[cdi-dev] Update on Async event doc

Jozef Hartinger jharting at redhat.com
Mon Mar 16 05:33:56 EDT 2015

On 03/12/2015 01:56 PM, Antoine Sabot-Durand wrote:
> Hi all,
> I compiled all the feedback and decision we took regarding async 
> events and updated the Google Doc : 
> https://docs.google.com/document/d/1Tm_fZWS6569YqCH9lGxuqBGzS9gfupic-vv1dyKCUxk/edit?usp=sharing
> The following point stays open. I’d like to close them (if possible) 
> during the next meeting on Tuesday
> 1) Async delivery mechanism (comment by Jozef)
> Should we write in the spec about how threads for events delivery 
> should be used? Personally I’d rather not: I think this should be let 
> to implementation, the specification should only describe the expected 
> behavior (concurrency or not). now I may have missed something.
We should not specify technical details assuming we clearly define what 
guarantees are/are not available regarding ordering, visibility, 
concurrency, effects of exceptions, CDI/security/transactional context 
propagation and perhaps others I missed.
> 2) Exception Handling (comment by Jozef)
> I didn’t write anything about exception and we should decide what 
> happens if an exception occurs in an observer during async event 
> dispatch. I think that it shouldn’t impact other observers and that we 
> should stick to the way CompletionStage API works today.

What does it mean? CompletionStage API allows you to handle (a single) 
Exception. Would those exceptions be swallowed or provided to the caller 
> 3) Async event activation on both ends
> We all agree that we need to explicitly fire event asynchronously on 
> the producer side (fireAsync()). The discussion in 8.1 is about adding 
> a way to accept async call on the consumer (observer) side.
> a) As events are often triggered in other parts of the application 
> than the parts that consume them (most CDI framework lib fire events 
> foe end user code) preventing user to decide if an observer can be 
> called asynchronously could lead to issues and will prevent library 
> developper to use fireAsync() (in a defensive coding approach).
> b) On the other hand, when placed in the same application, it’ll be 
> very confusing for user to have to fireAsync() and enable async 
> observer to activate this new feature.
> I propose an opt-out approach. We add ‘asyncSupported' member to 
> ‘@Observes' annotation with ‘true’ as default value. So in case of b) 
> the end user won’t have to explicitly activate async on observer and i 
> case of a) user detecting issue coming from async treatment of an 
> event can explicitly declares one or more observer not compatible with 
> async resolution with @Observes(asyncSupported=False)
> 4) Support observer ordering with async events
> I think we should keep event ordering for synchronous event and ignore 
> this feature in async event. I don’t see obvious use case to be async 
> and ordered.
if an observer O1 defines priority P1 and a different observer O2 
defines P2 where P2 > P1 then it probably does that for a reason. Most 
likely because O2 depends on the state changes possibly performed by O1. 
I think this holds true no matter if the event is fired synchronously or 
asynchronously. Therefore, I think we should respect priority in both cases.

In addition, the observer ordering concept will be simpler if we treat 
observers the same way in both sync and async.
> 5) Context propagation
>  I understand that propagating contexts in async event would impact 
> easily context API. My only concern here is to be define async event 
> to keep this feature possible.
> If I forgot points please comment this mail and the doc so we can take 
> final decision during next meeting.
> Thank you
> Antoine
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