[cdi-dev] On @Observes for async events

Pete Muir pmuir at redhat.com
Wed Mar 18 11:14:53 EDT 2015 

I think I have misunderstood the async model that is being proposed. We are proposing that all the observers of an event fired asynchronously are all run on a single thread? (Not the thread that fires the event, another thread, but the same thread for each observer).

This would be a somewhat odd limitation in my mind. It’s certainly *not* something that is semantically intuitive if you have something like @Observes @Asynchronously on event observers, which definitely implies the event observation is asynchronous, not the event firing.

I would definitely prefer an asynchronous observation model, which I can’t see as being remotely compatible with observer ordering. The classic approach of using callbacks to identify when a method is done is inherently more powerful, and better understood.

Perhaps I’ve missed this, but what are the use cases for wanting to specify that the observation is done async on the observer? And how does this relate to the event firing methods ability to get a callback? I suspect you would need to retrofit the entire firing system to always return a callback if you wanted to allow the observer to control whether it was async or not.

> On 18 Mar 2015, at 10:11, Antoine Sabot-Durand <antoine at sabot-durand.net> wrote:
> 
> Support of ordering in async event was last discussed  yesterday in this thread (http://cdi-development-mailing-list.1064426.n5.nabble.com/Update-on-Async-event-doc-td5711176.html <http://cdi-development-mailing-list.1064426.n5.nabble.com/Update-on-Async-event-doc-td5711176.html>)
> I tend to agree with Jozef. It’s more consistent for user to allow ordering in async event as in sync.
> 
> Antoine
> 
> 
>> Le 18 mars 2015 à 11:05, Romain Manni-Bucau <rmannibucau at gmail.com <mailto:rmannibucau at gmail.com>> a écrit :
>> 
>> 
>> 2015-03-18 11:03 GMT+01:00 Antoine Sabot-Durand <antoine at sabot-durand.net <mailto:antoine at sabot-durand.net>>:
>> 
>>> Le 18 mars 2015 à 10:53, Romain Manni-Bucau <rmannibucau at gmail.com <mailto:rmannibucau at gmail.com>> a écrit :
>>> 
>>> 
>>> 2015-03-18 10:33 GMT+01:00 Antoine Sabot-Durand <antoine at sabot-durand.net <mailto:antoine at sabot-durand.net>>:
>>> 
>>>> Le 18 mars 2015 à 09:58, Romain Manni-Bucau <rmannibucau at gmail.com <mailto:rmannibucau at gmail.com>> a écrit :
>>>> 
>>>> 
>>>> 2015-03-18 9:55 GMT+01:00 Antoine Sabot-Durand <antoine at sabot-durand.net <mailto:antoine at sabot-durand.net>>:
>>>>> 
>>>>> Le 18 mars 2015 à 09:42, Romain Manni-Bucau <rmannibucau at gmail.com <mailto:rmannibucau at gmail.com>> a écrit :
>>>>> 
>>>>> Hi guys,
>>>>> 
>>>>> think Mark is right and a new API (as fireAsync) would be better for users for:
>>>>> - understanding
>>>>> - compatibility (think to custom extensions using this flag)
>>>> 
>>>> My point here is to avoid making CDI the EJB next by introducing @Async or @Asynchronous annotation in the spec. This kind of annotation should be shared by other specs and would have a better fit in concurrency utilities or commons annotations spec.
>>>> 
>>>> 
>>>> +1
>>>>  
>>>>> that said if we have @Async methods I think async observers are really useless, isn't it?
>>>> 
>>>> @Async is rather useless IMO when you see how easy it is do async operation with Java 8. On the other hand Async observers are a at a higher level since they are called thru the Container and that the fire point must know what’s going on at the other side.
>>>> 
>>>> 
>>>> don't get it, observer can use j8 then to do its stuff asynchronously so maybe the feature if finally useless assuming you run on j8.
>>> 
>>> And how do you make your fire point know that one of its observer will be asynchronous ? How do you manage event ordering (as we plan to support it for async event as well). I think we shouldn’t see the event bus as a standard method call but something more featured (think about parameter injection in observer methods or transactional behavior)
>>> 
>>> 
>>> then this is not asynchronism and just an observer queue which is something different IMO.
>> 
>> From user pov it’s async : "I fire an event and get the control back right away". The impl will make an observer queue in an other thread, but the user exp is totally different than classic fire().
>> 
>> 
