Vert.x gRPC extends the grpc-java
project (Netty transport) and therefore reading its http://www.grpc.io/grpc-java/javadoc/[documentation] is
recommended.

== Native transports

The client and server can be deployed with Netty's native transports, this is achieved when
creating the Vert.x instance.

[source,$lang]
----
{@link examples.Examples#nativeTransport}
----

Please refer Vert.x Core documentation for more information about native transports.
