Set Up Standby Server

1. Base Backup: Take a base backup of the primary server.
   pg base backup -h <primary-ip> -D /var/lib/PostgreSQL/12/main -U <replication-user> -P --wal-method=stream
   Create Recovery Configuration: Create a recovery.conf file in the data directory of the standby server.
   standby mode = on
   primary con info = host=<primary-ip> port=5432 user=<replication-user> password=<password>
   trigger file = '/tmp/promote.trigger'
   Start the Standby Server:
   sudo systemctl start PostgreSQL

Verify Replication

On the primary server, check the replication status.
SELECT  FROM pg stat replication;

Monitoring and Maintenance of the High-Availability Cluster

Monitoring Tools

To ensure the cluster is running optimally, use monitoring tools like:

• pgAdmin: A web-based GUI to manage PostgreSQL.
• Prometheus & Grafana: For metrics collection and visualization.
• pg stat statements: To monitor query performance and execution.

Routine Maintenance Tasks

1. Regular Backups: Implement a backup strategy using tools like pg dump or pgBackRest to create regular backups of the primary server.

2. Update and Patch Management: Regularly update PostgreSQL to the latest stable version to benefit from security patches and new features.

3. Log Management: Configure PostgreSQL logging to capture important events and errors for troubleshooting.

4. Health Checks: Perform routine health checks to monitor the status of primary and standby servers.

Failover Testing

Regularly test failover procedures to ensure that the standby server can seamlessly take over if the primary server fails. This involves promoting the standby server to primary and verifying that applications can connect without issues.

Handling Failover and Recovery

Automatic Failover Solutions

To automate failover, consider using tools like:

• Patroni: A template for managing PostgreSQL HA with etcd or Consul.
• pg auto failover: An extension that provides automated failover and monitoring.

Manual Failover Steps

If automatic failover is not configured, follow these steps for manual failover:

1. Promote the Standby Server:
   SELECT pg promote();

2. Update Connection Strings: Modify your application connection strings to point to the new primary server.

3. Reconfigure the Old Primary: After recovering the old primary, reconfigure it as a standby server and resynchronize it with the new primary.

Disaster Recovery

In case of a catastrophic failure, have a disaster recovery plan that includes:

• Off-site backups.
• Procedures for restoring data from backups.
• Steps for rebuilding the cluster in a different environment if necessary.

Best Practices for High-Availability PostgreSQL Clusters

Network Configuration

Ensure that the network between the primary and standby servers is reliable, with minimal latency. Consider using dedicated private networks for replication traffic.

Load Balancing Reads

Use a load balancer to distribute read queries across multiple standby servers, improving performance and reducing the load on the primary server.

Capacity Planning

Regularly assess the database load and storage requirements to ensure the cluster can handle growth. Adjust resources accordingly.

Security Measures

• Implement SSL/TLS for secure communication between nodes.
• Use strong authentication methods for database connections.
• Regularly review access permissions and roles.

Setting up a high-availability PostgreSQL cluster is a critical step for organizations looking to ensure data availability and minimize downtime. By following the steps outlined in this article, you can create a robust PostgreSQL environment that withstands failures and provides reliable access to your data. Continuous monitoring, maintenance, and testing are essential to ensure that the cluster remains healthy and ready to handle any challenges that may arise.