This section describes the requirements that are common to both topologies:
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 Application Server instances must be in a cluster and satisfy all related requirements. For more information on configuring clusters, see Chapter 5, Using Application Server Clusters, in Sun Java System Application Server Enterprise Edition 8.1 2005Q2 High Availability Administration Guide.
Machines hosting the Application 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.
Each DRU contains a complete copy of the data in HADB and can continue servicing requests if the other DRU becomes unavailable. However, if a node in one DRU and its mirror in another DRU fail at the same time, some portion of data is lost. For this reason, it is important that the system is not set up so that both DRUs can be affected by a single failure such as a power failure or disk failure.
Follow these guidelines when setting up the HADB nodes and machines:
Set up each DRU with a number of spare nodes equal to the number of nodes running on each machine. This is because if each machine in the configuration runs n data nodes, the failure of a single machine brings down n nodes.
Run the same number of HADB nodes on all machines to balance load as evenly as possible.
Do not run nodes from different DRUs on the same machine. If you must run nodes from different DRUs on the same machine, ensure that the machine can handle any single point of failure (for failures related to disk, memory, CPU, power, operating system crashes, and so on).
Both the topologies have Application Server instances in a cluster. These instances persist session information to the HADB. Configure the load balancer to include configuration information for all the Application Server instances in the cluster.
For more information on setting up a cluster and adding Application Server instances to clusters, see Chapter 5, Using Application Server Clusters, in Sun Java System Application Server Enterprise Edition 8.1 2005Q2 High Availability Administration Guide.