2.2. Sample Two-Node Cluster

RAC clusters are often configured to be symmetrical; the type of workload presented to the nodes is similar. In this topology, the servers are also of the same relative computing strength. Typically, the servers are over-configured by 50% in order for the failover node to handle the work of both nodes. However, this assumes that the business requirements for degraded operation are identical to normal operation, which is not always the case.

An alternative configuration is to build asymmetric topologies. In our simple two-node case, one node might be used to update the database (ETL - Extract, Transform and Load), and the other node may be used to run queries. Some nodes could be significantly larger in order to be dedicated to just one form of processing (e.g., Parallel Queries). Oracle RAC is not universally transparent to SQL workloads; awareness of when and where writes occur (SQL inserts, updates and deletes) can dramatically improve scalability, even in the two-node case.

Asymmetrical RAC topologies do have implications for failover, as a set of nodes might be tuned for queries and now must handle other work on behalf of the failed node. This topology is more common with higher node counts.