Application Server Clustering

Though the clustering helps to improve the availability and scalability of the application, what extra benefits we get by doing the clustering at operating system level rather than doing application server level. What are the objectives we achieve by doing the clustering at OS level and application server level.

In general, operating-system level clustering (aka hardware clustering) is designed to manage hardware and os-level failures. These typically work by starting a backup server when a primary fails in such a way that it fully assumes the role of the primary. Failover generally involves re-assigning the failed server IP-Address to the backup (IP-takeover), re-permissioning file system access to the backup (if using a shared file system instead of replication) , and then running a script that you setup yourself to startup all your applications. This technology is older, takes more time to perform a failover, and is less able to fully utilize all of your hardware resources.

Application Server Clustering, or, more generally, software clustering, is far more capable and dynamic. First of all, the backup server is usually in at least a warm-standby mode and hopefully hot, meaning that it can immediately assume the primary’s responsibilities with very little delay. Second, advanced software-level clustering also supports load-balancing, so you never have “backup” hardware sitting idle. Instead of re-assigning IP addresses, applications that connect to your clustered environment must already be designed to check for service failure/availability on more than one destination host. Alternatively, you can use some kind of load balancer/traffic router that exposes the cluster as a single IP address. By focusing on the availability of the application service, any lower-level problems in the OS or hardware are automatically covered as well (assuming, of course, that they cause the software to crash).

A final difference is the dynamic nature of newer software clustering techniques. You can generally add or lose capacity on-the-fly with little or no visible impact to dependent applications. Hardware-level clustering is quite difficult to correctly setup and modify and requires fairly painful and regular testing.

Data Fabric technologies such as GemStone’s GemFire are designed to cluster-enable most types of applications with all of the positive benefits listed .

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