- A
Use sticky sessions (session affinity) on the ALB and configure the Auto Scaling group with a larger min size.
Why wrong: Sticky sessions ensure a user is always directed to the same instance, but if that instance fails or is terminated, the session data stored in memory is lost.
- B
Store session data in a shared Amazon ElastiCache cluster and modify the application to read/write session state to ElastiCache.
ElastiCache provides a centralized, highly available in-memory cache that decouples session state from individual EC2 instances. Instances can be replaced or scaled without losing session data.
- C
Deploy the application in multiple AWS Regions and use Amazon Route 53 with latency-based routing.
Why wrong: Multi-Region deployment addresses regional failures, not cross-AZ high availability. It is unnecessary for the stated requirement and does not solve the session data loss problem within a single Region.
- D
Store session data in an Amazon RDS for MySQL database and configure the application to read/write session state to the database.
Why wrong: While a database can persist session data, RDS is slower than an in-memory cache for this use case and requires more significant application changes. ElastiCache is a better fit.
SOA-C02 Reliability and Business Continuity Practice Question
This SOA-C02 practice question tests your understanding of reliability and business continuity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: elastiCache provides in-memory data stores (Redis/Memcached).. 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 Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer (ALB). The application stores session state in memory on each instance. The SysOps administrator wants to make the application highly available across multiple Availability Zones without losing session data when instances are terminated or replaced. The solution must minimize application changes. Which approach should the administrator take?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"minimum / minimize"Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
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 data in a shared Amazon ElastiCache cluster and modify the application to read/write session state to ElastiCache.
Option B is correct because storing session state in a shared Amazon ElastiCache cluster decouples session data from individual EC2 instances, allowing any instance in the Auto Scaling group to serve any user request without losing session data when instances are terminated or replaced. This approach requires minimal application changes (only modifying the session handler to point to ElastiCache) and supports high availability across multiple Availability Zones by using a replicated ElastiCache cluster (e.g., Redis with replication).
Key principle: ElastiCache provides in-memory data stores (Redis/Memcached).
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 and configure the Auto Scaling group with a larger min size.
Why it's wrong here
Sticky sessions ensure a user is always directed to the same instance, but if that instance fails or is terminated, the session data stored in memory is lost.
- ✓
Store session data in a shared Amazon ElastiCache cluster and modify the application to read/write session state to ElastiCache.
Why this is correct
ElastiCache provides a centralized, highly available in-memory cache that decouples session state from individual EC2 instances. Instances can be replaced or scaled without losing session data.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
ElastiCache provides in-memory data stores (Redis/Memcached).
- ✗
Deploy the application in multiple AWS Regions and use Amazon Route 53 with latency-based routing.
Why it's wrong here
Multi-Region deployment addresses regional failures, not cross-AZ high availability. It is unnecessary for the stated requirement and does not solve the session data loss problem within a single Region.
- ✗
Store session data in an Amazon RDS for MySQL database and configure the application to read/write session state to the database.
Why it's wrong here
While a database can persist session data, RDS is slower than an in-memory cache for this use case and requires more significant application changes. ElastiCache is a better fit.
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 they seem to solve session affinity without code changes, but they fail to realize that sticky sessions do not persist session data across instance terminations, which is the core requirement for high availability without data loss.
Detailed technical explanation
How to think about this question
Amazon ElastiCache for Redis provides sub-millisecond latency for session read/write operations and supports features like replication, automatic failover, and backup/restore, making it ideal for session state management in highly available architectures. The application can use a Redis client library (e.g., PhpRedis for PHP, redis-py for Python) to store session data with a configurable TTL, ensuring that sessions are automatically cleaned up. In a real-world scenario, if the Auto Scaling group scales in and terminates an instance, the session data remains accessible in ElastiCache, and the ALB can route subsequent requests from the same user to any healthy instance without data loss.
KKey Concepts to Remember
- ElastiCache provides in-memory data stores (Redis/Memcached).
- Decouples session state from individual EC2 instances.
- Supports multi-AZ deployments for high availability.
- Offers low-latency access for session data compared to databases.
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
ElastiCache provides in-memory data stores (Redis/Memcached).
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Review elastiCache provides in-memory data stores (Redis/Memcached)., then practise related SOA-C02 questions on the same topic to reinforce the concept.
- →
Reliability and Business Continuity — study guide chapter
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Reliability and Business Continuity practice questions
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FAQ
Questions learners often ask
What does this SOA-C02 question test?
Reliability and Business Continuity — This question tests Reliability and Business Continuity — ElastiCache provides in-memory data stores (Redis/Memcached)..
What is the correct answer to this question?
The correct answer is: Store session data in a shared Amazon ElastiCache cluster and modify the application to read/write session state to ElastiCache. — Option B is correct because storing session state in a shared Amazon ElastiCache cluster decouples session data from individual EC2 instances, allowing any instance in the Auto Scaling group to serve any user request without losing session data when instances are terminated or replaced. This approach requires minimal application changes (only modifying the session handler to point to ElastiCache) and supports high availability across multiple Availability Zones by using a replicated ElastiCache cluster (e.g., Redis with replication).
What should I do if I get this SOA-C02 question wrong?
Review elastiCache provides in-memory data stores (Redis/Memcached)., then practise related SOA-C02 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
ElastiCache provides in-memory data stores (Redis/Memcached).
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Last reviewed: Jun 11, 2026
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