SAP-C02 Design for New Solutions Practice Question
A company is running a stateful web application on Amazon EC2 instances behind an Application Load Balancer (ALB). The application stores session data locally on the instance. The company wants to make the application highly available and fault-tolerant without rewriting the application code. What should the solutions architect do?
⚠ Common exam trap
Test-takers frequently assume moving session state to an external store like ElastiCache is the only way to achieve fault tolerance, but the question explicitly prohibits rewriting application code, making sticky sessions the correct choice despite its limitations (e.g., uneven load distribution).
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
✓
Configure the ALB with sticky sessions (session affinity) and use a custom cookie.
Configuring the ALB with sticky sessions (session affinity) using a custom cookie allows the ALB to consistently route requests from a user's session to the same EC2 instance, preserving the locally stored session state without requiring any application code changes. This approach leverages the ALB's built-in session affinity feature, which is transparent to the application and ensures high availability by distributing traffic across healthy instances while maintaining session continuity.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Replace the ALB with a Network Load Balancer (NLB) and enable cross-zone load balancing.
Why it's wrong here
NLB does not natively support sticky sessions; also state is still local.
- ✗
Move session state to Amazon ElastiCache and configure the ALB to use sticky sessions.
Why it's wrong here
Moving session state to ElastiCache requires code modification.
- ✓
Configure the ALB with sticky sessions (session affinity) and use a custom cookie.
Why this is correct
Sticky sessions bind a user to a specific instance, preserving local state without code changes.
- ✗
Use an Auto Scaling group to add more instances and distribute load.
Why it's wrong here
Auto Scaling does not solve the session state problem; new instances won't have the session.
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