Akka（41）： Http：DBTable-rows streaming – 数据库表行交换详解编程语言

  在前面一篇讨论里我们介绍了通过http进行文件的交换。因为文件内容是以一堆bytes来表示的，而http消息的数据部分也是byte类型的，所以我们可以直接用Source[ByteString,_]来读取文件然后放进HttpEntity中。我们还提到：如果需要进行数据库数据交换的话，可以用Source[ROW,_]来表示库表行，但首先必须进行ROW - ByteString的转换。在上期讨论我们提到过这种转换其实是ROW- Json- ByteString或者反方向的转换，在Akka-http里称之为Marshalling和Unmarshalling。Akka-http的Marshalling实现采用了type-class编程模式，需要为每一种类型与Json的转换在可视域内提供Marshaller[A,B]类型的隐式实例。Akka-http默认的Json工具库是Spray-Json，着重case class，而且要提供JsonFormat?(case-class),其中?代表case class的参数个数，用起来略显复杂。不过因为是Akka-http的配套库，在将来Akka-http的持续发展中具有一定的优势，所以我们还是用它来进行下面的示范。

下面就让我们开始写些代码吧。首先，我们用一个case class代表数据库表行结构，然后用它作为流元素来构建一个Source，如下：

 case class County(id: Int, name: String) 

 val source: Source[County, NotUsed] = Source(1 to 5).map { i = County(i, s"中国广东省地区编号 #$i") }

我们先设计服务端的数据下载部分：

import akka.actor._ 

import akka.stream._ 

import akka.stream.scaladsl._ 

import akka.http.scaladsl.Http 

import akka.http.scaladsl.server.Directives._ 

import akka._ 

import akka.http.scaladsl.common._ 

import spray.json.DefaultJsonProtocol 

import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport 


trait MyFormats extends SprayJsonSupport with DefaultJsonProtocol 

object Converters extends MyFormats { 

 case class County(id: Int, name: String) 

 val source: Source[County, NotUsed] = Source(1 to 5).map { i = County(i, s"中国广东省地区编号 #$i") } 

 implicit val countyFormat = jsonFormat2(County) 

object HttpDBServer extends App { 

 import Converters._ 

 implicit val httpSys = ActorSystem("httpSystem") 

 implicit val httpMat = ActorMaterializer() 

 implicit val httpEC = httpSys.dispatcher 


 implicit val jsonStreamingSupport = EntityStreamingSupport.json() 

 .withParallelMarshalling(parallelism = 8, unordered = false) 

 val route = 

 path("rows") { 

 get { 

 complete { 

 source 

 val (port, host) = (8011,"localhost") 

 val bindingFuture = Http().bindAndHandle(route,host,port) 

 println(s"Server running at $host $port. Press any key to exit ...") 

 scala.io.StdIn.readLine() 

 bindingFuture.flatMap(_.unbind()) 

 .onComplete(_ = httpSys.terminate()) 

}

在上面的代码里我们直接把source放进了complete()，然后期望这个directive能通过ToEntityMarshaller[County]类实例用Spray-Json把Source[County,NotUsed]转换成Source[ByteString,NotUsed]然后放入HttpResponse的HttpEntity里。转换结果只能在客户端得到证实。我们知道HttpResponse里的Entity.dataBytes就是一个Source[ByteString,_]，我们可以把它Unmarshall成Source[County,_]，然后用Akka-stream来操作：

 case Success([email protected](StatusCodes.OK, _, entity, _)) = 

 val futSource = Unmarshal(entity).to[Source[County,NotUsed]] 

 futSource.onSuccess { 

 case source = source.runForeach(println) 

 

上面这个Unmarshal调用了下面这个FromEntityUnmarshaller[County]隐式实例:

 // support for as[Source[T, NotUsed]] 

 implicit def sprayJsonSourceReader[T](implicit reader: RootJsonReader[T], support: EntityStreamingSupport): FromEntityUnmarshaller[Source[T, NotUsed]] = 

