High Availability (HA) is an important concept in maintaining the availability of applications and databases in the event of a system failure or load spike. Load Balancers (LBs) are a key component in HA architectures that distribute traffic to various database servers to ensure optimal availability and performance.

Best Practices

Some best practices to keep in mind:

1. Load Balancer Setup:
   • Load Balancer Type: Use a load balancer that supports the distribution of traffic to the database with algorithms such as round-robin, least-connections, or IP hashes. Examples: HAProxy, NGINX, or AWS ELB.
   • Health Checks: Implement health checks on load balancers to monitor the health status of database nodes. If a node fails, the load balancer should automatically redirect traffic to healthy nodes.
2. Database Replication:
   • Master-Slave Replication: Configure master-slave replication where the master receives all write operations and the slave is used for read operations. This can reduce the load on the master server and increase HA.
   • Multi-Master Replication: For systems that require high write availability in multiple locations, multi-master replication may be considered. Make sure to handle conflicts effectively.
3. Automatic Failover:
   • Failover Mechanism: Use tools like Pacemaker or Patroni to detect failures on the primary database server and automatically failover to the secondary node without manual intervention.
   • DNS-based Failover: Use DNS-based failover to redirect traffic to another data center or region if the entire infrastructure goes down.
4. Data Consistency & Split-Brain Scenarios:
   • Quorum-based Systems: In a multi-node configuration, ensure that the system has a quorum mechanism in place to prevent split-brain scenarios, where two independent nodes consider themselves masters.

Using Oracle Connection Manager as a Load Balancer

Step 1: Installation of Oracle Connection Manager

Oracle Connection Manager (CMAN) is a proxy server that can be used to distribute client connections to Oracle databases.

1. CMAN Installation:Instalasi Oracle Connection Manager dilakukan melalui Oracle Universal Installer (OUI) yang tersedia dalam instalasi Oracle Database Client.
2. Connection Manager Configuration:After installation, the configuration file cman.ora which is usually in $ORACLE_HOME/network/admin/cman.ora.

Step 2: Using CMAN as a Load Balancer

On clients accessing the database, use the following configuration on the tnsnames.ora

Database Replication: Master-Slave Setup

Oracle provides the Data Guard feature for Master-Slave Replication implementations, which provides data replication from the primary database to the backup database.

Step 1: Configure the Primary Database

1. Enable Force Logging:To ensure all changes are logged in the redo logs.ALTER DATABASE FORCE LOGGING;
2. Enable Archivelog Mode:Make sure the database is running in archivelog mode.SHUTDOWN IMMEDIATE; STARTUP MOUNT; ALTER DATABASE ARCHIVELOG; ALTER DATABASE OPEN;
3. Create Standby Redo Logs:Standby redo logs are required for Data Guard configuration.ALTER DATABASE ADD STANDBY LOGFILE ('/u01/app/oracle/oradata/orcl/stdby01.log') SIZE 500M;
4. Create a TNS Entry for Standby:Add entries to the tnsnames.ora on the primary server that leads to standby.STDBY = (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.11)(PORT = 1521)) ) (CONNECT_DATA = (SERVICE_NAME = orcl_stdby) ) )
5. Backup Primary Database and Send to Standby:Use RMAN to create backups and send them to standby.
   RMAN> BACKUP DATABASE PLUS ARCHIVELOG;
   Then restore on standby.

Step 2: Configure Database Standby

1. Restore Backup on Standby:Restore Backup on Standby:RMAN> RESTORE DATABASE; RMAN> RECOVER DATABASE;
2. Start Standby Database:Start the database in standby mode.ALTER DATABASE MOUNT STANDBY DATABASE;
3. Enable Managed Recovery:Enable managed recovery for standby.ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT FROM SESSION;

Conclusion

• Load Balancer: Use Oracle Connection Manager to distribute the connection load evenly among database nodes.
• Master-Slave Replication: Use Oracle Data Guard to ensure real-time replication of data from primary to standby database.
• Multi-Master Replication: Using Oracle GoldenGate to implement bi-directional replication between multiple database masters, allowing each node to perform write operations and stay in sync with other nodes.
• Automatic Failover: Pacemaker and Corosync along with Oracle Data Guard provide an automated failover mechanism that can be installed in a self-managed VPS environment.
• DNS-based Failover: Bind9 can be used to manage DNS failover automatically by redirecting traffic to the standby node when the primary node fails.
• Data Consistency & Split-Brain Scenarios: Quorum-based systems configurations with STONITH protect databases from split-brains, maintaining data consistency across clusters.

Dengan konfigurasi ini, sistem database Oracle Anda akan memiliki tingkat ketersediaan tinggi dan perlindungan terhadap berbagai skenario kegagalan tanpa menggunakan layanan cloud eksternal.

Referensi External

• Oracle Documentation: High Availability
• Database Journal: Understanding Database Load Balancing
• Tech Target: Guide to Database High Availability

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