The correct answer is to configure the ALB with sticky sessions (session affinity) using a custom cookie. This approach directly solves the problem of making a stateful application highly available without code changes because sticky sessions ensure that all requests from a given user session are routed to the same EC2 instance, preserving the locally stored session data. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding of when to use application-layer load balancing features versus infrastructure-level changes; a common trap is assuming you must offload state to ElastiCache or DynamoDB, but the question explicitly forbids code rewrites. Remember that sticky sessions on an ALB are the only native, zero-code way to maintain session affinity for stateful applications that cannot be refactored. Memory tip: think “stick to the same box” — the ALB’s cookie keeps the user pinned to their original instance, avoiding the need to rewrite the app.
SAP-C02 Design for New Solutions Practice Question
This SAP-C02 practice question tests your understanding of design for new 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 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?
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.
Option D is correct. By using an ALB with sticky sessions (session affinity), requests from the same user go to the same instance, preserving local session state. Option A is incorrect because moving session state to ElastiCache would require code changes. Option B is incorrect because a Network Load Balancer does not support sticky sessions by default and also doesn't solve the state issue. Option C is incorrect because Auto Scaling would terminate instances, losing session data.
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.
✗
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.
Related concept
Static NAT maps one inside address to one outside address.
✗
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.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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.
Design for New Solutions — This question tests Design for New Solutions — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Configure the ALB with sticky sessions (session affinity) and use a custom cookie. — Option D is correct. By using an ALB with sticky sessions (session affinity), requests from the same user go to the same instance, preserving local session state. Option A is incorrect because moving session state to ElastiCache would require code changes. Option B is incorrect because a Network Load Balancer does not support sticky sessions by default and also doesn't solve the state issue. Option C is incorrect because Auto Scaling would terminate instances, losing session data.
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.
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Question Discussion
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