Akka-CQRS(11)- akka-http for http-web-service: Marshalling-資料序列化

雪川大蟲發表於2019-06-19

  前面幾篇討論了關於gRPC方式的前後端連線整合方式。gRPC也是一個開放的標準,但講到普及性就遠遠不及基於http/1.1協議的web-service了。特別是gRPC的前端程式設計還是有一定的門檻,所以作為一種開放的網路大平臺還是必須考慮用web-service方式的整合。平臺服務api可以有兩樣選擇:一種是傳統web-service方式,新的一種是rest api款式。rest api比較適合資料庫表的crud操作。在2017年我曾經寫了一系列部落格介紹akka-http,這裡就不再敘述它的細節了。這篇我們只聚焦在解決當前問題上。在POS控制平臺例子裡不會涉及到POST操作,應該全部是GET型別的,如:

http://192.168.11.189:2588/pos/logon?opr=1010
http://192.168.11.189:2588/pos/logoff
http://192.168.11.189:2588/pos/logsales?acct=001&dpt=01&code=978111&qty=3&price=1200
http://192.168.11.189:2588/pos/subtotal?level=0
http://192.168.11.189:2588/pos/discount?disctype=2&grouped=true&code=481&percent=20

可以看到,請求部分只是帶引數的uri,不含entity資料部分,資料通過querystring提供。但返回會有幾種資料型別:POSResponse,TxnsItems,vchState,這些都曾經在Protobuffer用IDL定義過: 

message PBVchState {      //單據狀態
    string opr  = 1;    //收款員
    int64  jseq = 2;    //begin journal sequence for read-side replay
    int32  num  = 3;    //當前單號
    int32  seq  = 4;    //當前序號
    bool   void = 5;    //取消模式
    bool   refd = 6;    //退款模式
    bool   susp = 7;    //掛單
    bool   canc = 8;    //廢單
    bool   due  = 9;    //當前餘額
    string su   = 10;   //主管編號
    string mbr  = 11;   //會員號
    int32  mode = 12;   //當前操作流程:0=logOff, 1=LogOn, 2=Payment
}

message PBTxnItem {       //交易記錄
    string txndate    = 1;   //交易日期
    string txntime    = 2;   //錄入時間
    string opr        = 3;   //操作員
    int32  num        = 4;   //銷售單號
    int32  seq        = 5;   //交易序號
    int32  txntype    = 6;   //交易型別
    int32  salestype  = 7;   //銷售型別
    int32  qty        = 8;   //交易數量
    int32  price      = 9;   //單價(分)
    int32  amount     = 10;  //碼洋(分)
    int32  disc       = 11;  //折扣率 (%)
    int32  dscamt     = 12;  //折扣額:負值  net實洋 = amount + dscamt
    string member     = 13;  //會員卡號
    string code       = 14;  //編號(商品、卡號...)
    string acct       = 15;  //賬號
    string dpt        = 16;  //部類
}

message PBPOSResponse {
    int32  sts                  = 1;
    string msg                  = 2;
    PBVchState voucher          = 3;
    repeated PBTxnItem txnitems   = 4;

}

那麼概括我們現在的主要工作包括:Uri解析,HttpResponse例項的構建和傳輸。

首先,用akka-http搭建一個http server框架:

import akka.actor._
import akka.stream._
import akka.http.scaladsl.Http
import akka.http.scaladsl.server.Directives._

object HttpServerDemo extends App {

  implicit val httpSys = ActorSystem("httpSystem")
  implicit val httpMat = ActorMaterializer()
  implicit val httpEC = httpSys.dispatcher

  val route =
    path("hello") {
      complete {"hello, http server "}
    }

  val (port, host) = (8011,"192.168.11.189")

  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())

/*
  bindingFuture.foreach(s => println(s.localAddress.getHostString))

  bindingFuture.foreach(_.unbind())

  bindingFuture.onComplete {
    case Success(value) => value.unbind()
  }
*/

}

用akka-http的server api很快就完成了一個簡單的http-server。下一步研究一下如何構建返回的HttpResponse:httpresponse是從server端傳送到client端的。這個過程包括把HttpResponse Entity裡的資料從某種型別轉換成通訊用的二進位制資料流、到了客戶端再轉換成目標型別。akka-http的資料轉換機制Marshaller/Unmarshaller是通過型別轉換的隱式例項來實現的,akka-http提供了多個標準型別資料轉換的隱式例項,如StringMarshaller: 

  implicit val ByteArrayMarshaller: ToEntityMarshaller[Array[Byte]] = byteArrayMarshaller(`application/octet-stream`)
  def byteArrayMarshaller(contentType: ContentType): ToEntityMarshaller[Array[Byte]] =
    Marshaller.withFixedContentType(contentType) { bytes => HttpEntity(contentType, bytes) }

  implicit val ByteStringMarshaller: ToEntityMarshaller[ByteString] = byteStringMarshaller(`application/octet-stream`)
  def byteStringMarshaller(contentType: ContentType): ToEntityMarshaller[ByteString] =
    Marshaller.withFixedContentType(contentType) { bytes => HttpEntity(contentType, bytes) }

  implicit val StringMarshaller: ToEntityMarshaller[String] = stringMarshaller(`text/plain`)
  def stringMarshaller(mediaType: MediaType.WithOpenCharset): ToEntityMarshaller[String] =
    Marshaller.withOpenCharset(mediaType) { (s, cs) => HttpEntity(mediaType withCharset cs, s) }
  def stringMarshaller(mediaType: MediaType.WithFixedCharset): ToEntityMarshaller[String] =
    Marshaller.withFixedContentType(mediaType) { s => HttpEntity(mediaType, s) }

...

