A Primer on J2EE Clusters for Webhosting - Multi-Tier Architecture With a Dispatcher
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A multi-tier architecture with a dispatcher can actually be called a cluster within the cluster, because there are separate server instances which host the Web presentation layer (HTTP pages, JSPs, and Servlets), while EJB and JDBC objects are located on separate machines. For big applications, a configuration of that kind could be the best choice, because it provides better load balancing and fail over. It also gives a chance to adjust to the application specifics. For instance, if your application relies heavily on HTTP but there are no frequent invocations of EJBs, you can have only two servers for EJBs, and perhaps five for the Web layer. But if there are a lot of static pages that need to be serviced, you can consider even a separate HTTP cluster.
Generally, the Web and the objects layer do not communicate intensively with each other. If they do communicate heavily, then a combined tier architecture could be more appropriate. The dispacher communicates with all server instances and redirects incoming and outgoing requests as necessary. All servers communicate with the database. For better redundancy, you can have a second, standby database.
The three architectures described here are neither the only ones, nor even the most typical ones. Since each application is unique, there is no single architecture that can be applied in all cases. The aim was more to give you a basic idea of how J2EE clusters work. When you come to the point of deploying your applications, you will no doubt have to read in more detail about the particular server you have chosen on which to deploy your applications.
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