[cdi-dev] async activation on observers, why not CompletableFuture

Wed Apr 1 03:55:28 EDT 2015

Hi Romain,

Intersting proposal. As I felt reading you that we misse the CompletableFuture stuff in Java 8, I just repeat here that the agreed on fireAsync signatures

<U extends T> CompletionStage<U> fireAsync(U event);
and
<U extends T> CompletionStage<U> fireAsync(U event, Executor executor);

Yeah it’s CompletionStage because Jozef preferred using interfaces in our API, but I guess implementation will use CompletableFuture under the hood to avoid reinventing the wheel.

With this approach your example:

event.fireAsync(new LetTheWorldKnow()).thenRun(() -> System.out.println("We did it!"));

will work without adding constraint on observer signature.

Regarding the observer part, we already discuss similar approach. In a former version of my async event doc I proposed using return type on observer to do discrimination between async and sync and Mark made a suggestion near yours during this meeting:

http://transcripts.jboss.org/meeting/irc.freenode.org/cdi-dev/2015/cdi-dev.2015-02-25-17.06.log.html <http://transcripts.jboss.org/meeting/irc.freenode.org/cdi-dev/2015/cdi-dev.2015-02-25-17.06.log.html>

(search for the first “signature” in text)

The main drawback of this approach is to let end user generate the returned CompletableFuture. So each async observer should provide a way to construct this completableFuture. The second question is the type param of the returned CompletableFuture. Should we use raw type? Now we could imagine helped to do that but...

Don’t get me wrong, I’d love to find a solution based on this kind of idea, but I fear it will add more complexity than double activation.

Antoine

> Le 1 avr. 2015 à 09:15, Romain Manni-Bucau <rmannibucau at gmail.com> a écrit :
> 
> No, fireAsync is still needed for all the reason we mzntionned - strongest one being the fact we need a return type and cant change fire - but using the return type we have the double activation without introducing a new API. Said otherwise API stays natural on both sides which was my main fear with a fireAsync and an @ObservesAsync (or any other new api we talked about). And we have the bonus to be aligned on SE async which sounds quite interesting for the future.
> 
> Le 1 avr. 2015 08:48, "Jozef Hartinger" <jharting at redhat.com <mailto:jharting at redhat.com>> a écrit :
> So instead of calling observers asynchronously you suggest turning observers into producers of CompletableFuture that will then be completed asynchronously?
> 
> On 03/31/2015 06:21 PM, Romain Manni-Bucau wrote:
>> // fire side
>> event.fireAsync(new LetTheWorldKnow()).thenRun(() -> System.out.println("We did it!"));
>> 
>> // observer side
>> CompletableFuture iWantToKnow(@Observes LetTheWorldKnow event) {}
>> 
>> // impl behavior would be like CompletableFuture.allOf(allObserverReturnedInstances) to be aligned on CompletableFuture behavior I think
>> 
>> Am I clearer?
>> 
>> 
>> 
>> 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/>
>> 2015-03-31 18:15 GMT+02:00 Sven Linstaedt <sven.linstaedt at gmail.com <mailto:sven.linstaedt at gmail.com>>:
>> Hi Romain,
>> 
>> I am not sure, I have fully understand how an observer with CompletableFuture could look like. Could you give us an example?
>> 
>> Afair CompletableFuture was considered to be used in the "trigger" side in order to track async event invocation completion.
>> 
>> br, Sven
>> 
>> 2015-03-31 18:00 GMT+02:00 Romain Manni-Bucau <rmannibucau at gmail.com <mailto:rmannibucau at gmail.com>>:
>> Hi guys,
>> 
>> on async topic if I followed we are at the point where we are looking for an activation on the observer side.
>> 
>> Since Java 8 has now CompletableFuture it would be great to use it. Today the spec doesnt use observer returned values so it is mainly a bad practise to have one even if not strictly forbidden - BTW never saw it in real applications - plus spec is not compatible - not specified at all - with CompletableFuture since it is a new API so we can use it as a marker.
>> 
>> This is quite interesting for few reasons:
>> 1- we have our double activation
>> 2- API is user friendly (observer is async and has an async signature)
>> 3- open door for future async enhancements (hopefully not in CDI) with composition of these observers
>> 
>> 
>> Only point I'm not sure is should these observers support sync events. I don't see anything blocking to do it but can have missed something.
>> 
>> 
>> wdyt?
>> 
>> 
>> 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/>
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