Validating Multi-Region DR for Terraform Using AWS FIS

Back in October 2025, Athenahealth, a big name in North American Electronic Health Records, faced a major hiccup. An AWS regional service disruption in us-east-1 knocked out their single-region HashiCorp Terraform Enterprise deployment. This was a wake-up call about needing a strong multi-region disaster recovery (DR) setup to keep vital workloads going when things go south. The real fix was testing their multi-region DR approach with AWS Fault Injection Service (FIS), as laid out in the source article.

Why Multi-Region DR Matters

Right now, Terraform Enterprise is only set up for single AWS regions. If a region goes down and you haven't tested your DR plan, teams might get stuck unable to deploy, change, or recover infrastructure. That's a risk you don't want to take. A multi-region DR plan helps, but you have to make sure regional failover works before you're in trouble. AWS FIS can help you stress test this by creating real failures in your AWS setup, which reveals hidden issues and dependencies.

Setting Up Multi-Region DR

Athenahealth teamed up with AWS and HashiCorp to build a user-operated multi-region DR model. They went with an active-passive design spanning us-east-1 as primary and us-west-2 as a DR site. They aimed for a recovery time objective (RTO) of 12-14 minutes and a recovery point objective (RPO) of under a minute. Their plan involves constantly replicating data to the DR spot, while keeping compute costs low by running resources at zero until needed.

Core Architecture Components

• Traffic Routing: Uses Amazon Route 53 DNS alias records aimed at an ELB Network Load Balancer. The records have a TTL of 60 seconds, so clients switch to the DR region within a minute, instead of waiting out long cache times.
• Compute Management: Keeps the DR site's compute power at zero during normal times with Amazon EC2 Auto Scaling groups managing TFE instances. By setting the minimum capacity to 0, they save on costs but can quickly scale up during failover.
• Database Replication: Uses Aurora PostgreSQL-Compatible global databases for cross-region replication, giving sub-second lag and managed failover. The main cluster has one writer and two readers across three Availability Zones; the backup cluster holds an inactive writer ready for promotion.
• State File Replication: Replicates Terraform workspace state files between the main and DR region's S3 buckets with S3 cross-region replication. This two-way replication helps with failback without needing data resync.
• Credential and Encryption Key Management: Handled by AWS Secrets Manager and AWS Key Management Service (KMS) across the regions. It's crucial to copy the TFE encryption password, which protects the internal Vault unseal key and root token, to the DR region and ensure it's in the DR launch setup.

Failover Process

The failover involves four steps:

1. Activate DR Auto Scaling Group: Scale up from 0 to 1 instance and do health checks. TFE's /health_check endpoint gives a 200 OK when it's up, and this is what the load balancer and Route 53 health check use to verify instance health.
2. Promote Database: Boost the DR writer in Aurora to stop split-brain issues. This step must happen before DNS switches traffic to the DR site to make sure only one region can handle writes.
3. DNS Failover: Pre-set Route 53 failover routing policy directs traffic to the DR region’s Network Load Balancer. This happens internally in Route 53, so there's no need for control plane API calls during events, which you're supposed to avoid.
4. Scale Out: Increase Auto Scaling group capacity for the production load while keeping an eye on things with Amazon CloudWatch. This ensures the DR site can take the full production load after failover finishes.

Validating with AWS Fault Injection Service

To prove the multi-region setup works under real failure conditions, AWS FIS was used in the DR workflow. Athenahealth confirmed their architecture's resilience by running tests in three phases: compute failures, database failover, and a simulated S3 outage.

Phase A: EC2 and Auto Scaling Group Failure

They expected TFE instances to bounce back in five minutes if stopped or if EC2 capacity was lost. AWS FIS proved this right, with the Auto Scaling group triggering replacements in 2-3 minutes. This phase caught an outdated AMI reference in the DR region’s launch template, which is a big find for keeping configurations updated. In my view, keeping AMI references current is a must to ensure that features and security updates are covered during launches.

Phase B: Aurora Database Cluster Failover

If the Aurora cluster failed, TFE was supposed to restart writes in a minute. AWS FIS verified this expectation, promoting the secondary cluster’s writer in 58 seconds. Yet, it found that reconnection took longer because of TFE's connection pool settings, leading to tweaks that sped up recovery. From what I've seen, regularly checking and adjusting connection settings helps reduce recovery times significantly.

Phase C: Simulated S3 Connectivity Loss

This phase checked how S3 access loss affected Terraform state file availability. AWS FIS simulated a network partition cutting S3 access in the primary region, proving that cross-region replication upheld state file availability at the DR site. This showed how critical it is to set up and monitor replication policies correctly.

Practical Steps for Implementing Multi-Region DR

If you're thinking about setting up a similar multi-region DR plan for Terraform Enterprise, here are some concrete steps:

• Evaluate Current Architecture: Look at your Terraform setup and find gaps in regional resilience. Understand which parts are regional or global and how they work together during failover.
• Design DR Strategy: Craft a multi-region architecture following AWS Well-Architected Framework and HashiCorp best practices. Aim for operational excellence and dependability, making sure all critical components are included.
• Integrate AWS FIS: Employ AWS FIS to test your DR strategy by simulating failures and tracking recovery times. This uncovers hidden dependencies and configuration issues that aren't obvious when everything's working fine.
• Automate Failover Processes: Set up automation for failover steps to ensure quick activation during regional disruptions. Automation cuts down on human error and ensures a consistent response to failures.
• Continuously Monitor and Update: Regularly review and update configurations, especially AMIs and connection settings, to prevent drift. In my experience, configuration drift often causes surprise failures during DR events.

Limitations and Considerations

This multi-region DR approach really boosts resilience, but it's not a cure-all. The pilot light strategy is budget-friendly but results in longer RTO compared to a warm standby. Also, the failover process requires strict step-by-step enforcement, which might not work for everyone. For more control, think about using Amazon Application Recovery Controller for orchestrating region switches.

The success of AWS FIS relies heavily on how thorough your failure scenarios are. You need to think about both planned and unexpected failover paths, especially with Aurora Global Database. Based on my experience, extensive testing and validation of all potential failure scenarios can significantly improve the real-world effectiveness of a DR strategy.