So far in JBoss, the standard and easy way to deploy a service as a Highly Available (HA) Singleton (meaning it will run only on the master node) is to copy it on all nodes of the cluster, in the ./deploy-hasingleton directory. The way it works, is that a special service (hint: look in ./deploy/deploy-hasingleton-service.xml) will essentially deploy whatever is under ./deploy-hasingleton, on the master node only. This is actually how HA-JMS will run in a clustered setup.

Since JBoss 4.0.4/3.2.8 there is an alternative way to do this. You can deploy your service as you would normally do by copying it to ./deploy or ./farm, but remember to add a dependency on:

jboss.ha:service=HASingletonDeployer,type=Barrier

The service will be deployed normally on all nodes, up to the create step, but will only be started on the master node! The advantage comparing to the previous method is that you can use farming, i.e. automatic distribution of the deployment through the ./farm directory. Or do a normal deployment (copy to ./deploy) on all nodes or just the few ones that will potentially run the singleton service. Note that in both cases you get hot-redeployment (which you don't get with the standard approach).

On the other hand, a disadvantage, or better a difference is that the singleton service is actually deployed (up to the create step) on all the non-master nodes, whereas with the ./deploy-hasingleton approach, it's not deployed at all. So any singleton logic will have to be coded in the start()/stop() methods of the singleton service.

If you want to find out more details of how this is implemented, take a look here.

Cheers
dimitris@jboss

I'm very pleased to announce that the long awaited final (GA) release of the JBoss Application Server 4.0.4 is now available for download!!!

This is really a gargantuan release considering the number of bug fixes (290) and minor new features (70) that were incorporated, as well as other tasks (180) that were dealt with. Just have a look at the length of the Release Notes.

The .zip distro is the good and old canonical jboss server distribution with the three configurations that everyone is familiar with: minimal, default and all. Just unzip and you are all set. The installer version (which can now run headless, too) can be used to produce customized configurations and previews to latest technologies, like an EJB3 RC7+ preview that matches the current EJB3 draft standard.

A lot of interesting features come through updates to the various independent JBoss projects integrated under the JBossAS umbrella, the most prominent being the replacement of the axis-based jboss-ws4ee stack with the first official release of the all-new JBoss Web Services stack. This is developed using jdk5 and retrofitted for jdk1.4 JVMs using JBossRetro.

Tomcat is updated to version 5.5.17 and JBossCache now supports AOP-based, http field-level session replication. There are also updates to JBossAOP, Hibernate, JBossRemoting, JBossSerialization and more. (Even my pet project got a very interesting contribution). As usual, we expect your online feedback in the JBoss User Forums.

If you can make it, JBossWorld in Vegas will be an excellent opportunity to meet with the development team, discuss what you like or not, share experiences and learn about innovative uses of JBoss, while getting insights into current and future JBoss technologies and roadmaps (the conference agenda is loaded, take a look here).

A really big thank you to everyone that worked on this release and to the JBoss community as a whole for providing valuable feedback during the two CR stages we went through.

Enjoy & hope to see you soon!
dimitris@jboss