Hmm I don't know why you would think there is a race condition. The code
calls an Await() on all the futures from the ASK to Order. Furthermore, the
method NEEDS the result to the call to Order to finish the JSON map. How
would you do this without a list of futures? Remember all of the data from
the Order needs to be in the map before its returned.
On Wednesday, August 26, 2015 at 1:53:19 PM UTC-5, Adam wrote:
>
> Compiler or not, the code above has a race condition and I'd bet that it
> would return an empty map in most cases. The actor is sending the reply
> without waiting fir the future to end. (It should not deadlock BTW). If
> anything, I'd pipe the result to the sender.
>
> But a future is not needed at all in this case. Once an actor exits the
> receive method it's free to process the next messages in it's mailbox. As
> you haven't written any code that waits, a deadlock is impossible. That
> said, it is possible to create a deadlock in akka (or any other
> multithreaded framework). It's just harder, because everything is more
> explicit and "waiting" is not even one if the primitives supported by
> actors - you have to implement them.
>
> For this use case you need to define a protocol of messages between the
> two actors (if they can be remote I'd include handling of dropped
> messages). Such a protocol can distinguish between the initial request to
> the customer actor and subsequent requests if that turns out to be needed.
>
> BTW, you should always send immutable data between actors and there's no
> need for the default case where you call unhandled, if your using the Scala
> API.
> On Aug 26, 2015 9:35 PM, "kraythe" <kra...@gmail.com <javascript:>> wrote:
>
>> Either way I have to return just the map to the sender that called with
>> OrderHistory. In fact the sender could be on another node in the cluster so
>> sending the futures is not a viable solution. This is a multiple node
>> environment where the actors MUST run local to where Hazelcast stores the
>> object to avoid heavy serialization overhead. In fact each of the orders
>> themselves might be on another node.
>>
>> However the principle question remains, "Will what I posted deadlock?"
>>
>> -- Robert
>>
>> On Wednesday, August 26, 2015 at 1:27:38 PM UTC-5, rrodseth wrote:
>>>
>>> Have you looked at Future.sequence? It turns a List[Future[A]] into a
>>> Future[List[A]].
>>>
>>> Then you can pipe that result to the same actor or another, or add a
>>> complete handler as you have done.
>>>
>>> On Wed, Aug 26, 2015 at 11:17 AM, kraythe <kra...@gmail.com> wrote:
>>>
>>>> I am using play framework with a mix of scala and java. I can return a
>>>> promise to the play framework and that promise can call the actor to get
>>>> the information in a future. No Problem there. The problem is when an
>>>> actor
>>>> (A) needs something from another actor (B) and B needs something from A. A
>>>> sends to a list of Bs and B sends a different message to A and then
>>>> composes it. Think of generating JSON from a Customer -> Order Situation
>>>> where Orders can have several different customers. The order JSON needs to
>>>> have info from the customer in detail but for each of the orders we have
>>>> to
>>>> return all customer ids involved in the order. At some point I need to
>>>> collect the results from all the calls to B and shove them into the result
>>>> map. I can pass the map in the message to B and have it add the data but
>>>> then if I have a pool of 5 actors and 5 customers are processing then when
>>>> it calls back into A to get the customer ids of other customers involved
>>>> in
>>>> the order, it needs to use an instance of actor A in the pool to
>>>> accomplish
>>>> this. If they are waiting on the completion of a future then that cant
>>>> happen.
>>>>
>>>> An example of what I am talking about is below. Please excuse that I
>>>> coded this cold without compiling but hopefully you get the idea.
>>>>
>>>> class CustomerActor extends Actor {
>>>> override def receive: Receive = {
>>>> case msg: OrderHistoryJsonMsg =>
>>>> val customer = Customer.findInCacheById(msg.customerId)
>>>> val jsonMap = scala.collection.mutable.Map()
>>>> // init JSON with customer data.
>>>> val selection = Akka.system().actorSelection("/user/orderActors")
>>>> val futures = customer.orderIds.map(orderId => {
>>>> val future = selection ? new OrderJsonMsg(orderId)
>>>> future.onComplete(r => {
>>>> // add data to JSON.
>>>> })
>>>> future
>>>> })
>>>> futures.foreach(f => Await.result(f, 1 seconds))
>>>> sender ! jsonMap
>>>> case msg =>
>>>> unhandled(msg)
>>>> }
>>>> }
>>>>
>>>> class OrderActor extends Actor {
>>>> override def receive: Receive = {
>>>> case msg: OrderJsonMsg =>
>>>> val jsonMap = scala.collection.mutable.Map()
>>>> // init JSON with order data.
