ScalaPB（2）： 在scala中用gRPC实现微服务

   gRPC是google开源提供的一个RPC软件框架，它的特点是极大简化了传统RPC的开发流程和代码量，使用户可以免除许多陷阱并聚焦于实际应用逻辑中。作为一种google的最新RPC解决方案，gRPC具备了以下这些强项：

1、gRPC在HTTP/2协议上用protobuf取代了json实现了最佳效率

2、用IDL（Interface Definition Language），一种简单的描述语言来自动产生RPC的api源代码

3、支持blocking/non-blocking双向数据流交互，适合程序的流程控制

gRPC的使用非常简单，具体流程如下：

1、在一个.proto字符类文件中用IDL来描述用户自定义的数据类型和服务

2、用protoc编译器编译文件并产生自定义数据类型和服务的api源代码

3、在server端实现.proto中定义的服务函数

4、在client端通过自动产生的stub来调用服务

下面我们就来示范gRPC的编程流程。gRPC支持下面这几种服务类型：

1、Unary：独立的一对client-request/server-response，是我们常用的http交互模式

2、Server-Streaming：client发出一个request后从server端接收一串多个response

3、Client-Streaming：client向server发送一串多个request后从server接收一个response

4、Bidirectional-Streaming：由client首先发送request启动连接，然后在这个连接上两端可以不断交互信息。

在本篇讨论中我们先示范Unary-service的编程流程，下面是.proto文件内容：

syntax = "proto3";

import "google/protobuf/wrappers.proto";
import "scalapb/scalapb.proto";

package learn.grpc.services;

/*
 * Returns a greeting for the given person optionally including a custom message.
 */
service HelloWorld {
  rpc SayHello(ToBeGreeted) returns (Greeting) {}
}

message Person {
   string name = 1;
}

message ToBeGreeted {
  Person person = 1;
  google.protobuf.StringValue msg = 2;
}

message Greeting {
  string message = 1;
}

这段IDL描述了一个HelloWorld服务，包括了一个服务函数SayHello。三种数据类型：Person,ToBeGreeted,Greeting。通过对.proto文件进行编译后产生文件中包括一个HelloWorldGrpc.scala文件，里面提供了一些重要的api：

trait HelloWorld -> 用于实现HelloWorld服务的trait
trait HelloWorldBlockingClient -> 用于实现客户端stub
class HelloWorldBlockingStub -> blocking客户端stub
class HelloWorldStub -> non-blocking客户端stub
def bindService -> 服务类型绑带方法

我们先实现HelloWorld服务：

 class HelloService extends HelloWorldGrpc.HelloWorld {
    override def sayHello(request: ToBeGreeted): Future[Greeting] = {
      val greeter = request.person match {
        case Some(p) => p.name
        case None => "friendo"
      }
      Future.successful(Greeting(message = s"Hello $greeter, ${request.msg}"))
    }
  }

可以看到我们直接使用了IDL描述的自定义数据类型如：ToBeGreeted,Greeting。在客户端调用服务并输出返回结果response：

    //build connection channel
    val channel = io.grpc.ManagedChannelBuilder
      .forAddress("LocalHost",50051)
        .usePlaintext(true)
      .build()

    //construct requestHelloService
    val greeter = ToBeGreeted()
      .withMsg("remote greetings!")
      .withPerson(ToBeGreeted.Person("mickey"))

    //async call
    val asyncStub: HelloWorldGrpc.HelloWorldStub = HelloWorldGrpc.stub(channel)
    val futResponse: Future[Greeting] = asyncStub.sayHello(greeter)

    import scala.concurrent.ExecutionContext.Implicits.global
    futResponse.foreach(greeting => println(greeting.message))

    val greeter2 = ToBeGreeted(person = Some(Person("jacky")),msg = Some("how are you?"))
    //sync call
    val syncStub: HelloWorldGrpc.HelloWorldBlockingClient = HelloWorldGrpc.blockingStub(channel)
    val response: Greeting = syncStub.sayHello(greeter2)

    println(s"${response.message}")

下面是bindService方法的使用示范：

  def main(args: Array[String]): Unit = {
    val service = HelloWorldGrpc.bindService(new HelloService,ExecutionContext.global)
    runServer(service)
  }

runServer函数定义如下：

package learn.grpc.server
import io.grpc.{ServerBuilder,ServerServiceDefinition}

trait gRPCServer {
  def runServer(service: ServerServiceDefinition): Unit = {
    val server = ServerBuilder
      .forPort(50051)
      .addService(service)
      .build
      .start

