Both topologies must meet the following general requirements:
Machines that host HADB nodes must be in pairs. That is, there must be an even number of them.
Machines that host the HADB nodes must run the same operating system. It is best to use identical or nearly identical machines, in terms of configuration and performance.
For HTTP and SFSB session information to be persisted to the HADB, the Enterprise Server instances must be in a cluster and satisfy all related requirements. For more information on configuring clusters, see Chapter 2, Setting Up Clusters in Enterprise Server, in Sun GlassFish Enterprise Server 2.1 High Availability Administration Guide.
Machines hosting the Enterprise Server instances must be as identical as possible, in terms of configuration and performance. This is because the load balancer plug-in uses a round-robin policy for load balancing, and if machines of different classes host instances, then the load will not be balanced in the most optimum way across these machines.
Preferably have a separate uninterruptible power supply (UPS) for each DRU.