Akka（41）： Http：DBTable-rows streaming

  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(r@HttpResponse(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(r@HttpResponse(StatusCodes.OK, _, entity, _)) =>
          val futSource = Unmarshal(entity).to[Source[County,NotUsed]]
          futSource.onSuccess {
            case source => source.runForeach(println)
          }
        case Success(r@HttpResponse(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(r@HttpResponse(StatusCodes.OK, _, entity, _)) =>
          entity.dataBytes.map(_.utf8String).runForeach(println)
        case Success(r@HttpResponse(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(r@HttpResponse(StatusCodes.OK, _, entity, _)) =>
          val futSource = Unmarshal(entity).to[Source[County,NotUsed]]
          futSource.onSuccess {
            case source => source.runForeach(println)
          }
        case Success(r@HttpResponse(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(r@HttpResponse(StatusCodes.OK, _, entity, _)) =>
          entity.dataBytes.map(_.utf8String).runForeach(println)
        case Success(r@HttpResponse(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()

}