>>>> val order = Order.findInCacheById(msg.orderId)
>>>> // NOTE HERE WE SEND MESSAGE BACK TO CUSTOMER ACTOR
>>>> val selection = Akka.system().actorSelection("/user/orderActors")
>>>> val futures = order.customers.foreach(customerId => {
>>>> val future = selection ? new CustomerNameMsg(customerId)
>>>> future.onComplete(r => {
>>>> // add data to JSON.
>>>> sender ! jsonMap
>>>> })
>>>> })
>>>> case msg =>
>>>> unhandled(msg)
>>>> }
>>>> }
>>>>
>>>>
>>>> On Wednesday, August 26, 2015 at 12:22:02 PM UTC-5, rrodseth wrote:
>>>>>
>>>>> Robert, in my case the REST endoint is using Spray. The per-request
>>>>> actor has a reference to the RequestContext, and calls complete() on it,
>>>>> before stopping itself.
>>>>>
>>>>> I don't have time to check, but it might be modelled on this Activator
>>>>> Template (which I think is referenced in the net-a-porter blog post).
>>>>>
>>>>> http://www.typesafe.com/activator/template/spray-actor-per-request
>>>>>
>>>>> On Wed, Aug 26, 2015 at 9:37 AM, Viktor Klang <viktor...@gmail.com>
>>>>> wrote:
>>>>>
>>>>>> I still have no idea what you mean by "wait" here. Do you mean
>>>>>> "block"?
>>>>>> Please either show some code to illustrate or be more specific, it's
>>>>>> impossible to answer otherwise and it just ends up being a lot of
>>>>>> replying
>>>>>> for nothing. :(
>>>>>>
>>>>>> --
>>>>>> Cheers,
>>>>>> √
>>>>>> On 26 Aug 2015 08:28, "kraythe" <kra...@gmail.com> wrote:
>>>>>>
>>>>>>> If you need the results of the generated futures then it waits for
>>>>>>> them all to be completed. Process A needs the results of Process B in
>>>>>>> order
>>>>>>> to complete. This is a pretty common paradigm in our software.
>>>>>>>
>>>>>>> -- Robert
>>>>>>>
>>>>>>> On Tuesday, August 25, 2015 at 9:43:11 PM UTC-5, √ wrote:
>>>>>>>>
>>>>>>>> how does that wait?
>>>>>>>>
>>>>>>>> --
>>>>>>>> Cheers,
>>>>>>>> √
>>>>>>>> On 25 Aug 2015 18:17, "kraythe" <kra...@gmail.com> wrote:
>>>>>>>>
>>>>>>>>> Call another actor using an ask pattern.
>>>>>>>>>
>>>>>>>>> On Tuesday, August 25, 2015 at 7:22:00 PM UTC-5, √ wrote:
>>>>>>>>>>
>>>>>>>>>> define "wait for the responses to send back to the caller"
>>>>>>>>>>
>>>>>>>>>> On Tue, Aug 25, 2015 at 12:04 PM, kraythe <kra...@gmail.com>
>>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>>> Lets say that we create a pool of 5 actors using round robin
>>>>>>>>>>> logic and then we send 5 messages at the same time and those
>>>>>>>>>>> messages in
>>>>>>>>>>> the course of being processed send another 5 messages each to the
>>>>>>>>>>> actor and
>>>>>>>>>>> wait for the responses to send back to the caller. If this was a
>>>>>>>>>>> java
>>>>>>>>>>> executor it would eventually deadlock waiting for the second set of
>>>>>>>>>>> calls
>>>>>>>>>>> to complete. Is the same deadlock possible with Akka? If not then
>>>>>>>>>>> what
>>>>>>>>>>> would prevent it?
>>>>>>>>>>>
>>>>>>>>>>> Thanks in advance.
>>>>>>>>>>>
>>>>>>>>>>> --
>>>>>>>>>>> >>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>>>>>>> >>>>>>>>>> Check the FAQ:
>>>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
>>>>>>>>>>> >>>>>>>>>> Search the archives:
>>>>>>>>>>> https://groups.google.com/group/akka-user
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>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>> Cheers,
>>>>>>>>>> √
>>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> >>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>>>>> >>>>>>>>>> Check the FAQ:
>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
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>>>>>>>>>
>>>>>>>> --
>>>>>>> >>>>>>>>>> Read the docs: http://akka.io/docs/
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>>>>>> --
>>>>>> >>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>> >>>>>>>>>> Check the FAQ:
>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
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>>>>>>
>>>>>
>>>>> --
>>>> >>>>>>>>>> Read the docs: http://akka.io/docs/
>>>> >>>>>>>>>> Check the FAQ:
>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
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>>>>
>>>
>>> --
>> >>>>>>>>>> Read the docs: http://akka.io/docs/
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>> http://doc.akka.io/docs/akka/current/additional/faq.html
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>
--
>>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>>>>>> Check the FAQ:
>>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
>>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user
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