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.