因為akka-http提供了implicit val StringMarshaller,所以在上面的例子裡我可以直接寫成: complete("hello world!"),然後系統自動構建一個含字元型別資料entity的HttpResponse。Entity.dataBytes中的資料型別是由Entity.contentType指明的: 

object ContentTypes {
  val `application/json` = ContentType(MediaTypes.`application/json`)
  val `application/octet-stream` = ContentType(MediaTypes.`application/octet-stream`)
  val `application/x-www-form-urlencoded` = ContentType(MediaTypes.`application/x-www-form-urlencoded`)
  val `text/plain(UTF-8)` = MediaTypes.`text/plain` withCharset HttpCharsets.`UTF-8`
  val `text/html(UTF-8)` = MediaTypes.`text/html` withCharset HttpCharsets.`UTF-8`
  val `text/xml(UTF-8)` = MediaTypes.`text/xml` withCharset HttpCharsets.`UTF-8`
  val `text/csv(UTF-8)` = MediaTypes.`text/csv` withCharset HttpCharsets.`UTF-8`

  val `application/grpc+proto` = ContentType(MediaTypes.`application/grpc+proto`)

  // used for explicitly suppressing the rendering of Content-Type headers on requests and responses
  val NoContentType = ContentType(MediaTypes.NoMediaType)
}

客戶端收到HttpResponse後把收到的二進位制資料流轉換成MediaTypes指定的型別。當然,最基本的資料型別就是String了。所有客戶端都提供String型別的反序列化deserialization。理論上來講,我們可以用字元形式來描述任何型別資料,這樣我們可以把一個特殊型別例項轉成String,然後傳送給客戶端。客戶端再按照協議好的型別轉換規則把字元轉換成目標型別: 

  case class TextMessage(msg: String)
  val helloMsg: String = TextMessage("hello string message converter").toString
  val route =
    path("hello") {
      complete {helloMsg}
    }

不過,這種情況只適用於內部系統的資料交換,因為資料型別轉換的規則方式都是內部私有的。xml,json是開放平臺系統資料交換的標準資料型別描述語言,本身是字元String形式的,只是它用String描述型別的語法是行業標準的。客戶端可以按行業標準從一個xml/json檔案裡提取裡面的資料型別和例項。所以,自定義型別的資料轉換主要包括  型別->jsonstring->bytestring->jsonstring->型別。換句話說我們只要有隱式JsonMarshaller例項就可以完成大部分的資料交換工作了。

spray-json是akka-http自帶預設的一個json工具庫,它提供了通用的針對任何型別T的Marshaller/Unmarshaller: ToEntityMarshaller[T] 和 FromEntityUnmarshaller[T]。使用spay-json很簡單,如下:

import akka.http.scaladsl.marshallers.sprayjson._
import spray.json._

object JsonMarshaller extends  SprayJsonSupport with DefaultJsonProtocol {
  //domain models
  case class Person(name:String, age: Int)
  case class Location(province: String, city: String, zipcode: Int)
  case class Employee(person: Person, loccation: Location)

  //collect your json format instances
  implicit val fmtPerson = jsonFormat2(Person.apply)
  implicit val fmtLocation = jsonFormat3(Location.apply)
  implicit val fmtEmployee = jsonFormat2(Employee.apply)
}

使用Marshaller時只要import JsonMarshaller._ 把幾個型別的隱式轉換例項帶進可視域即可,如下: 

  import JsonMarshaller._

  val person = Person("Jonh Doe", 23)
  val location = Location("GuangDong","ShenZhen",10223)
  val employee = Employee(person,location)

  val route =
    path("json") {
      complete {employee}
    }

就這麼簡單,試試看:

http://192.168.11.189:8011/json

{"loccation":{"city":"ShenZhen","province":"GuangDong","zipcode":10223},"person":{"age":23,"name":"Jonh Doe"}}

沒錯,客戶端的確收到正確的json資料。還有一項需求是在Reponse裡返回一個資料流(多條資料),如當前交易專案清單。這個也比較容易:akka-http本身支援json-streaming。具體使用方法如下: 

  import akka.http.scaladsl.common.EntityStreamingSupport
  import akka.stream.scaladsl._

  implicit val jsonStreamingSupport = EntityStreamingSupport.json()
    .withParallelMarshalling(parallelism = 4, unordered = false)

  val persons = List(person,Person("Peter Kung",28), Person("Ketty Wang",16))
  val personDataSource: Source[Person,Any] = Source.fromIterator(() => persons.iterator)

  val route =
    path("json") {
      complete {employee}
    } ~
    path("stream") {
      complete(personDataSource)
    }

在客戶端browser上測試: 

http://192.168.11.189:8011/stream

[{"age":23,"name":"Jonh Doe"},{"age":28,"name":"Peter Kung"},{"age":16,"name":"Ketty Wang"}]

也沒問題。下面是本次示範中使用的依賴和它們的版本: 

libraryDependencies ++= Seq(
  "de.heikoseeberger" %% "akka-http-json4s" % "1.26.0",
  "org.json4s" %% "json4s-jackson" % "3.6.6",
  "org.json4s" %% "json4s-ext" % "3.6.6",
  "com.typesafe.akka" %% "akka-http" % "10.1.8" ,
  "com.typesafe.akka" %% "akka-http-spray-json" % "10.1.8",
  "com.typesafe.akka" %% "akka-stream" % "2.5.23"
)

 

 

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