Is your Java Application Server clustered? Why not? It should be, and your reasons for NOT clustering have vanished, so there's no excuse. Cluster. Now.
When designing a web architecture for an application, this question always comes up: "To cluster or not to cluster?" Clustering allows two or more app servers to act as one. This has many advantages, such as high availability if one server goes down, or the cluster can enable seamless upgrades to software or hardware. Clustering also allows a successful application to scale by spreading users across more and more servers. Unfortunately, clustered servers are much too rare today. But they shouldn't be rare, clustering should be the default, the normal case.
Question: To cluster or not to cluster?
At first the answer is "no", I mean... clustering make the installation more complex, there are performance impacts, and configuration problems and... well... RISK! But the benefits can be pretty great for an app that is growing. So if we can eliminate the RISK, then we have no reason NOT to cluster.
Answer: Always Cluster.
The time for clustering every Java application has come. Worrying about statefullness was a habit we acquired when SFSBs in EJB2 were dangerously bad at clustering (and a bad solution for many applications). But today, JBoss, Tomcat, and other Java app servers have matured to make clustering a solid part of any server. Terracotta has even come in to save the day for clusters with large node counts, or with huge memory needs, so the reasons not to cluster become smaller.
RedHat is telling everyone to virtualize Linux.. ALWAYS! And it makes sense. Servers out there need the flexibility of virtualization, and the reasons to NOT virtualize have vanished. It's time has come. Technologies like Xen and VMWare have improved, but more than the technologies, the people have matured to understand the benefits and risks of virtualization. So now it's packaging and marketing that are driving virtualization as the default answer.
Java application server clustering should now follow and become the default installation. The technologies are solid, so the clustering idea is just in need of good packaging such as easier installation and configuration utilities, and good marketing in the form of endorsements and documentation about the many benefits of clustering.
- High Availability - Server crashes, app stays up, 'nuf said, this bene' is obvious.
- Scalability - more users? add more servers, clustering lets your app grow - also an obvious bene'
- Hot Deployment - this bene' is rarely understood, but it will save your weekend! Stop deploying on a weekend at 2am! Deploy on Tuesday at 11am, yes, while users are still using the system.... You see, most code changes do not require huge data conversions and outages (although those cases do happen). Most deployments are to fix a bug, change a label, add a brand new feature, or add more memory to the server - and those changes do not require downtime if I have a cluster and a deployment process that can make it HOT. Its quite simple - 1)take a server node out of the cluster, 2) upgrade the code, 3) put it back in the cluster, 4)repeat on other nodes. No late nights, no weekends -and no lost sleep. Developers and admins are handy and awake if anything goes wrong, no need to start that late-night conference call. Freedom...
Believe it? You may not, but I've witnessed it. My system took a millions of hits a day, and we deployed twice a day at times - between 8 and 5. We had a cluster of JBoss application servers and did the deployments in broad daylight without any affect on users. And all this with out-of-the-box clustering from JBoss.
If you want to talk RISK - try taking an outage at 2am on a Sunday for 5 hours after 3 months of intense development by a large team. Put in a massive code change and pray that the deployment goes well... Now that's the stuff that nightmares are made of.
So cluster your Java servers, the benefits are big, and clustering comes with every application server - it probably came with the server you've got. No excuses, do it now.