[seam-dev] [weld-dev] startup beans

[Denis Forveille]

  In fact, the real need we have is to have "ClusterScope" components that has a method annotated with "@Create" that "fires" only 
once per application in the cluster.

For example to a post a -unique per application- message in a JMS queue or send a mail saying "hey, the app is up", or do something 
in teh database *once* or this kind of thing

I thought that EJB 3.1 solved the problem..

Today as application scope components runs in all member of the cluster, we have 1 message in the JMS queue par cluster, and we have 
to develop code to deal with that.

So the questions is: Will a method annotated with @Create (seam 2 notation here) in a Singleton EJB fire only once for the cluster 
or not?

Denis

On 10/20/2010 12:53 PM, Dan Allen wrote:
> On Wed, Oct 20, 2010 at 12:41 PM, Denis Forveille <denis.forveille at gmail.com <mailto:denis.forveille at gmail.com>> wrote:
>
>      The main difference between EJB 3.1 singleton and servlet listener is that the EJB 3.1 solution starts only one component in one
>     member of the cluster.. Not the latter.
>
>
> I've seen questions about whether a singleton is in fact a unique member in a cluster. Since the Java EE spec doesn't specifically 
> address clustering (right?), this requirement is open for loose interpretation. In fact, section 4.8 of the EJB 3.1 spec even 
> suggests that this is not a guarantee:
>
> /A Singleton session bean is a session bean component that is instantiated once per application. In cases
> //where the container is distributed over many virtual machines, each application will have one bean
> //instance of the Singleton for each JVM./
>
>
>     So there is a difference between "application scope" and "application-as-deployed-in-cluster" scope.
>     Most of the so-called "ApplicationScope" components are not well named IMHO as there is one component per member in a cluster,
>     not 1
>     per application as deployed in a cluster...
>
>
> I would definitely agree with you there. The CDI tutorial in the Weld reference guide makes mention of a theoretical 
> @ClusterScoped which would be a more apt choice.
>
>
>     We have a complex custom in-house solution for this with seam 2 and jEE5/WebSphere 7 (based on distributed map in WAS)  and we are
>     waiting for EJB3.1 and WAS v8 to have an elegant solution for this and we hope that seam 3 will have a solution also
>
>
> And that's precisely the type of solution we want to provide in Seam 3 so that duplication of effort for such custom solutions can 
> be avoided. We certainly welcome your review, feedback or contribution in tackling this scope.
>
> Given that singleton session beans aren't likely unique in a cluster, then we really need to get serious about offering a 
> cluster-scoped annotation. Pete, JIRA?
>
> -Dan
>
> -- 
> Dan Allen
> Principal Software Engineer, Red Hat | Author of Seam in Action
> Registered Linux User #231597
>
> http://mojavelinux.com
> http://mojavelinux.com/seaminaction
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