 Unmarshaller.withMaterializer { implicit ec ⇒ implicit mat ⇒ e ⇒ 

 if (support.supported.matches(e.contentType)) { 

 val frames = e.dataBytes.via(support.framingDecoder) 

 val unmarshal = sprayJsonByteStringUnmarshaller(reader)(_) 

 val unmarshallingFlow = 

 if (support.unordered) Flow[ByteString].mapAsyncUnordered(support.parallelism)(unmarshal) 

 else Flow[ByteString].mapAsync(support.parallelism)(unmarshal) 

 val elements = frames.viaMat(unmarshallingFlow)(Keep.right) 

 FastFuture.successful(elements) 

 } else FastFuture.failed(Unmarshaller.UnsupportedContentTypeException(support.supported)) 

 }

这个隐式实例是由Spray-Jason提供的，在SprayJsonSupport.scala里。
下面是这部分客户端的完整代码： 

import akka.actor._ 

import akka.stream._ 

import akka.stream.scaladsl._ 

import akka.http.scaladsl.Http 

import akka.http.scaladsl.model._ 

import scala.util._ 

import akka._ 

import akka.http.scaladsl.common._ 

import spray.json.DefaultJsonProtocol 

import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport 

import akka.http.scaladsl.unmarshalling.Unmarshal 

trait MyFormats extends SprayJsonSupport with DefaultJsonProtocol 

object Converters extends MyFormats { 

 case class County(id: Int, name: String) 

 implicit val countyFormat = jsonFormat2(County) 

object HttpDBClient extends App { 

 import Converters._ 

 implicit val sys = ActorSystem("ClientSys") 

 implicit val mat = ActorMaterializer() 

 implicit val ec = sys.dispatcher 

 implicit val jsonStreamingSupport: JsonEntityStreamingSupport = EntityStreamingSupport.json() 

 def downloadRows(request: HttpRequest) = { 

 val futResp = Http(sys).singleRequest(request) 

 futResp 

 .andThen { 

 case Success([email protected](StatusCodes.OK, _, entity, _)) = 

 val futSource = Unmarshal(entity).to[Source[County,NotUsed]] 

 futSource.onSuccess { 

 case source = source.runForeach(println) 

 case Success([email protected](code, _, _, _)) = 

 println(s"download request failed, response code: $code") 

 r.discardEntityBytes() 

 case Success(_) = println("Unable to download rows!") 

 case Failure(err) = println(s"download failed: ${err.getMessage}") 

 downloadRows(HttpRequest(HttpMethods.GET,uri = s"http://localhost:8011/rows")) 

 scala.io.StdIn.readLine() 

 sys.terminate() 

}

以上我们已经实现了客户端从服务端下载一段数据库表行，然后以Akka-stream的操作方式来处理下载数据。那么反向交换即从客户端上传一段表行的话就需要把一个Source[T,_]转换成Source[ByteString,_]然后放进HttpRequest的HttpEntity里。服务端收到数据后又要进行反向的转换即把Request.Entity.dataBytes从Source[ByteString,_]转回Source[T,_]。Akka-http在客户端没有提供像complete这样的强大的自动化功能。我们可能需要自定义并提供像ToRequestMarshaller[Source[T,_]]这样的隐式实例。但Akka-http的Marshalling-type-class是个非常复杂的系统。如果我们的目的是简单提供一个Source[ByteString,_]，我们是否可以直接调用Spray-Json的函数来进行ROW- Son- ByteString转换呢？如下：

 import akka.util.ByteString 

 import akka.http.scaladsl.model.HttpEntity.limitableByteSource 

 val source: Source[County,NotUsed] = Source(1 to 5).map {i = County(i, s"广西壮族自治区地市县编号 #$i")} 

 def countyToByteString(c: County) = { 

 ByteString(c.toJson.toString) 

 val flowCountyToByteString : Flow[County,ByteString,NotUsed] = Flow.fromFunction(countyToByteString) 

 val rowBytes = limitableByteSource(source via flowCountyToByteString) 

 val request = HttpRequest(HttpMethods.POST,uri = s"http://localhost:8011/rows") 

