- A
Create a Multi-AZ RDS instance and attach it to the existing EC2 instance.
Why wrong: Does not address EC2 fault tolerance.
- B
Add a second EC2 instance in the same Availability Zone and use a Network Load Balancer.
Why wrong: Single AZ is a single point of failure.
- C
Use an Auto Scaling group with a launch configuration that stores session data on instance store.
Why wrong: Instance store is ephemeral and lost on instance termination.
- D
Place instances in an Auto Scaling group across two Availability Zones, use an Application Load Balancer, and store session state in ElastiCache.
Provides high availability and fault tolerance.
Question 602 of 1,546
Fault Tolerance for Stateful Web Applications
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. 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 stateful web application on a single EC2 instance. The SysOps Administrator wants to improve fault tolerance. Which design should they implement?
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
✓
Place instances in an Auto Scaling group across two Availability Zones, use an Application Load Balancer, and store session state in ElastiCache.
Option D is correct because it distributes the web application across multiple Availability Zones for high availability, uses an Application Load Balancer to route traffic to healthy instances, and stores session state externally in ElastiCache. This decouples the stateful session data from the EC2 instances, allowing any instance to handle any request without losing session context, which is essential for fault tolerance in a stateful application.
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.
- ✗
Create a Multi-AZ RDS instance and attach it to the existing EC2 instance.
Why it's wrong here
Does not address EC2 fault tolerance.
- ✗
Add a second EC2 instance in the same Availability Zone and use a Network Load Balancer.
Why it's wrong here
Single AZ is a single point of failure.
- ✗
Use an Auto Scaling group with a launch configuration that stores session data on instance store.
Why it's wrong here
Instance store is ephemeral and lost on instance termination.
- ✓
Place instances in an Auto Scaling group across two Availability Zones, use an Application Load Balancer, and store session state in ElastiCache.
Why this is correct
Provides high availability and fault tolerance.
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 confuse high availability with fault tolerance, choosing a single-AZ solution (Option B) or a database-only fix (Option A), failing to recognize that stateful applications require externalized session state to survive instance or AZ failures.
Detailed technical explanation
How to think about this question
Under the hood, an Application Load Balancer operates at Layer 7 and can perform content-based routing and health checks, while ElastiCache (e.g., Redis) provides sub-millisecond session retrieval with replication across nodes. In a real-world scenario, if an EC2 instance fails, the ALB automatically reroutes traffic to healthy instances, and ElastiCache ensures the user's session data (e.g., shopping cart) is preserved and accessible from any surviving instance, maintaining a seamless user experience.
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
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
Reliability and Business Continuity — study guide chapter
Learn the concepts, then practise the 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 — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Place instances in an Auto Scaling group across two Availability Zones, use an Application Load Balancer, and store session state in ElastiCache. — Option D is correct because it distributes the web application across multiple Availability Zones for high availability, uses an Application Load Balancer to route traffic to healthy instances, and stores session state externally in ElastiCache. This decouples the stateful session data from the EC2 instances, allowing any instance to handle any request without losing session context, which is essential for fault tolerance in a stateful application.
What should I do if I get this SOA-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.
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Last reviewed: Jul 4, 2026
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