The correct answer is to enable sticky sessions on the Application Load Balancer and place the ElastiCache for Redis cluster outside the Auto Scaling group. Sticky sessions, also known as session affinity, ensure that a user’s requests are routed to the same EC2 instance during its lifetime, while keeping ElastiCache external to the Auto Scaling group guarantees the session data persists even when instances are terminated or replaced during scaling events. This combination directly addresses session loss after scaling events because the external Redis store remains available regardless of instance churn. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding of stateful application design in elastic environments—a common trap is assuming that ElastiCache replication alone prevents session loss, but replication does not protect against cache deletion when the cluster is tied to terminating instances. A useful memory tip: “Stick the session, persist the cache” reminds you to pair ALB stickiness with a durable, external data store like ElastiCache outside the ASG.
SAP-C02 Continuous Improvement for Existing Solutions Practice Question
This SAP-C02 practice question tests your understanding of continuous improvement for existing solutions. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company runs a web application on EC2 instances in an Auto Scaling group. The application stores session data in an Amazon ElastiCache for Redis cluster. The operations team reports that after scaling events, users lose their sessions. Which TWO actions should the Solutions Architect take to resolve this issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable sticky sessions (session affinity) on the Application Load Balancer.
Options A and C are correct: Sticky sessions ensure requests from a user go to the same instance, and placing ElastiCache outside the ASG ensures it persists. Option B is unnecessary because Redis replication does not address session loss during scaling. Option D is wrong because session stickiness at ALB is already implied but not sufficient without persistent cache. Option E is wrong because DynamoDB is not necessary.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 a Network Load Balancer instead of an Application Load Balancer.
Why it's wrong here
NLB does not have session affinity features.
✓
Enable sticky sessions (session affinity) on the Application Load Balancer.
Why this is correct
Sticky sessions route user to same instance.
Related concept
Static NAT maps one inside address to one outside address.
✓
Move session data storage from ElastiCache to an external store that persists across instance termination.
Why this is correct
External store like ElastiCache persists across scaling.
Related concept
Static NAT maps one inside address to one outside address.
✗
Store session data in Amazon DynamoDB.
Why it's wrong here
DynamoDB is an option but not required; ElastiCache can work if configured correctly.
✗
Enable replication in the ElastiCache cluster to handle failover.
Why it's wrong here
Replication doesn't prevent session loss during scale-in.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SAP-C02 NAT questions on configuration and troubleshooting.
Continuous Improvement for Existing Solutions — This question tests Continuous Improvement for Existing Solutions — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Enable sticky sessions (session affinity) on the Application Load Balancer. — Options A and C are correct: Sticky sessions ensure requests from a user go to the same instance, and placing ElastiCache outside the ASG ensures it persists. Option B is unnecessary because Redis replication does not address session loss during scaling. Option D is wrong because session stickiness at ALB is already implied but not sufficient without persistent cache. Option E is wrong because DynamoDB is not necessary.
What should I do if I get this SAP-C02 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SAP-C02 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a web application behind an Application Load Balancer that uses sticky sessions. The application is deployed on EC2 instances in an Auto Scaling group. During a deployment, the team notices that users are experiencing errors after new instances are launched. What is the MOST likely cause?
hard
A.The target group's deregistration delay is too short.
B.The stickiness duration is set too long, causing requests to be routed to terminated instances.
✓ C.The Auto Scaling group's scale-in policy is terminating instances with active sessions.
D.The ALB health check is not configured for the new instances.
Why C: Option A is correct because stickiness binds users to specific instances; new instances don't have session data, causing errors. Option B is wrong because deregistration delay helps drain connections. Option C is wrong because health checks would detect unhealthy instances. Option D is wrong because scaling policies don't affect session stickiness.
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Question Discussion
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