 val data = HttpEntity( 

 ContentTypes.`application/json`, 

 rowBytes 

我们直接用toJson函数进行County- Json转换实现了flowCountyToByteString。toJason是Spray-Json提供的一个函数： 

package json { 

 case class DeserializationException(msg: String, cause: Throwable = null, fieldNames: List[String] = Nil) extends RuntimeException(msg, cause) 

 class SerializationException(msg: String) extends RuntimeException(msg) 

 private[json] class PimpedAny[T](any: T) { 

 def toJson(implicit writer: JsonWriter[T]): JsValue = writer.write(any) 

 private[json] class PimpedString(string: String) { 

 @deprecated("deprecated in favor of parseJson", "1.2.6") 

 def asJson: JsValue = parseJson 

 def parseJson: JsValue = JsonParser(string) 

}

假设服务端收到数据后以Akka-stream方式再转换成一个List返回，我们用下面的方法来测试功能：

 def uploadRows(request: HttpRequest, dataEntity: RequestEntity) = { 

 val futResp = Http(sys).singleRequest( 

 request.copy(entity = dataEntity) 

 futResp 

 .andThen { 

 case Success([email protected](StatusCodes.OK, _, entity, _)) = 

 entity.dataBytes.map(_.utf8String).runForeach(println) 

 case Success([email protected](code, _, _, _)) = 

 println(s"Upload request failed, response code: $code") 

 r.discardEntityBytes() 

 case Success(_) = println("Unable to Upload file!") 

 case Failure(err) = println(s"Upload failed: ${err.getMessage}") 

 }

服务端接收数据处理方法如下：

 post { 

 withoutSizeLimit { 

 entity(asSourceOf[County]) { source = 

 val futofNames: Future[List[String]] = 

 source.runFold(List[String](""))((acc, b) = acc ++ List(b.name)) 

 complete { 

 futofNames 

 }

考虑到在数据转换的过程中可能会出现异常。需要异常处理方法来释放backpressure：

 def postExceptionHandler: ExceptionHandler = 

 ExceptionHandler { 

 case _: RuntimeException = 

 extractRequest { req = 

 req.discardEntityBytes() 

 complete((StatusCodes.InternalServerError.intValue,"Upload Failed!")) 

 post { 

 withoutSizeLimit { 

 handleExceptions(postExceptionHandler) { 

 entity(asSourceOf[County]) { source = 

 val futofNames: Future[List[String]] = 

 source.runFold(List[String](""))((acc, b) = acc ++ List(b.name)) 

 complete { 

 futofNames 

 }

在客户端试运行返回结果显示：

 uploadRows(request,data) 

["","广西壮族自治区地市县编号 #1","广西壮族自治区地市县编号 #2","广西壮族自治区地市县编号 #3","广西壮族自治区地市县编号 #4","广西壮族自治区地市县编号 #5"]

正是我们期待的结果。

下面是本次讨论的示范代码：

服务端：

import akka.actor._ 

import akka.stream._ 

import akka.stream.scaladsl._ 

import akka.http.scaladsl.Http 

import akka._ 

import akka.http.scaladsl.common._ 

import spray.json.DefaultJsonProtocol 

import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport 

import scala.concurrent._ 

import akka.http.scaladsl.server._ 

import akka.http.scaladsl.server.Directives._ 

import akka.http.scaladsl.model._ 

trait MyFormats extends SprayJsonSupport with DefaultJsonProtocol 

object Converters extends MyFormats { 

 case class County(id: Int, name: String) 

 val source: Source[County, NotUsed] = Source(1 to 5).map { i = County(i, s"中国广东省地区编号 #$i") } 

 implicit val countyFormat = jsonFormat2(County) 

object HttpDBServer extends App { 

 import Converters._ 

 implicit val httpSys = ActorSystem("httpSystem") 

 implicit val httpMat = ActorMaterializer() 

 implicit val httpEC = httpSys.dispatcher 


 implicit val jsonStreamingSupport = EntityStreamingSupport.json() 

