[cdi-dev] On @Observes for async events
Jozef Hartinger
jharting at redhat.com
Thu Mar 19 08:09:34 EDT 2015
On 03/18/2015 04:14 PM, Pete Muir wrote:
> 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).
No, the current proposal is not that specific at this point do define a
particular architecture. The one you mentioned is one of the possible
options.
It has certain advantages:
- the caller of fireAsync() does not wait for observers to complete so
it is async for the caller
- observer notification works the same as way as we know today except
that in a different thread
- the common pattern of using mutable event payload on which observers
contribute would work fine - the payload does not need to be thread-safe
- it would be safely published to the notification thread by the CDI
implementation and there are no races between observers
- ordering (if needed) fits well
Disadvantages
- it does not eliminate the backward-compatibility problem completely
(different thread means different transaction context, CDI contexts and
thread locals)
- observers are not called in parallel
Another option would be to call the observers in parallel
Advantages:
- the caller of fireAsync() does not wait for observers to complete so
it is async for the caller
- observers are independent of each other
Disadvantages:
- it does not eliminate the backward-compatibility problem completely
(different thread means different transaction context, CDI contexts and
thread locals)
- the payload needs to be thread-safe
- ordering would not work here
- single event firing possibly results in multiple exceptions (tricky
for the caller to handle with CompletionStage API)
Another option is to combine those two and by default call the observers
in parallel. If for some reason an observer depends on a different
observer (by means of priorities) then the container guarantees that the
partial ordering of observer notification is met.
Advantages:
- the caller of fireAsync() does not wait for observers to complete so
it is async for the caller
- observers are independent of each other
- priority works the same in sync and async
Disadvantages:
- it does not eliminate the backward-compatibility problem completely
(different thread means different transaction context, CDI contexts and
thread locals)
- the payload needs to be thread-safe
- single event firing possibly results in multiple exceptions (tricky
for the caller to handle with CompletionStage API)
>
> 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.
In the current proposal, @Observes @Asynchronously is just an enabler
(for backward compatibility) of async event firing. It alone does not
mean anything.
>
> 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.
So basically kind of EJB's @Asynchronous methods applied to observers +
a callback on event.fire()? The downside I can see here is that the
event.fire() caller never knows whether the call will block or not
whereas with event.fireAsync() it would be guaranteed not to block.
>
> 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 <mailto: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)
>> 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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