why ?

The difference between persist and persistAsync is that,

persist "stash"es (callback) handler execution
- then, state becomes from processingCommands to persistingEvents
persistAsyc does not stash (callback) handler execution
- then, state stays in persistingCommands un-changed

So if you mix up persist and persistAsyc (inside a single case of the receiveCommand method), persistAsync doesn't fulfill its purpose - persist will make your PersistentActor wait before moving back to processingCommand state although persistAsync is supposed to make PersistentActor come back to processingCommand asap.

what to do?

I found test cases ([1], [2]) of the mixed use cases. Can we remove these mixed-up test cases from PersistentActorSpec?

For the original purpose of this issue, we can add sample code like these in the doc:

//Actor code with persist
  var internalState: Int = 0
  override def receiveCommand : Receive = {
    case Evt(i) =>
      persistAsync(Evt(i))( _ => { incrementStateBy(i); } )
      persistAsync(Evt(i))( _ => { incrementStateBy(i); } )
      deferAsync(Evt(i))( _ => { sender() ! internalState } )
  }

//Test code
  "A persistent actor" must {
    "keep the order of handler execution with deferAsync" in {
      val persistentActor = system.actorOf(Props[MyPersistentActor])
      persistentActor ! Evt(1)
      expectMsg(2)
      persistentActor ! Evt(1)
      expectMsg(4)
    }
  }

//Actor code with persistAsync
  override def receiveCommand : Receive = {
    case Evt(i) =>
      persistAsync(Evt(i))( _ => { incrementStateBy(i); } )
      persistAsync(Evt(i))( _ => { incrementStateBy(i); } )
      deferAsync(Evt(i))( _ => { sender() ! internalState } )
  }

//Test code is omitted, as it must be same as above

Improve documentation on deferAsync #20873

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