Design query: using getContext().getChild() & stop()

[Mahesh Govind]

Dear Experts ,

Could you please help me with a right design choice for the following scenario.

Use case  : 1000's of Network device   are controlled by a network  .

• Network devices will join and leave the network by sending  Join and Leave messages respectively to the network controller.

Joining scenario

• When controller get a Join message from a network device , 
  
• it will check whether an FSM actor exists for that network device by calling getContext().getChild("networkdevice_IP").
  
• "networkdevice_IP" is used to identify the actor.
  
• if the child lookup  returns null  , a new FSMActor will be spawned and join message is processed by the new FSMActor
  

Leaving scenario

• Network device will send a Leave message  to the controller
  
• controller will do getContext().getChild("networkdevice_IP") and get the right FSMActor.
  
• sends “leave message" to  this actor.
• While processing Leave message , FSM actor will terminate itself by calling stop() .
  

Complication

• Now there is a possibility  that while the stop() is being processed by FSMActor , a new Join may come from same network device (networkdevice_IP).
  
• Since stop() is asynchronous  getContext().getChild(network device) will still return the FSMActor (networkdevice_IP) , but if we send message to this actor , the message will go to DeadMessages.
• 
  

Design question .

• How to handle such scenario  using AKKA ? So that  we will not return a stale Actor using getContext().getChild()
  
• [one possiblity might be to leave this corner scenario and let the network protocol to handle this  with retransmission .But if retransmission is not there what to do ?]

With thanks and regards

Mahesh

[Michael Frank]

I can think of two solutions.

1. in your controller actor, maintain a separate mapping of "networkdevice_IP" -> ActorRef.  when you create a new child actor, don't give it a name, let akka pick a random unique name, then add the mapping.  when routing messages to the child, instead of using getChild(), look up the actor ref in your map.  when a child leaves, remove it from the map immediately, then stop it asynchronously by sending the 'leave' message.

2. in your controller actor, track child actors which are leaving, and stash messages destined to these actors until you receive termination confirmation.  you could again keep a mapping of "networkdevice_IP" -> ActorRef, but this mapping would only hold actors which are leaving.  when you receive a message for a child, first check if the child is leaving: if so stash() the message.  if the child is not in the leaving map, if the child exists, send the message to it.  if the child doesn't exist, create it.  when a child is leaving, put a watch on the child using context.watch(), add the child name to the leaving map, then send the 'leave' message to the actor. when you receive the deathwatch Terminated() message, call unstashAll() to flush any stashed messages.

i have used the first solution many times, its quick and easy.  solution two is applicable however if you are modeling a resource which cannot have two separate incarnations running at the same time.

-Michael

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[Mahesh Govind]

Thank You .

So in case 1 , you are recommending the child to send a message to  controller when it gets a leave message . 

The map(IP->actor) is maintained by controller .Controller will remove the child from map and kill the child ?

Since controller is processing messages one by one there will not  be any inconsistency ?

Please let me know whether my understanding is right .

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[Michael Frank]

reply inline.  hopefully this makes my solution more clear.

-Michael

On 04/14/16 02:37, Mahesh Govind wrote:

Thank You .

So in case 1 , you are recommending the child to send a message to  controller when it gets a leave message .

no.  i would use the same message flow as you described in your original email.  the only change i would make is instead of:

"controller will do getContext().getChild("networkdevice_IP") and get the right FSMActor."

i would instead get the child ActorRef by consulting the map, then immediately remove the child from the map.  by 'immediately' i mean remove the child from the map in this iteration of the actor receive loop.  since actors process their messages synchronously, there is no chance of inconsistency.

The map(IP->actor) is maintained by controller .Controller will remove the child from map and kill the child ?

yes.

Since controller is processing messages one by one there will not  be any inconsistency ?

yes.

[Mahesh Govind]

Thank you

[Michael Frank]

You're welcome :)  hope it helps.

-Michael