Today most of projects are build upon a composite structure that has multiple different APIs. Since many of them is unaware of the others and has its of implementation and dependency , every one of them should be have its own configuration which will make them work as a single application.
This is the hard point in fact "
To bundle them in single application"
An Example:
in a sample medium size application like AppFuse would be using
Hibernate/JPA,
Acegi,SpringFramework,JasperReports, JSF, Tomcat/Jetty and so on. it is all good even perfect when you are run in a single JVM. but if you want to build a cluster for only a load balancing you should configure every one by one.
In other world you be building a multi tier cluster for each APIs , which would be harder rhan implemation for some old fashioned singleton type application (a type of application which has lots of static variable and caches)
your applcation would be looking like this.
what should be your success criteria
performance : cluster should not slow down the application in a critical point
transaction: any rollback should be noticed
failover: in case of any instance fail over any other one can replace it.
seamless: the cluster mode should be seamless. should be hidden behind proper interfaces
today most open and commercial API support clusters. but in most cases you would implement a yous own schema for your own needs . before start you should read some articles about "Distributed Application" , you can use any distributable caching, maps, or JMS bridges between your components
