Akka持久性查询事件流和CQRS

我正在尝试在我的ES-CQRS架构中实现读取端。假设我有一个像这样的执着演员：

object UserWrite {

  sealed trait UserEvent
  sealed trait State
  case object Uninitialized extends State
  case class User(username: String, password: String) extends State
  case class AddUser(user: User)
  case class UserAdded(user: User) extends UserEvent
  case class UserEvents(userEvents: Source[(Long, UserEvent), NotUsed])
  case class UsersStream(fromSeqNo: Long)
  case object GetCurrentUser

  def props = Props(new UserWrite)
}

class UserWrite extends PersistentActor {

  import UserWrite._

  private var currentUser: State = Uninitialized

  override def persistenceId: String = "user-write"

  override def receiveRecover: Receive = {
    case UserAdded(user) => currentUser = user
  }

  override def receiveCommand: Receive = {
    case AddUser(user: User) => persist(UserAdded(user)) {
      case UserAdded(`user`) => currentUser = user
    }
    case UsersStream(fromSeqNo: Long) => publishUserEvents(fromSeqNo)
    case GetCurrentUser => sender() ! currentUser
  }

  def publishUserEvents(fromSeqNo: Long) = {
    val readJournal = PersistenceQuery(context.system).readJournalFor[CassandraReadJournal](CassandraReadJournal.Identifier)
    val userEvents = readJournal
      .eventsByPersistenceId("user-write", fromSeqNo, Long.MaxValue)
      .map { case EventEnvelope(_, _, seqNo, event: UserEvent) => seqNo -> event }
    sender() ! UserEvents(userEvents)
  }
}

据我了解，每次事件持续存在时，我们都可以通过发布Akka Persistence Query。现在，我不确定订阅这些事件的正确方法是什么，以便可以将其持久保存在我的读取方数据库中？一种想法是首先UsersStream从我的阅读方演员向UserWrite演员发送消息，并在该阅读演员中“下沉”事件。

编辑

根据@cmbaxter的建议，我以这种方式实现了read side：

object UserRead {

  case object GetUsers
  case class GetUserByUsername(username: String)
  case class LastProcessedEventOffset(seqNo: Long)
  case object StreamCompleted

  def props = Props(new UserRead)
}

class UserRead extends PersistentActor {
  import UserRead._

  var inMemoryUsers = Set.empty[User]
  var offset        = 0L

  override val persistenceId: String = "user-read"

  override def receiveRecover: Receive = {
    // Recovery from snapshot will always give us last sequence number
    case SnapshotOffer(_, LastProcessedEventOffset(seqNo)) => offset = seqNo
    case RecoveryCompleted                                 => recoveryCompleted()
  }

  // After recovery is being completed, events will be projected to UserRead actor
  def recoveryCompleted(): Unit = {
    implicit val materializer = ActorMaterializer()
    PersistenceQuery(context.system)
      .readJournalFor[CassandraReadJournal](CassandraReadJournal.Identifier)
      .eventsByPersistenceId("user-write", offset + 1, Long.MaxValue)
      .map {
        case EventEnvelope(_, _, seqNo, event: UserEvent) => seqNo -> event
      }
      .runWith(Sink.actorRef(self, StreamCompleted))
  }

  override def receiveCommand: Receive = {
    case GetUsers                    => sender() ! inMemoryUsers
    case GetUserByUsername(username) => sender() ! inMemoryUsers.find(_.username == username)
    // Match projected event and update offset
    case (seqNo: Long, UserAdded(user)) =>
      saveSnapshot(LastProcessedEventOffset(seqNo))
      inMemoryUsers += user
  }
}

有一些问题，例如：事件流似乎很慢。即UserRead演员可以在保存新添加的用户之前与一组用户进行应答。

编辑2

我增加了cassandra查询日志的刷新间隔，从而减少了事件流缓慢带来的问题。默认情况下，似乎Cassandra事件日志是每3秒进行一次轮询。在我的application.conf补充中：

cassandra-query-journal {
  refresh-interval = 20ms
}

编辑3

实际上，请勿减少刷新间隔。这将增加内存使用量，但这并不危险，一点也不重要。CQRS的一般概念是写入和读取端是异步的。因此，写入后，数据将永远无法立即读取。处理UI？在读取端确认后，我只是打开流并通过服务器发送的事件推送数据。