Akka流在流中使用HttpResponse

Question

我想利用一个简单的流从http服务收集一些额外的数据，并用这些结果来增强我的数据对象。下面说明了这一想法：

val httpClient = Http().superPool[User]()

val cityRequest = Flow[User].map { user=>
  (HttpRequest(uri=Uri(config.getString("cityRequestEndpoint"))), User)
}

val cityResponse = Flow[(Try[HttpResponse], User)].map {
  case (Failure(ex), user) => user
  case (Success(resp), user) => {
    // << What to do here to get the value >> //
    val responseData = processResponseSomehowToGetAValue?
    val enhancedUser = new EnhancedUser(user.data, responseData)
    enhancedUser
  }
}

val processEnhancedUser = Flow[EnhancedUser].map {
  // e.g.: Asynchronously save user to a database
}

val useEnhancementGraph = userSource
  .via(getRequest)
  .via(httpClient)
  .via(getResponse)
  .via(processEnhancedUser)
  .to(Sink.foreach(println))

我有一个问题，要理解流的本质和流内部的物化/未来之间的机制和区别。

以下想法并没有向我解释：

null

val cityResponse = Flow[(Try[HttpResponse], User)].map {
  case (Failure(ex), member) => member
  case (Success(response), member) => {
    Unmarshal(response.entity).to[String] onComplete {
      case Success(s) =>  member.city = Some(s)
      case Failure(ex) => member.city = None
    }
  }
  member
}.async  // <<-- This changed the behavior to be correct, why?

Answer 1

根据从“CityRequestEndpoint”获取的实体的性质，您可以使用两种不同的策略：

基于流

处理这种情况的典型方法是始终假设来自源endpoint的实体可以包含N段数据，其中N段是事先不知道的。这通常是要遵循的模式，因为它在现实世界中是最通用的，因此也是“最安全的”。

val convertResponseToByteStrSource : (Try[HttpResponse], User) => Source[(Option[ByteString], User), _] = 
  (response, user) => response match {
    case Failure(_) => Source single (None -> user)
    case Success(r) => r.entity.dataBytes map (byteStr => Some(byteStr) -> user)
  }

val cityByteStrFlow : Flow[(Try[HttpResponse], User), (Option[ByteString], User), _] = 
  Flow[(Try[HttpResponse], User)] flatMapConcat convertResponseToByteStrSource

val convertByteStringToUser : (Option[ByteString], User) => EnhancedUser = 
  (byteStr, user) => 
    byteStr
      .map(s => EnhancedUser(user.data, s))
      .getOrElse(user)

val cityUserFlow : Flow[(Option[ByteString], User), EnhancedUser, _] = 
  Flow[(ByteString, User)] map convertByteStringToUser

val useEnhancementGraph =
  userSource
    .via(cityRequest)
    .via(httpClient)
    .via(cityByteStrFlow)
    .via(cityUserFlow)
    .via(processEnhancedUser)
    .to(Sink foreach println)

null

val parallelism = 10

val timeout : FiniteDuration = ??? //you need to specify the timeout limit

val convertResponseToFutureByteStr : (Try[HttpResponse], User) => Future[EnhancedUser] = 
  _ match {
    case (Failure(ex), user)   => 
      Future successful user
    case (Success(resp), user) => 
      resp
        .entity
        .toStrict(timeout)
        .map(byteStr => new EnhancedUser(user.data, byteStr))
  }    

val cityResponse : Flow[(Try[HttpResponse], User), EnhancedUser, _] =
  Flow[(Try[HttpResponse], User)].mapAsync(parallelism)(convertResponseToFutureByteStr)