[cdi-dev] Async events. We need ideas to improve CDI-499

[Jozef Hartinger]

Right, these usecases won't work together with async delivery. In an 
ideal world an event payload would contain all the state possibly needed 
by an observer and the payload would be documented to be fired async. In 
an ideal world users would not rely on any of those bullets you wrote as 
they would understand async. Same as you would not use of those bullets 
in MDB's onReceive(), @Asynchronous method or Akka actor's receive(). Or 
would you? Now obviously that is an ideal world far from the real one in 
which this is a risk.

On the other hand, the "async switch on both sides" option, which you 
prefer, is a huge barrier to adoption.

Seems that we currently have two options out of which none is a good one.

On 02/09/2015 11:12 AM, Mark Struberg wrote:
> Hi Jozef, here we go!
>
>
> 1.) accessing @RequestScoped beans in your Observer
>
> 2.) accessing @SessionScoped beans in your Observer
> 3.) accessing/relying on any transactional behaviour. This is really a boomer. Basically you break transactions that way.
>
> 4.) accessing @TransactionScoped beans in your Observer
>
> 5.) access/relying on any ThreadLocal in your Observer
> 6.) accessing attached entities in your Observers (they must only get accessed from a very single Thread according to the JPA spec)
> 7.) using an EntityManager in a parallel thread might give you unexpected results.
>
>
> There might be quite a few more. E.g. we need to specify that EJBs and other EE features need to work in such a new Thread, etc
>
>
> LieGrue,
> strub
>
>
> On Tuesday, 3 February 2015, 9:40, Jozef Hartinger <jharting at redhat.com> wrote:
>
>
>>
>> We should enumerate all the arguments supporting async flag on an observer. So far I have only seen one:
>>
>> - an observer accessing @RequestScoped state would no longer be able
>      to do so since it would be run in a worker thread
>> I am eager to hear more arguments as this single one may not be
>      enough to justify the observer-async-flag design decision.
>> Remember that introducing fireAsync() itself does not break any
>      existing application because existing applications are using fire().
>      It's when an existing application / library is modified to use
>      fireAsync() when the problem may occur. Such change should not be
>      done blindly. As with any other change, an author should consider
>      possible consequences of the change. Clearly documenting the fact
>      that fireAsync() processing is done in a different thread with a
>      different @RequestScoped state may be sufficient.
>> Jozef
>>
>>
>> On 02/02/2015 03:43 PM, Antoine Sabot-Durand wrote:
>>
>> Hi all,
>>>
>>>
>>>
>>> https://issues.jboss.org/browse/CDI-499 comes after a lot of discussion about async events.
>>>
>>>
>>> I think the solution exposed here is quite satisfying, yet the idea to need to activate async behaviour on the observer side doesn’t please a lot of us. It’ll be confusing for users to have to activate async from the firing end and consuming end to see it work :-(.
>>>
>>>
>>> I’d like to see alternative proposal to have this new feature as user friendly as possible and keep the retro-compatibility aspect.  We should find a better solution on our next meeting on wednesday.
>>>
>>>
>>> Antoine
>>>
>>>
>>> _______________________________________________
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