 .withParallelMarshalling(parallelism = 8, unordered = false) 

 def postExceptionHandler: ExceptionHandler = 

 ExceptionHandler { 

 case _: RuntimeException = 

 extractRequest { req = 

 req.discardEntityBytes() 

 complete((StatusCodes.InternalServerError.intValue,"Upload Failed!")) 

 val route = 

 path("rows") { 

 get { 

 complete { 

 source 

 } ~ 

 post { 

 withoutSizeLimit { 

 handleExceptions(postExceptionHandler) { 

 entity(asSourceOf[County]) { source = 

 val futofNames: Future[List[String]] = 

 source.runFold(List[String](""))((acc, b) = acc ++ List(b.name)) 

 complete { 

 futofNames 

 val (port, host) = (8011,"localhost") 

 val bindingFuture = Http().bindAndHandle(route,host,port) 

 println(s"Server running at $host $port. Press any key to exit ...") 

 scala.io.StdIn.readLine() 

 bindingFuture.flatMap(_.unbind()) 

 .onComplete(_ = httpSys.terminate()) 

}

客户端：

import akka.actor._ 

import akka.stream._ 

import akka.stream.scaladsl._ 

import akka.http.scaladsl.Http 

import akka.http.scaladsl.model._ 

import scala.util._ 

import akka._ 

import akka.http.scaladsl.common._ 

import spray.json.DefaultJsonProtocol 

import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport 

import akka.http.scaladsl.unmarshalling._ 

trait MyFormats extends SprayJsonSupport with DefaultJsonProtocol 

object Converters extends MyFormats { 

 case class County(id: Int, name: String) 

 implicit val countyFormat = jsonFormat2(County) 

object HttpDBClient extends App { 

 import Converters._ 

 implicit val sys = ActorSystem("ClientSys") 

 implicit val mat = ActorMaterializer() 

 implicit val ec = sys.dispatcher 

 implicit val jsonStreamingSupport: JsonEntityStreamingSupport = EntityStreamingSupport.json() 

 def downloadRows(request: HttpRequest) = { 

 val futResp = Http(sys).singleRequest(request) 

 futResp 

 .andThen { 

 case Success([email protected](StatusCodes.OK, _, entity, _)) = 

 val futSource = Unmarshal(entity).to[Source[County,NotUsed]] 

 futSource.onSuccess { 

 case source = source.runForeach(println) 

 case Success([email protected](code, _, _, _)) = 

 println(s"download request failed, response code: $code") 

 r.discardEntityBytes() 

 case Success(_) = println("Unable to download rows!") 

 case Failure(err) = println(s"download failed: ${err.getMessage}") 

 downloadRows(HttpRequest(HttpMethods.GET,uri = s"http://localhost:8011/rows")) 


 import akka.util.ByteString 

 import akka.http.scaladsl.model.HttpEntity.limitableByteSource 

 val source: Source[County,NotUsed] = Source(1 to 5).map {i = County(i, s"广西壮族自治区地市县编号 #$i")} 

 def countyToByteString(c: County) = { 

 ByteString(c.toJson.toString) 

 val flowCountyToByteString : Flow[County,ByteString,NotUsed] = Flow.fromFunction(countyToByteString) 

 val rowBytes = limitableByteSource(source via flowCountyToByteString) 

 val request = HttpRequest(HttpMethods.POST,uri = s"http://localhost:8011/rows") 

 val data = HttpEntity( 

 ContentTypes.`application/json`, 

 rowBytes 

 def uploadRows(request: HttpRequest, dataEntity: RequestEntity) = { 

 val futResp = Http(sys).singleRequest( 

 request.copy(entity = dataEntity) 

 futResp 

 .andThen { 

 case Success([email protected](StatusCodes.OK, _, entity, _)) = 

 entity.dataBytes.map(_.utf8String).runForeach(println) 

 case Success([email protected](code, _, _, _)) = 

 println(s"Upload request failed, response code: $code") 

 r.discardEntityBytes() 

 case Success(_) = println("Unable to Upload file!") 

 case Failure(err) = println(s"Upload failed: ${err.getMessage}") 

 uploadRows(request,data) 

 scala.io.StdIn.readLine() 

 sys.terminate() 

}

12820.html