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}") } }