SAA-C03 Design Resilient Architectures Practice Question
A company runs a stateful web application on a fleet of Amazon EC2 instances in an Auto Scaling group. The application stores session state locally on each instance. During an Availability Zone failure, the Auto Scaling group replaces the unhealthy instances in a different AZ, but users lose their sessions and must log in again. The company wants to make the application resilient to AZ failures without requiring users to re-authenticate. Which solution should a solutions architect recommend?
⚠ Common exam trap
The trap here is thinking that sticky sessions or increased capacity can preserve session state when an Availability Zone fails.
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
✓
Store session state in an Amazon ElastiCache for Redis cluster with Multi-AZ enabled, and configure the application to use it.
Externalizing session state to a Multi-AZ ElastiCache for Redis cluster removes the dependency on local instance storage. When an instance is replaced after an AZ failure, the new instance can retrieve the session from Redis, so users stay logged in. This decouples state from compute and provides automatic failover, meeting the resilience requirement.
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 S3 to store session data and configure the application to read and write session files directly to an S3 bucket.
Why it's wrong here
Amazon S3 is object storage and not designed for low-latency, high-frequency session updates. While it is durable and cross-AZ, using it for session state would introduce latency and potential consistency issues. It is not a suitable replacement for an in-memory session store, and the application would need significant changes to handle S3 semantics.
- ✗
Configure the Auto Scaling group to launch instances in only one Availability Zone and use a larger instance type to handle the load.
Why it's wrong here
Confining instances to a single AZ increases the blast radius of an AZ failure and does not address session persistence. A larger instance type does not provide resilience. This option contradicts the goal of surviving an AZ failure and would worsen the outage impact.
- ✓
Store session state in an Amazon ElastiCache for Redis cluster with Multi-AZ enabled, and configure the application to use it.
Why this is correct
ElastiCache for Redis with Multi-AZ provides automatic failover to a replica in another AZ, and the session data is externalized from the EC2 instances. When an instance is replaced, it can retrieve the session from the Redis cluster, so users remain authenticated. This design decouples session state from the compute layer and survives an AZ failure.
- ✗
Enable sticky sessions on the Application Load Balancer and increase the Auto Scaling group's desired capacity.
Why it's wrong here
Sticky sessions bind a user to a specific instance, but if that instance or its AZ fails, the session is lost. Increasing capacity does not preserve session state across instance replacement. This approach does not prevent re-authentication after an AZ failure and is not a resilient solution for stateful sessions.
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Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
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