A database node consists of a set of processes, a dedicated area of shared memory, and one or more secondary storage devices. The database stores, updates, and retrieves session data. Each node has a mirror node, therefore nodes occur in pairs. In addition, to maximize availability, include two or more spare nodes, one in each DRU, so if a node fails a spare can take over while the node is repaired.
For an explanation of node topology alternatives, see Chapter 3, Selecting a Topology, in Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Deployment Planning Guide.