One minute of system downtime can cost an organization anywhere from$2,500 to $10,000 per minute. Using that metric, even 99.9 data availabilitycan cost a company $5 million a year.
The Standish Group
Internet commerce demands 24x7 Web site availability. We need systems thatprovide zero planned downtime, so during routine maintenance, systemupgrades, etc., the system can still serve client requests. We also needsystems with zero unplanned downtime - one application server crashing orone computer accidentally turned off should not stop the system as a wholefrom working. We need to be able to handle far more concurrent requests thanone server can handle. To do all this we need clustering.
These objectives cannot be met by strict adherence to J2EE APIs. It'snot something the application does, but something the container orinfrastructure provides.
A cluster is a set of server nodes that cooperates to provide a morescalable and fault-tolerant server infrastructure for stateful and statelesscomponents. To external clients, a cluster appears as a single server thatservices requests with a single point of entry.
In this article we'll discuss clustering with regard to the J2EEplatform, focusing on the Web tier. We'll discuss the various tiers ofclustering, the mechanisms, and their performance/scalability ramifications,and illustrate how to design highly available, scalable, fault-tolerant, andperformant systems.
Tiers of J2EE Clustering
Clustering is not limited to any single tier. It can be achieved inevery tier - cache, Web (servlet, JSP), EJB, and even at the database (seeFigure 1).
Caches sit in front of Web servers caching Web content and acting asvirtual servers. They reduce the response time, offload the back-endservers, and help the Web tier handle the huge volume of client requests,thus increasing scalability.
The Web tier sits behind the cache layer. In a typical J2EE solution,the Web tier is made up of servlets and JSPs that are responsible fordynamic content generation.
Beyond the Web server tier is the EJB tier that runs stateless sessionbeans, stateful session beans, and entity beans.
Even the last layer of the database can now be clustered using adatabase cluster, as with Oracle9i Real Application Clusters.
Aspects/Facets of J2EE Clustering
The two major aspects of clustering are:
- Load balancing or front ending
- Fault tolerance or reliability
The idea behind load balancing is to distribute the load (from clientrequests) to multiple back ends. This enables a cluster (collection ofcooperating servers) to handle more requests/clients than an individualserver, thus providing scalability. A load balancer sits in front of theserver nodes and receives requests, then redirects these client requests tothe various back ends.
