[cdi-dev] [JBoss JIRA] (CDI-563) Event.fireAsync() - clarify the usage of the returned CompletionStage

Antoine Sabot-Durand (JIRA) issues at jboss.org
Tue Oct 20 07:54:00 EDT 2015

    [ https://issues.jboss.org/browse/CDI-563?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13119861#comment-13119861 ] 

Antoine Sabot-Durand commented on CDI-563:

{quote}bq. What do you think we should add?
I don't like the wording "called in a new lifecycle contexts". What exactly does it mean? Maybe we should just rephrase this.{quote}

I'm not a big fan of this wording either. But it came from discussion with Jozef, and the fact that for the sake of non-denormalize the spec we shouldn't repeat what is said in [6.7|http://docs.jboss.org/cdi/spec/2.0.EDR1/cdi-spec.html#builtin_contexts]:

bq. The context associated with a built-in normal scope propagates across local, synchronous Java method calls. The context does not propagate across remote method invocations or to asynchronous processes.

Perhaps we could find a way to have a clearer phrase pointing to this assertion. It lets the possibility for a third party to create a normal scope supporting propagation.

> Event.fireAsync() - clarify the usage of the returned CompletionStage
> ---------------------------------------------------------------------
>                 Key: CDI-563
>                 URL: https://issues.jboss.org/browse/CDI-563
>             Project: CDI Specification Issues
>          Issue Type: Clarification
>          Components: Events
>    Affects Versions: 2.0-EDR1
>            Reporter: Martin Kouba
> So far {{CompletionStage}} is only mentioned in "10.5.1. Handling multiple exceptions thrown during an asynchronous event" and the {{Event.fireAsync()}} javadoc is too general. However, the {{CompletionStage}} itself does not define an unambiguous contract for its methods. E.g. what thread is used to execute a given callback? Or what's the _"stage's default asynchronous execution facility"_? I believe this is left on implementors. In Weld 3.0 Alpha we're using {{CompletableFuture}} under the hood, and its more concrete in this area, e.g.:
> {quote}
> * Actions supplied for dependent completions of _non-async_ methods may be performed by the thread that completes the current CompletableFuture, or by any other caller of a completion method.
> {quote}
> So as a result, if an async delivery is finished before a sync dependent action is registered, the callback is executed in the caller thread:
> {code:java}
> event.fireAsync(new Message()).thenAccept((m) -> System.out.println("This might be executed in a caller thread or in a different thread!"));
> {code}
> And this might be confusing. Especially from the context propagation point of view. I think the spec should clarify the contract of a returned {{CompletionStage}}.

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