RAC

RAC (Real Application Clusters) is Oracle’s technology that allows multiple database instances to simultaneously access the same shared storage. If a node fails, the others continue serving requests without interruption — failover is transparent to applications.

How it works #

A RAC cluster consists of two or more servers (nodes) connected via a high-speed private network (interconnect) and shared storage (typically ASM — Automatic Storage Management). Each node runs its own Oracle instance, but all access the same datafiles.

The Cache Fusion protocol manages data coherence across nodes: when a block modified by one node is needed by another, it’s transferred directly via the interconnect without going through disk.

High availability #

If a node goes down, active sessions are automatically transferred to the remaining nodes via TAF (Transparent Application Failover) or Application Continuity. Failover time depends on configuration but is typically in the order of seconds.

Licensing implications #

RAC is an Enterprise Edition option with significant licensing costs. During cloud migration, RAC license counting is one of the most sensitive aspects: on OCI with BYOL, on-premises licenses are reused; on other cloud providers, the cost can multiply.

When it’s really needed #

RAC is justified when automatic failover high availability and horizontal scalability are required. For environments with few users or standard uptime requirements, a single node with Data Guard is often a simpler and less expensive solution.