Akka 持久化状态恢复

Akka 框架提供了一种称为持久化的机制，可确保消息和状态在系统崩溃或重启时不会丢失。在 Akka 的持久化中，状态存储在持久化存储区中，以便在发生故障时可以恢复。此外，Akka 还提供了一个称为 Akka Persistence Query 的组件，它允许在持久化存储区中查询和检索数据，以便在恢复时使用。

下面是一个基于 Akka Persistence 的状态恢复的示例：

1. 定义一个 actor，该 actor 会接收一些消息并更新状态：

import akka.actor.{ActorLogging, Props}
import akka.persistence.{PersistentActor, SnapshotOffer}

object CounterActor {
  def props(): Props = Props(new CounterActor())
  
  // 将消息包装到事件中
  sealed trait Event
  case object Increment extends Event
  case object Decrement extends Event
  
  // 定义状态
  case class CounterState(counter: Int)
}

class CounterActor extends PersistentActor with ActorLogging {
  import CounterActor._

  private var state = CounterState(0)

  override def persistenceId: String = "counter-persistence-id"

  // 更新状态
  private def updateState(event: Event): Unit =
    state = event match {
      case Increment => CounterState(state.counter + 1)
      case Decrement => CounterState(state.counter - 1)
    }

  override def receiveCommand: Receive = {
    case Increment =>
      log.info("Received Increment")
      persist(Increment)(event => {
        updateState(event)
        saveSnapshot(state)
      })
    case Decrement =>
      log.info("Received Decrement")
      persist(Decrement)(event => {
        updateState(event)
        saveSnapshot(state)
      })
    case "print" =>
      log.info(s"Current count is ${state.counter}")
    case SaveSnapshotSuccess(metadata) =>
      log.info(s"Snapshot saved successfully: $metadata")
    case SaveSnapshotFailure(metadata,