Application Availability

Availability should be defined not just at the application level scope but also at each functionality level. It should be further quantified in terms of acceptable response time.

Availability of an application is increased by getting rid of single point of failure and this is done by converting this point into a pool of points so that if one goes down, the request is served by another point. Pooling the point is more commonly referred as horizontal scaling and building for it requires load balancing the requests hitting the application plus a provision for fail over support.

Load balancing and fail over support is done through deploying the application in a clustered environment. Clustering not only ensures high availability but also make the application more scalable. Clustering is the term which describes the was to provide redundant servers to ensure high availability.

Clustering is OK but what to cluster, anything where we are calling a distributed object, so need to cluster those components that can be deployed in a distributed way.

How to scale a J2EE application?

Some quite simple and basic points.

• Get rid of single point of bottleneck : There can be any point across the whole architecture of a layered lava application which can act as a constraint from the application to scale . This point is know as the single point of bottleneck, Single point of bottleneck can be any service, resource or server that all the request shares and goes through, examples can be connection pools, singletons, databases, JMS queues, web services etc. First we need to identify that constraint and then identify the ways to remove that constraint. This should be done till we end up finding another constraint.

 

• Do not use databases for everything : Do not persist high volume stuff which is of very little return like logging info, http session state (there are much better performing options for caching stuff like these) into the database. Put only those things which are persistent and transactional.