Configuring High Availability for Amazon RDS Clusters with Terraform: Avoiding Resource Recreation on Terraform Apply

Introduction

Amazon Web Services (AWS) offers a range of database solutions to meet your application’s data storage needs, and Amazon RDS (Relational Database Service) is one of the most popular choices. When using Terraform to provision and manage your AWS resources, it’s crucial to understand the impact of Availability Zones (AZs) on your RDS clusters. In this blog, we will explore why having less than three AZs configured for your aws_rds_cluster resource can lead to resource recreation on your next Terraform apply.

Understanding Amazon RDS Multi-AZ Deployments

High availability is a critical consideration for databases in production environments. Amazon RDS offers a Multi-AZ deployment option that leverages multiple Availability Zones to provide failover support and improve availability. When you create an Amazon RDS Multi-AZ instance, it automatically replicates your database across at least two AZs.

Why Three AZs Matter

To fully leverage the high availability features of Amazon RDS and to avoid resource recreation in Terraform, it’s essential to configure your RDS clusters to use at least three Availability Zones. Here’s why:

1. High Availability: Configuring your RDS cluster across three or more AZs ensures high availability. In the event of an AZ failure, Amazon RDS can promote a standby instance in another AZ to become the primary instance. This minimizes downtime and ensures uninterrupted database service.
2. Resource Recreation: Terraform is designed to ensure that your infrastructure is aligned with your desired configuration. When you change the number of configured AZs from less than three to three or more, Terraform recognizes this as a significant difference and plans to recreate the resources accordingly.
3. Resource Distribution: To meet the high availability requirements of a three-AZ configuration, your RDS cluster’s read replicas, failover instances, and other components may need to be redistributed across the new AZs. Terraform will handle this by recreating and reconfiguring the resources as necessary.

Handling Resource Recreation

When Terraform identifies that a change in the number of AZs for your RDS cluster requires resource recreation, it’s essential to manage the transition carefully:

1. Backup Data: Ensure you have recent and reliable backups of your RDS cluster data to prevent data loss during the resource recreation process.
2. Plan the Change: Carefully plan how you will transition from a configuration with less than three AZs to one with three or more. Understand how it will impact your RDS cluster and its availability.
3. Terraform State Management: Use Terraform state management practices to keep track of the changes and ensure that Terraform understands the new desired state of your RDS cluster.
4. Rolling Updates: Consider a phased approach to the resource recreation to minimize service disruption and potential downtime.

Conclusion

Configuring your Amazon RDS clusters to use at least three Availability Zones is a best practice for achieving high availability and fault tolerance. If you find yourself in a situation where you need to change the number of AZs, Terraform will flag this as a significant difference, and resource recreation will be required. Proper planning and execution of this change are essential to maintain the health and resilience of your RDS clusters. Always prioritize high availability when configuring your RDS resources with Terraform in AWS.