AWS Design Resilient Architectures 1

Question 1

A company runs a critical e-commerce application on a single EC2 instance in one Availability Zone. During a recent AZ outage, the entire application was unavailable for several hours. Which architecture change would BEST address this single point of failure?

Accepted Answer

Deploy the application across multiple EC2 instances in at least two Availability Zones behind an Application Load Balancer

Answer

Deploying across multiple Availability Zones behind an Application Load Balancer eliminates the single point of failure by distributing traffic across redundant infrastructure. If one AZ fails, the ALB automatically routes traffic to healthy instances in other AZs, ensuring high availability. Increasing instance size, enabling monitoring, or creating AMI backups do not address the fundamental issue of having a single point of failure in one AZ.

Question 2

An application tier communicates with a backend database. During peak traffic, the database becomes overwhelmed with connection requests, causing timeouts. The database itself has sufficient CPU and memory. Which solution would MOST effectively resolve this bottleneck without modifying the application code?

Accepted Answer

Deploy Amazon RDS Proxy between the application and the database

Answer

Amazon RDS Proxy sits between the application and the database and maintains a pool of established database connections. It allows many application connections to share fewer actual database connections, dramatically reducing connection overhead. Since the database has sufficient CPU/memory but is overwhelmed by connection volume, RDS Proxy directly addresses the root cause. Upgrading instance size won't fix connection exhaustion, Multi-AZ adds failover capability (not connection pooling), and read replicas handle read scaling but not connection pooling for the primary.

Question 3

A financial services company needs to replicate an RDS database to a second AWS Region for disaster recovery with a Recovery Point Objective (RPO) of under 1 second. Which approach meets this requirement?

Accepted Answer

Set up an RDS read replica in the DR region using cross-region replication

Answer

RDS cross-region read replicas use asynchronous replication and typically achieve replication lag well under 1 second for most workloads, satisfying a sub-1-second RPO. Hourly snapshot copies would result in up to 1-hour RPO. RDS Multi-AZ provides high availability within a single region, not cross-region DR. AWS DMS is primarily a migration tool and introduces additional complexity without matching native read replica replication performance for ongoing DR.

Question 4

A company uses SQS to decouple its order processing system. Occasionally, messages fail processing repeatedly and never complete. These failed messages are blocking queue workers and consuming unnecessary resources. What is the recommended AWS solution?

Accepted Answer

Configure a Dead Letter Queue (DLQ) with a maxReceiveCount to capture repeatedly failed messages

Answer

A Dead Letter Queue (DLQ) is specifically designed to handle messages that cannot be successfully processed after a configurable number of attempts (maxReceiveCount). When a message exceeds this threshold, SQS automatically moves it to the DLQ, preventing it from blocking the main queue and allowing engineers to inspect and reprocess failed messages separately. Increasing visibility timeout delays reprocessing but doesn't remove persistent failures. Long polling optimizes empty receive costs but doesn't handle failures. FIFO queues address ordering, not failure isolation.

Question 5

A web application hosted on EC2 instances behind an ALB experiences occasional traffic spikes that exceed current capacity, resulting in degraded performance. The load is highly variable and unpredictable. Which combination of services provides the MOST cost-effective and resilient solution?

Accepted Answer

Configure an Auto Scaling Group with a target tracking scaling policy based on average CPU utilization

Answer

An Auto Scaling Group with target tracking scaling automatically adds or removes EC2 instances based on real-time metrics like average CPU utilization. This provides elastic capacity that matches actual demand, ensuring resilience during spikes while minimizing cost during low-traffic periods. Reserved Instances for peak load means paying for peak capacity 24/7 even during off-peak times — very expensive. Manual scaling is error-prone and can't respond to unpredictable spikes. Dedicated Hosts are for licensing compliance, not elasticity.

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AWS Design Resilient Architectures 1