    // make sure our server is stopped when jvm is shut down
    Runtime.getRuntime.addShutdownHook(new Thread() {
      override def run(): Unit = server.shutdown()
    })

    server.awaitTermination()
  }

}

注意我们还使用了io.grpc库里的类型和方法，这是protobuf项目提供的一个标准库。在客户端也需要使用它来构建通道：

   //build connection channel
    val channel = io.grpc.ManagedChannelBuilder
      .forAddress("LocalHost",50051)
        .usePlaintext(true)
      .build()

我们将在后面的讨论里介绍gRPC的streaming编程方法。下面是本次示范的源代码：

project/scalapb.sbt

addSbtPlugin("com.thesamet" % "sbt-protoc" % "0.99.18")
libraryDependencies += "com.thesamet.scalapb" %% "compilerplugin" % "0.7.1"

build.sbt

import scalapb.compiler.Version.scalapbVersion
import scalapb.compiler.Version.grpcJavaVersion
name := "learn-gRPC"
version := "0.1"
scalaVersion := "2.12.6"
libraryDependencies ++= Seq(
  "com.thesamet.scalapb" %% "scalapb-runtime" % scalapbVersion % "protobuf",
  "io.grpc" % "grpc-netty" % grpcJavaVersion,
  "com.thesamet.scalapb" %% "scalapb-runtime-grpc" % scalapbVersion
)
PB.targets in Compile := Seq(
  scalapb.gen() -> (sourceManaged in Compile).value
)

src/main/protobuf/hello.proto

syntax = "proto3";

import "google/protobuf/wrappers.proto";
import "scalapb/scalapb.proto";

package learn.grpc.services;

/*
 * Returns a greeting for the given person optionally including a custom message.
 */
service HelloWorld {
  rpc SayHello(ToBeGreeted) returns (Greeting) {}
}

message Person {
   string name = 1;
}

message ToBeGreeted {
  Person person = 1;
  google.protobuf.StringValue msg = 2;
}

message Greeting {
  string message = 1;
}

src/main/scala/gRPCServer.scala

package learn.grpc.server
import io.grpc.{ServerBuilder,ServerServiceDefinition}

trait gRPCServer {
  def runServer(service: ServerServiceDefinition): Unit = {
    val server = ServerBuilder
      .forPort(50051)
      .addService(service)
      .build
      .start

    // make sure our server is stopped when jvm is shut down
    Runtime.getRuntime.addShutdownHook(new Thread() {
      override def run(): Unit = server.shutdown()
    })

    server.awaitTermination()
  }

}

src/main/scala/HelloServer.scala

package learn.grpc.hello.server
import learn.grpc.services.hello._
import learn.grpc.server.gRPCServer
import scala.concurrent._

object HelloServer extends gRPCServer {

  class HelloService extends HelloWorldGrpc.HelloWorld {
    override def sayHello(request: ToBeGreeted): Future[Greeting] = {
      val greeter = request.person match {
        case Some(p) => p.name
        case None => "friendo"
      }
      Future.successful(Greeting(message = s"Hello $greeter, ${request.msg}"))
    }
  }

  def main(args: Array[String]): Unit = {
    val service = HelloWorldGrpc.bindService(new HelloService,ExecutionContext.global)
    runServer(service)
  }
}

src/main/scala/HelloClient.scala

package learn.grpc.hello.client
import learn.grpc.services.hello.ToBeGreeted.Person
import learn.grpc.services.hello._

import scala.concurrent.Future
object HelloClient {
  def main(args: Array[String]): Unit = {

    //build connection channel
    val channel = io.grpc.ManagedChannelBuilder
      .forAddress("LocalHost",50051)
        .usePlaintext(true)
      .build()

    //construct requestHelloService
    val greeter = ToBeGreeted()
      .withMsg("remote greetings!")
      .withPerson(ToBeGreeted.Person("mickey"))

    //async call
    val asyncStub: HelloWorldGrpc.HelloWorldStub = HelloWorldGrpc.stub(channel)
    val futResponse: Future[Greeting] = asyncStub.sayHello(greeter)

    import scala.concurrent.ExecutionContext.Implicits.global
    futResponse.foreach(greeting => println(greeting.message))

    val greeter2 = ToBeGreeted(person = Some(Person("jacky")),msg = Some("how are you?"))
    //sync call
    val syncStub: HelloWorldGrpc.HelloWorldBlockingClient = HelloWorldGrpc.blockingStub(channel)
    val response: Greeting = syncStub.sayHello(greeter2)

    println(s"${response.message}")


  }

}