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Difficulty: MediumHigh Availability and Disaster Recovery Design

An organization is designing a multi-region disaster recovery (DR) architecture for a critical internal inventory management system. The primary workload runs in the us-east-1 Region, and the DR environment will be established in the us-west-2 Region. The application layer runs on Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer (ALB). The instances require outbound internet access to download vendor updates. The database tier uses Amazon RDS for PostgreSQL. The company requires a Recovery Point Objective (RPO) of 5 minutes and a Recovery Time Objective (RTO) of 15 minutes, while keeping standby costs minimal. During normal operations, internal client services in both Regions must resolve the application's internal endpoint, api.inventory.local. Which solution meets these requirements at the lowest cost?

  1. Implement a warm standby architecture in us-west-2 by deploying the application EC2 instances at minimum capacity. Configure an Amazon RDS for PostgreSQL cross-region read replica in us-west-2, and promote it during failover. Route traffic using Amazon Route 53 failover routing. Set up NAT Gateways in each Availability Zone of the VPCs in both Regions for outbound internet connectivity. Create a Route 53 Private Hosted Zone for inventory.local and associate it with the VPCs in both Regions.Answer
  2. B
    Implement a backup and restore architecture. Configure automated snapshots of the primary Amazon RDS DB instance and copy them to us-west-2 every 4 hours. During failover, restore the database from the latest copied snapshot and launch the application EC2 instances using AWS CloudFormation. Route traffic using Amazon Route 53 geolocation routing. Deploy a single NAT Gateway in each Region to reduce outbound network gateway costs.
  3. C
    Implement a warm standby architecture in us-west-2 with application EC2 instances running at minimum capacity. Configure an Amazon RDS for PostgreSQL cross-region read replica in us-west-2. Route traffic using Amazon Route 53 failover routing. Deploy a single NAT Gateway in the us-west-2 VPC to minimize idle outbound gateway costs. Create a Route 53 Private Hosted Zone for inventory.local and associate it with the VPCs in both Regions.
  4. D
    Implement an active-active multi-region architecture. Configure Amazon RDS for PostgreSQL with a Multi-AZ deployment spanning across us-east-1 and us-west-2. Configure the standby DB instance in us-west-2 to serve read-only queries during normal operations to offset costs. Create a Route 53 Private Hosted Zone for inventory.local and associate it with both VPCs. Route incoming traffic using Route 53 latency-based routing.

Answer

Implement a warm standby architecture in the secondary region with minimum EC2 capacity, configure an Amazon RDS cross-region read replica, set up redundant NAT Gateways in each Availability Zone of both VPCs, route traffic using Route 53 failover routing, and associate the Private Hosted Zone with both VPCs.
The correct solution uses a Warm Standby disaster recovery strategy, deploying application EC2 instances at minimum capacity to control costs while ensuring the system can scale within the 15-minute RTO. An Amazon RDS for PostgreSQL cross-region read replica satisfies the 5-minute RPO because database changes are replicated asynchronously in near-real-time. High availability is maintained by using multi-AZ NAT Gateways, and the Route 53 Private Hosted Zone is associated with both VPCs to allow internal clients in both Regions to resolve the local application endpoint.

Step-by-Step Solution

1
Determine the database replication strategy to satisfy the 5-minute RPO.
Identify that snapshot-based copy strategies (e.g., 4-hour intervals) are too slow. Choose cross-region database replication via an Amazon RDS for PostgreSQL cross-region read replica, which operates asynchronously with a lag of seconds.
Asynchronous replication satisfies the tight RPO constraint of 5 minutes without the cost or complexity of synchronous multi-region setups.
2
Select the application deployment strategy to meet the 15-minute RTO.
Choose a Warm Standby strategy where application servers run at minimum capacity (e.g., 1 instance) in the secondary Region and scale up via Auto Scaling during a failover event.
This keeps costs minimal compared to Active-Active while ensuring the application can be online and scaled within the 15-minute RTO, which is difficult to guarantee with pure Backup & Restore.
3
Configure internal DNS resolution and redundant network access.
Associate the Route 53 Private Hosted Zone with both VPCs so that clients in both Regions can resolve the internal domain. Deploy NAT Gateways in multiple Availability Zones in both VPCs.
Failing to associate the PHZ prevents DNS resolution in the failover VPC. Deploying only a single NAT Gateway in a Region creates a single point of failure that compromises high availability.

Key Concept

High Availability and Disaster Recovery Design
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High Availability and Disaster Recovery Design

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