Yes, you are right that these events should be persisted atomically
Dne neděle 25. října 2015 0:25:02 UTC+2 Michael Omann napsal(a):
>
> You can (and should) avoid this by emitting 2 events during command
> handling instead if one.
>
> So instead of :
>
> AddExperience(10) -> ExperienceAdded(10) -> EarnLevel(1) -> LevelEarned(1)
>
> your should derive both Events from your player's state and atomically
> persist 2 Events using persistAll(...)
>
> AddExperience(10) -> Seq(ExperienceAdded(10), LevelEarned(1))
>
>
> It fundamental that those events are stored atomically (either both or no
> at all), otherwise your players state could be corrupted. Imagine the first
> event (ExperienceAdded(10)) is saved successfully while LevelEarned(10)fails
> to persist properly. During recovery your player would end up in an
> inconsistent state. Those two events must be stored in a single write
> operation.
>
>
>
>
> I also believe that using an observable member inside an actor breaks the
> actor model. Events received in the subscribe callback (e.g. onNext()) are
> bypassing the mailbox, adding an additional inlet to the actor. That leads
> to many problems, shared mutable state and multithreading inside the actor
> to name just two. I may be wrong. Maybe Patrik can comment on that.
>
>
> Mike
>
>
>
> On Tuesday, October 20, 2015 at 8:32:50 AM UTC+2, Adam Dohnal wrote:
>>
>> You understand it right. Unfortunately, your solution won't work. Suppose
>> player has 30/100 health (current/max) and 99/100 experience and two
>> messages are sent to him with this order:
>>
>> AddExperience(10)
>> TakeDamage(40)
>>
>> Correct behaviour is that first 10 xp is added to 109/100 ... level is
>> earned which cause to gain full hp 100/100. Only after then TakeDamage can
>> by applied which causes to change player's hp to 60/100.
>>
>> If you would send LevelEarned to self, it will be processed after
>> TakeDamage message and player will die.
>>
>> I would like to process each command at the time and persist ALL its
>> consequence. Problem is that my consequences are found asynchronously :(
>>
>> Dne pondělí 19. října 2015 12:15:33 UTC+2 Patrik Nordwall napsal(a):
>>>
>>> If I understand it correctly your question is about how to persist the
>>> LevelEarned event which comes via the
>>> asyncrounous player.observable.subscribe callback. Since this is an actor
>>> you must turn that callback into a message that is sent to self and when
>>> receiving that message you can persist it.
>>>
>>> /Patrik
>>>
>>> On Mon, Oct 12, 2015 at 3:17 PM, Adam Dohnal <dohna...@gmail.com> wrote:
>>>
>>>> Hello,
>>>>
>>>> I am new in Akka and I am trying to develop some mini project to test
>>>> some things (akka, rx, cqrs, es etc..)
>>>>
>>>> I think I understand the concept of cqrs/es and how it fit to akka. My
>>>> problem is probably more theoretical than technical.
>>>>
>>>> Let's say, I have class Player, which has several Attribute[T]
>>>> attributes ... these attributes can be for example player's current
>>>> health,
>>>> vitality, experience, level etc ... These attributes have dependencies ...
>>>> for example currentHealth is defined as 10 * vitality ... if experience is
>>>> > 100 new level is earns, which causes to add 1 vitality, which causes to
>>>> add 10 health ... Hopefully, you get the point.
>>>>
>>>> I like the idea to implement this using reactive streams. Each
>>>> Attribute[T] should be considered as Observable[T] and with all these
>>>> excellent operators I can model dependencies I have described before.
>>>>
>>>> Now I am trying to model each Player as actor, which can handle
>>>> commands (AddExperience(...), AddVitality(...) etc...) and produce events
>>>> (ExperienceAdded(...), LevelEarned(...)) which should be event sourced.
>>>>
>>>> Problem is, that when I am handling command I directly don't know what
>>>> events should be generated ... for example AddExperience(100) is handled
>>>> and after some validation ExperienceAdded(100) should be persisted to
>>>> event
>>>> store. When it success, it should be applied to my domain, which emit
>>>> value
>>>> change in that attribute observable and new level can be earned ... but
>>>> how
>>>> can I persist that event?
>>>>
>>>> I try to write some pseudo-code
>>>>
>>>> class ExamplePersistentActor extends PersistentActor {
>>>> val Player player = ... // domain
>>>>
>>>> val subscription = player.observable.subscribe {
>>>> // here I get asynchronously information about that level is increased
>>>> ... should I call persist here?
>>>> }
>>>>
>>>> val receiveCommand: Receive = {
>>>> case AddExperience(xp: Int) => {
>>>> persist(ExperienceAdded(xp)) { event =>
>>>> player.addExperience(xp) // should produce LevelEarned(1) event,
>>>> which should be also persisted
>>>> }
>>>> }
>>>> }
>>>>
>>>> So I have reference to Player which has observable of all events that
>>>> are produced ... but I don't how to persist them :/
>>>>
>>>> --
>>>> >>>>>>>>>> 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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>>>>
>>>
>>>
>>>
>>> --
>>>
>>> Patrik Nordwall
>>> Typesafe <http://typesafe.com/> - Reactive apps on the JVM
>>> Twitter: @patriknw
>>>
>>>
--
>>>>>>>>>> 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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