The primary requirements are automatic scaling, resilience to traffic spikes, and guaranteed order durability without losing customer orders. Option D follows a well-established, AWS-recommended decoupled architecture pattern that meets all these requirements.
Using an Application Load Balancer (ALB) provides Layer 7 request routing and distributes incoming traffic across multiple EC2 instances in Auto Scaling groups, allowing the application layer to scale horizontally as traffic increases. This ensures that spikes in web traffic do not overwhelm individual instances.
The critical resilience feature in this design is Amazon SQS, which acts as a durable buffer for unprocessed customer orders. If the application experiences a sudden traffic surge or if downstream processing slows, SQS stores incoming orders reliably until they can be processed. This decoupling prevents request loss and protects the user experience by absorbing load bursts.
Processed orders are stored in Amazon RDS with a Multi-AZ deployment, which provides synchronous replication to a standby instance in another Availability Zone and automatic failover. This ensures high availability and data durability for transactional order data.
The other options misuse services or introduce unnecessary complexity. NAT gateways (Option A) do not manage web traffic or provide resiliency for application workloads. Redshift (Option B) is an analytics data warehouse and not suitable for transactional order processing. Gateway Load Balancer (Option C) is designed for network appliances, not application traffic management or order capture.
Therefore, D is the correct solution because it combines load balancing, auto scaling, message queuing, and highly available databases to deliver a resilient, scalable, and fault-tolerant order processing architecture.
