如何使用 Akka Persistence 保存流式数据

Question

我使用 StreamRefs 在集群中的参与者之间建立流连接。目前，在写入节点中，我手动将传入消息保存到日志文件中，但我想知道是否可以将其替换为用于写入的持久性 Sink 和用于在 actor 启动时读取的持久性 Source 来自 Akka Persistence 杂志。我一直在考虑用 Persistent actor 的 persist { evt => ... } 替换日志文件接收器，但由于它是异步执行的，所以我将失去背压。那么是否可以将带有背压的流式数据写入Akka Persistence journal，并在actor recover上以流式方式读取这些数据？

当前实施：

object Writer {
  case class WriteSinkRequest(userId: String) 
  case class WriteSinkReady(userId: String, sinkRef: SinkRef[ByteString])
  case class ReadSourceRequest(userId: String)
  case class ReadSourceReady(userId: String, sourceRef: SourceRef[ByteString])
}

class Writer extends Actor {

    // code omitted

    val logsDir = "logs"

    val path = Files.createDirectories(FileSystems.getDefault.getPath(logsDir))

    def logFile(id: String) = {
        path.resolve(id)
    }

    def logFileSink(logId: String): Sink[ByteString, Future[IOResult]] = FileIO.toPath(logFile(logId), Set(CREATE, WRITE, APPEND))
    def logFileSource(logId: String): Source[ByteString, Future[IOResult]] = FileIO.fromPath(logFile(logId))

    override def receive: Receive = {
        case WriteSinkRequest(userId) => 
            // obtain the source you want to offer:
            val sink = logFileSink(userId)
            // materialize the SinkRef (the remote is like a source of data for us):
            val ref: Future[SinkRef[ByteString]] = StreamRefs.sinkRef[ByteString]().to(sink).run()
            // wrap the SinkRef in some domain message, such that the sender knows what source it is
            val reply: Future[WriteSinkReady] = ref.map(WriteSinkReady(userId, _))
            // reply to sender
            reply.pipeTo(sender())

        case ReadSourceRequest(userId) =>
            val source = logFileSource(userId)
            val ref: Future[SourceRef[ByteString]] = source.runWith(StreamRefs.sourceRef())
            val reply: Future[ReadSourceReady] = ref.map(ReadSourceReady(userId, _))
            reply pipeTo sender()

    }
}

P.S。是否可以创建不是 "save-to-journal" 接收器，而是流： incoming data to write ~> save to persistence journal ~> data that was written?

Answer 1

以背压方式将数据流式传输到持久性 actor 的一个想法是使用 Sink.actorRefWithAck：让 actor 在持久化消息时发送确认消息。这看起来像下面这样：

// ...
case class WriteSinkReady(userId: String, sinkRef: SinkRef[MyMsg])    
// ...

def receive = {
  case WriteSinkRequest(userId) =>
    val persistentActor: ActorRef = ??? // a persistent actor that handles MyMsg messages
                                        // as well as the messages used in persistentSink

    val persistentSink: Sink[MyMsg, NotUsed] = Sink.actorRefWithAck[MyMsg](
      persistentActor,
      /* additional parameters: see the docs */
    )

    val ref: Future[SinkRef[MyMsg]] = StreamRefs.sinkRef[MyMsg]().to(persistentSink).run()
    val reply: Future[WriteSinkReady] = ref.map(WriteSinkReady(userId, _))
    reply.pipeTo(sender())

  case ReadSourceRequest(userId) =>
    // ...
}

以上示例使用自定义案例 class 命名为 MyMsg 而不是 ByteString.

在发件人中，假设它是一个演员：

def receive = {
  case WriteSinkReady(userId, sinkRef) =>
    source.runWith(sinkRef) // source is a Source[MyMsg, _]

  // ...
}

发送方中的物化流将消息发送给持久化 actor。

如何使用 Akka Persistence 保存流式数据

How to save streaming data using Akka Persistence

scala

akka

akka-stream

akka-persistence