>> this is discussable since it is not for 2 observers. If you use async it can be - often - cause of a long processing, if you have 2 long computing methods then you sequentiallize it which breaks the reason why you wanted async, no? 
>>>  
>>>> 
>>>> That said saying a method is async is still more elegant than firing it in a pool you don't control.
>>> 
>>> fireAsync will have a signature allowing you to pass your own Executor to give you better control on thread pool
>>> 
>>> 
>>> a default one should be bound to the bm - main case IMO. having an app executor is not a very nice solution in term of API. 
>> 
>> Yes it was already discussed, there will be 2 signatures for fireAsync().
>> 
>> 
>> 
>>>>  
>>>>> 
>>>>> 
>>>>> 
>>>>> Romain Manni-Bucau
>>>>> @rmannibucau <https://twitter.com/rmannibucau> |  Blog <http://rmannibucau.wordpress.com/> | Github <https://github.com/rmannibucau> | LinkedIn <https://www.linkedin.com/in/rmannibucau> | Tomitriber <http://www.tomitribe.com/>
>>>>> 2015-03-18 9:30 GMT+01:00 Arne Limburg <arne.limburg at openknowledge.de <mailto:arne.limburg at openknowledge.de>>:
>>>>> Hi Antoine,
>>>>> 
>>>>> The third bullet point in 10.5.1 of the CDI 1.1 spec states that the
>>>>> observer method must be called in the same transaction context as the
>>>>> event.fire(...) if it is no transactional observer (that is
>>>>> TransactionPhase.IN_PROGRESS).
>>>>> If the default behavior would be async, we would have to move the
>>>>> transaction context to another thread. To my best knowledge this would be
>>>>> the only situation in EE where this is the case.
>>>>> 
>>>>> Cheers,
>>>>> Arne
>>>>> 
>>>>> Am 18.03.15 09:21 schrieb "Antoine Sabot-Durand" unter
>>>>> <antoine at sabot-durand.net <mailto:antoine at sabot-durand.net>>:
>>>>> 
>>>>> >Hi Arne,
>>>>> >
>>>>> >Sorry can you explain why? This value allows observer to be called inside
>>>>> >or outside a transaction. What will be the compatibility issue?
>>>>> >
>>>>> >Antoine
>>>>> >
>>>>> >
>>>>> >> Le 18 mars 2015 à 09:05, Arne Limburg <arne.limburg at openknowledge.de <mailto:arne.limburg at openknowledge.de>> a
>>>>> >>écrit :
>>>>> >>
>>>>> >> Hi to all,
>>>>> >>
>>>>> >> I think the biggest issue with backward compatibility is, that the
>>>>> >>current
>>>>> >> @Observes annotation by default has TransactionPhase.IN_PROGRESS.
>>>>> >> I think we can¹t deal with this, if the default for observers would be
>>>>> >> async. So I think there is no way to specify async as default without
>>>>> >> loosing backward compatibility.
>>>>> >> Any other thoughts?
>>>>> >>
>>>>> >> Cheers,
>>>>> >> Arne
>>>>> >>
>>>>> >>
>>>>> >> Am 18.03.15 08:48 schrieb "Antoine Sabot-Durand" unter
>>>>> >> <antoine at sabot-durand.net <mailto:antoine at sabot-durand.net>>:
>>>>> >>
>>>>> >>> Hi all,
>>>>> >>>
>>>>> >>> Yesterday we had another meeting to try to find a better solution than
>>>>> >>> explicitly activating async event on observer, without no success. I
>>>>> >>> understand that we should go on on this feature so what I suggest is to
>>>>> >>> have a meeting (an hangout) with people that want to try to find a
>>>>> >>>better
>>>>> >>> solution. If we find something we¹ll do a last proposal, and in all
>>>>> >>>case
>>>>> >>> we¹ll adopt the woking solution next week for this point. People
>>>>> >>> interested with this please manifest yourself.
>>>>> >>>
>>>>> >>> If we have to go with opt-in (have to explicitly declare an observer
>>>>> >>> supporting async event) we also have to validate the decision to use a
>>>>> >>> member in @Observes (as it was decided before) or go back on that as
>>>>> >>> mMark keep asking by introducing a new annotation to add on the
>>>>> >>>observer
>>>>> >>> (@Async or something similar). As I said when we discussed this point,
>>>>> >>>I
>>>>> >>> prefer the member in @Observes but we may have overlooked issues linked
>>>>> >>> to backward compatibility.
>>>>> >>> A third solution might be to introduce an @ObserveAsync to declare an
>>>>> >>> asynchronous capable observerŠ
>>>>> >>>
>>>>> >>> I¹m waiting for active feedback from you to find the best solution
>>>>> >>>taking
>>>>> >>> ALL aspects (not only the technicals one) into account.
>>>>> >>>
>>>>> >>> Thanks,
>>>>> >>>
>>>>> >>> Antoine
>>>>> >>
>>>>> >
>>>>> 
>>>>> 
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