- A
Use sticky sessions (session affinity) on the ALB with a session cookie.
Why wrong: Sticky sessions reduce scalability as instances cannot be terminated easily.
- B
Use Amazon ElastiCache for Memcached to store session data.
Why wrong: Memcached is simpler but less feature-rich; Redis is better for persistence.
- C
Store session data in Amazon DynamoDB tables.
Why wrong: DynamoDB is more expensive and higher latency for session storage.
- D
Use Amazon ElastiCache for Redis to store session data externally.
Redis provides a scalable, highly available session store.
Question 776 of 1,746
External Session Store with ElastiCache Redis — AWS Solutions Architect Professional Explained
This SAP-C02 practice question tests your understanding of design for new solutions. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 designing a new application that will run on Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer. The application must maintain session state. The company expects steady traffic with occasional spikes. Which solution is MOST scalable and cost-effective?
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
✓
Use Amazon ElastiCache for Redis to store session data externally.
Option D is correct because Amazon ElastiCache for Redis provides a highly scalable, low-latency, and durable external session store that decouples session state from EC2 instances. This allows the Auto Scaling group to add or remove instances freely without losing session data, and Redis supports replication and persistence for reliability. Compared to DynamoDB, Redis offers lower latency for session access, and compared to Memcached, it provides data structures and persistence that are beneficial for session management.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 sticky sessions (session affinity) on the ALB with a session cookie.
Why it's wrong here
Sticky sessions reduce scalability as instances cannot be terminated easily.
- ✗
Use Amazon ElastiCache for Memcached to store session data.
Why it's wrong here
Memcached is simpler but less feature-rich; Redis is better for persistence.
- ✗
Store session data in Amazon DynamoDB tables.
Why it's wrong here
DynamoDB is more expensive and higher latency for session storage.
- ✓
Use Amazon ElastiCache for Redis to store session data externally.
Why this is correct
Redis provides a scalable, highly available session store.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often choose sticky sessions (Option A) because it seems simple and directly supported by ALB, but they overlook the fundamental scalability and resilience issues it introduces in an Auto Scaling environment.
Detailed technical explanation
How to think about this question
Redis supports data persistence via snapshots (RDB) and append-only files (AOF), ensuring session data survives restarts, and its replication across multiple nodes provides high availability. Under the hood, Redis uses a single-threaded event loop for sub-millisecond latency, making it ideal for session state lookups that require consistent performance. In a real-world scenario, a gaming application with sudden traffic spikes can use Redis Cluster to shard session data across nodes, scaling horizontally without downtime.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Design for New Solutions — study guide chapter
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FAQ
Questions learners often ask
What does this SAP-C02 question test?
Design for New Solutions — This question tests Design for New Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use Amazon ElastiCache for Redis to store session data externally. — Option D is correct because Amazon ElastiCache for Redis provides a highly scalable, low-latency, and durable external session store that decouples session state from EC2 instances. This allows the Auto Scaling group to add or remove instances freely without losing session data, and Redis supports replication and persistence for reliability. Compared to DynamoDB, Redis offers lower latency for session access, and compared to Memcached, it provides data structures and persistence that are beneficial for session management.
What should I do if I get this SAP-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
3 more ways this is tested on SAP-C02
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 solutions architect is designing a web application that will run on Amazon EC2 instances behind an Application Load Balancer (ALB). The application requires that users' session data be stored and made available across all instances. Which solution is MOST cost-effective and scalable?
easy- ✓ A.Use Amazon ElastiCache for Redis to store session data
- B.Store session data on an Amazon EBS volume attached to each instance
- C.Store session data in an Amazon RDS database
- D.Enable sticky sessions (session affinity) on the ALB
Why A: Amazon ElastiCache for Redis provides a fully managed, in-memory data store that is ideal for storing session state externally from the EC2 instances. This decouples session data from the compute layer, allowing any instance to retrieve the same session data regardless of which instance originally handled the request. Redis offers sub-millisecond latency, built-in replication, and automatic failover, making it both highly scalable and cost-effective for session management at scale.
Variation 2. A company is designing a new application that will run on Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer. The application requires that users' session data be stored durably and with low latency. The session data is accessed frequently but is rarely updated. Which solution is MOST cost-effective?
medium- ✓ A.Use ElastiCache for Redis with replication.
- B.Store session data in Amazon S3 with reduced redundancy.
- C.Use DynamoDB with on-demand capacity.
- D.Use ElastiCache for Memcached with multiple nodes.
Why A: ElastiCache for Redis with replication is the most cost-effective solution because it provides durable, low-latency session storage with built-in replication for high availability. Redis supports persistence (e.g., snapshots or AOF logs) to ensure session data survives node failures, and its in-memory nature delivers sub-millisecond latency for frequent reads with rare updates, avoiding the higher costs of DynamoDB on-demand or the lack of durability in Memcached.
Variation 3. A company is designing a new application that will run on Amazon EC2 instances behind an Application Load Balancer. The application needs to store session state. Which AWS service provides a fully managed, highly scalable solution for session state management?
easy- A.Amazon DynamoDB
- ✓ B.Amazon ElastiCache for Redis
- C.Amazon S3
- D.Amazon RDS for MySQL
Why B: Amazon ElastiCache for Redis is the correct choice because it provides a fully managed, in-memory data store that is ideal for session state management. Redis offers sub-millisecond latency, built-in replication, and automatic failover, making it highly scalable and durable for session data. The Application Load Balancer can be configured with stickiness (session affinity) to route requests to the same EC2 instance, but using ElastiCache for Redis decouples session state from the compute layer, enabling stateless application tiers and seamless scaling.
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Last reviewed: Jul 4, 2026
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