What is Oracle RAC?

Oracle RAC (Real Application Clusters) is a high-availability and scalability solution that allows multiple Oracle database instances running on different servers (nodes) to access the same shared database storage simultaneously.

In a RAC environment:

• One database
• Multiple instances
• Multiple servers (nodes)
• Shared storage

This provides:

• High Availability – if one node fails, other nodes continue serving users.
• Load Balancing – user connections can be distributed across nodes.
• Scalability – add more nodes to increase database processing capacity.

Key Components

1. Oracle Instance

Each node has its own:

• SGA (System Global Area)
• Background Processes (PMON, SMON, DBWn, LGWR)

2. Shared Storage

All RAC nodes access the same:

• Data Files
• Control Files
• Redo Logs (configured for RAC)

3. Clusterware

Oracle Clusterware:

• Monitors node health
• Manages cluster resources
• Handles failover

4. Interconnect

A private high-speed network between RAC nodes used for:

• Cache synchronization
• Cluster communication

How RAC Works

Suppose:

• Node 1 and Node 2 are active.
• Users connect through SCAN listeners.
• Sessions are distributed across nodes.

If Node 1 fails:

• Node 2 continues processing.
• Connected applications can reconnect automatically (depending on configuration).
• Database remains available.

RAC vs Single Instance