Chapter 22. EJB Clustering Concepts

Too Much Isn't Always a Good Thing

As builders of B2C and B2B e-commerce applications, we set out hoping that our applications work well and are well received by the intended audience. Sometimes, what gets overlooked is what happens if too many users like and use the applications that we build. For a brick-and-mortar business such as fast food, people will stand in line for a good hamburger or milkshake, but usually this is not true for users of Internet sites. One of the major conveniences of using a company's Web site is so you don't have to stand in line or wait for service. When users have to wait or be told to come back later, often they don't come back and they will just go down the virtual street to spend their money with another site.

As an enterprise application developer, you really can't afford not to think about how the system will react when the user load becomes too high. In reality, no Internet application can sustain an infinite user load. Even the best Internet applications have a breaking point where the end users notice the response time slowing so much that the visit is unpleasant. At that point, you might have just lost a customer. The idea is to design a system where that breaking point takes longer and longer to get to and the system can sustain a higher and higher load.

The other big concern is that of system failure. Enterprise applications generally are very complicated systems to build. Many components along the way can give out and cause a failure of the system. When this happens, an end user typically experiences a problem. Depending on the type and seriousness of the failure, the user might not be able to submit the order or browse the catalog, or maybe the user won't be able to view the site at all. In any case, you can probably bet that revenue has just been lost. Sometimes as developers, we get caught in the whole technical aspects of the application and forget that revenue is the main purpose for building the application in the first place. It's not about which enterprise technology is the coolest or easiest to build and deploy; the point of building the system is to fulfill a business requirement so that customers can view, buy, and sell their products over the Internet. When this is prevented from happening, the business is missing a huge market for its products and the objective of the application is not being fulfilled.

One of the primary solutions to deal with unexpected user load and possible system failure is to provide some type of load balancing in your enterprise application. Load balancing, replication, and failover are important characteristics of any scalable and reliable enterprise application. Although not the only way to achieve load balancing in an EJB application, clustering is one of the most supported ways of achieving this goal.