我正在尝试在我的 ES-CQRS 架构中实现读取端。假设我有一个这样的持久演员：
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，以便我可以将它保存在我的读取端数据库中？其中一个想法是最初UsersStream从我的读取方actor向UserWriteactor发送消息，并在该读取actor中“接收”事件。
编辑
根据@cmbaxter 的建议，我以这种方式实现了读取端：
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用一组用户回答。
编辑 2
我增加了 cassandra 查询日志的刷新间隔，这更多地解决了慢事件流的问题。默认情况下，Cassandra 事件日志似乎每 3 秒轮询一次。在我的application.conf我补充说：
cassandra-query-journal {
refresh-interval = 20ms
}
编辑 3
实际上，不要减少刷新间隔。这会增加内存使用量，但这并不危险，也不是一点。CQRS 的一般概念是写入端和读取端是异步的。因此，在您写入数据后将永远无法立即读取。处理用户界面？在读取端确认它们之后，我只需打开流并通过服务器发送的事件推送数据。