SOA-C02 Reliability and Business Continuity Practice Question
A company runs a critical stateful web application on Amazon EC2 instances in a single AWS region. The application stores user session data in an Amazon ElastiCache for Redis cluster. The SysOps administrator must design a disaster recovery (DR) strategy that can survive a complete regional outage with a Recovery Point Objective (RPO) of 15 minutes and a Recovery Time Objective (RTO) of 1 hour. The application must be able to redirect users to the DR region with minimal manual effort. Which combination of actions meets these requirements?
⚠ Common exam trap
Test-takers frequently assume snapshot-based replication (Option D) is sufficient for a 15-minute RPO, but they overlook the inherent latency and potential data loss from periodic snapshots, and that latency routing (Option D) does not provide health-based failover, while weighted routing (Option A) lacks automatic failover capability.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a read replica of the ElastiCache Redis cluster in the DR region using the native cross-region replication feature. Use Route 53 with failover routing to point to the DR region ALB when the primary health check fails. Pre-configure EC2 instances in an Auto Scaling group in the DR region.
ElastiCache for Redis supports cross-region replication via a read replica in the DR region, which can keep session data synchronized with minimal lag, meeting the 15-minute RPO. Route 53 failover routing with health checks on the primary region's ALB automatically redirects traffic to the pre-configured DR region EC2 instances and ALB, achieving the 1-hour RTO with minimal manual effort. Pre-configuring the DR region with an Auto Scaling group ensures compute capacity is ready, while the read replica provides the required data availability.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use Amazon Route 53 with weighted routing to distribute traffic between the two regions. Use a global DynamoDB table for session data, and launch EC2 instances in the DR region only when a failure is detected using AWS CloudFormation StackSets.
Why it's wrong here
Weighted routing in Route 53 distributes traffic based on weights and does not perform automatic health-based failover; when the primary region fails, you would need to manually adjust weights or DNS records to redirect traffic, which extends recovery time. Additionally, a global DynamoDB table replicates NoSQL data, but this application's session state resides in ElastiCache Redis, and DynamoDB does not provide the required Redis data replication, so session data would not be available in the DR region. Finally, using CloudFormation StackSets to launch EC2 instances only after a failure is detected introduces significant provisioning and instance boot time, which would almost certainly violate the 1-hour RTO because no pre-warmed capacity exists in the DR region.
- ✓
Create a read replica of the ElastiCache Redis cluster in the DR region using the native cross-region replication feature. Use Route 53 with failover routing to point to the DR region ALB when the primary health check fails. Pre-configure EC2 instances in an Auto Scaling group in the DR region.
Why this is correct
Global Datastore for Redis provides cross-Region replication with low RPO. Pre-configured Auto Scaling groups in the DR region ensure that compute capacity is ready. Route 53 failover routing automatically redirects traffic when the primary ALB health check fails. This combination meets the RPO and RTO requirements with minimal manual effort.
- ✗
Use an Amazon CloudFront distribution with multiple origins (primary and DR). Enable session stickiness at the CloudFront level. Use EC2 instances in both regions behind separate ALBs. No special data replication is needed because sessions are stored in Redis.
Why it's wrong here
CloudFront does not provide cross-region failover for origins in the way Route 53 does. Session stickiness is not sufficient for DR because Redis data is not replicated across regions, so users would lose sessions after failover. RPO would be effectively longer than 15 minutes without replication.
- ✗
Use EC2 instances with an Auto Scaling group in both regions. Schedule a Lambda function to take snapshots of the Redis cluster every 15 minutes and copy them to the DR region. Use Route 53 latency routing to direct users to the nearest region.
Why it's wrong here
Snapshot-based replication every 15 minutes could meet the RPO, but copying snapshots and restoring them during a disaster would take more than 1 hour, exceeding the RTO. Latency routing does not provide automatic failover in a regional outage; it routes based on latency, which could still send traffic to the failed region.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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