Introduction

Oracle Exadata Exascale Database Service on Oracle Cloud Infrastructure (OCI) enables elastic scaling of Oracle Real Application Clusters (RAC) databases. This document explains the architectural transition from a single-node RAC deployment to a two-node RAC deployment running Oracle Database 26ai.

Architecture Evolution

Benefits

• Active-active processing
• Rolling maintenance
• Runtime Load Balancing
• Higher availability

SCAN Architecture Deep Dive

Client
   |
SCAN DNS
   |
SCAN Listener
   |
Service Lookup
   |
Local Listener
   |
Database Instance

Components

• SCAN Name
• SCAN VIPs
• SCAN Listeners
• Node VIPs
• Local Listeners
• Services
• FAN
• ONS

High Availability Components

High Availability Infographic

CLIENTS
   |
SCAN
   |
+-----------------------------+
| SCAN VIP1 / SCAN LISTENER1 |
| SCAN VIP2 / SCAN LISTENER2 |
| SCAN VIP3 / SCAN LISTENER3 |
+-----------------------------+
              |
      Service Resolution
              |
     +--------+--------+
     |                 |
  DB26AI1          DB26AI2

 FAN / ONS / RLB / TAC

Availability Technology Comparison

Application Continuity (AC)

srvctl modify service \
-db MYCDB \
-service pdb1 \
-failovertype TRANSACTION \
-commit_outcome TRUE \
-replay_init_time 1800 \
-retention 86400

Transparent Application Continuity (TAC)

srvctl modify service \
-db MYCDB \
-service pdb1 \
-failovertype AUTO \
-session_state DYNAMIC \
-commit_outcome TRUE \
-replay_init_time 1800 \
-retention 86400

Service Configuration and Load Balancing

PDB1 =
 (DESCRIPTION=
   (LOAD_BALANCE=ON)
   (FAILOVER=ON)
   (ADDRESS=
      (PROTOCOL=TCP)
      (HOST=salesdb-scan.company.com)
      (PORT=1521))
   (CONNECT_DATA=
      (SERVICE_NAME=pdb1)))

srvctl modify service \
-db MYCDB \
-service pdb1 \
-clbgoal SHORT \
-rlbgoal SERVICE_TIME

RAC Validation Commands

crsctl check cluster -all
crsctl stat res -t
olsnodes -n

srvctl status database -db MYCDB
srvctl status service -db MYCDB
srvctl status scan
srvctl status scan_listener
srvctl status listener

Session and Memory Distribution Queries

These Structured Query Language (SQL) queries use Oracle dynamic performance views to compare user sessions and process memory across RAC instances and PDBs.

SELECT inst_id, COUNT(*)
FROM gv$session
GROUP BY inst_id;

SELECT s.inst_id,
       p.name pdb_name,
       COUNT(*) sessions
FROM gv$session s
JOIN v$containers p
  ON s.con_id=p.con_id
WHERE s.type='USER'
GROUP BY s.inst_id,p.name
ORDER BY p.name,s.inst_id;

SELECT
    s.inst_id,
    c.name pdb_name,
    ROUND(SUM(p.pga_alloc_mem)/1024/1024,2) pga_mb,
    COUNT(*) sessions
FROM gv$session s
JOIN gv$process p
  ON s.inst_id=p.inst_id
 AND s.paddr=p.addr
JOIN v$containers c
  ON s.con_id=c.con_id
WHERE s.type='USER'
GROUP BY s.inst_id,c.name
ORDER BY c.name,s.inst_id;

SELECT
    c.name pdb_name,
    ROUND(SUM(p.pga_alloc_mem)/1024/1024,2) total_pga_mb,
    COUNT(*) sessions
FROM gv$session s
JOIN gv$process p
  ON s.inst_id=p.inst_id
 AND s.paddr=p.addr
JOIN v$containers c
  ON s.con_id=c.con_id
WHERE s.type='USER'
GROUP BY c.name
ORDER BY total_pga_mb DESC;

SELECT
    s.inst_id,
    c.name pdb_name,
    COUNT(*) sessions,
    ROUND(SUM(p.pga_alloc_mem)/1024/1024,2) total_pga_mb,
    ROUND(SUM(p.pga_alloc_mem)/COUNT(*)/1024/1024,2) avg_pga_mb_per_session
FROM gv$session s
JOIN gv$process p
  ON s.inst_id=p.inst_id
 AND s.paddr=p.addr
JOIN v$containers c
  ON s.con_id=c.con_id
WHERE s.type='USER'
GROUP BY s.inst_id,c.name
ORDER BY c.name,s.inst_id;

High Availability Testing

srvctl relocate service \
-db MYCDB \
-service pdb1 \
-oldinst DB26AI1 \
-newinst DB26AI2

Best Practices

Conclusion

A two-node Oracle RAC deployment on Exadata Exascale provides active-active scalability, workload balancing, and high availability. Combining SCAN, FAN, RLB, AC, and TAC delivers a resilient architecture for Oracle Database 26ai workloads.

Glossary

These definitions summarize the acronyms used in this article and point to official Oracle documentation for deeper reference.

Official Oracle references

• Oracle Exadata Database Service on Exascale Infrastructure Overview
• What is Oracle Exadata Exascale?
• Oracle Real Application Clusters Administration and Deployment Guide, 26ai
• About the SCAN
• Ensuring Application Continuity
• Load Balancing of Connections to Oracle RAC Databases
• Oracle Real Application Clusters and Oracle Clusterware