← SOA-C02·Amazon Web Services

Question 560 of 1,546

Reliability and Business Continuity →mediumMultiple ChoiceObjective-mapped

Making Stateful Web Applications Highly Available on EC2

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. To improve reliability, the company wants to implement a highly available architecture. What should the SysOps administrator do?

A
Refactor the application to store session state externally (e.g., ElastiCache), then deploy it across multiple AZs with an Application Load Balancer.
Correct: Externalizing session state allows the application to be stateless, enabling horizontal scaling and high availability.
B
Migrate the application to a larger instance type.
Why wrong: Incorrect: A larger instance still represents a single point of failure.
C
Create a standby EC2 instance and use an Elastic IP to fail over manually.
Why wrong: Incorrect: Manual failover is slow and not highly available.
D
Use Route 53 health checks to route traffic to a secondary instance if the primary fails.
Why wrong: Incorrect: Without externalizing state, session data is lost on failover; also, secondary instance is not automatically provisioned.

Full breakdown with real-world context →

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

✓

Refactor the application to store session state externally (e.g., ElastiCache), then deploy it across multiple AZs with an Application Load Balancer.

Option A is correct because it addresses the core challenge of making a stateful web application highly available. By storing session state externally in ElastiCache, the application becomes stateless from a networking perspective, allowing any EC2 instance to handle any request. Deploying these instances across multiple Availability Zones (AZs) behind an Application Load Balancer (ALB) provides fault tolerance and automatic traffic distribution, eliminating the single point of failure.

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.

✓
Refactor the application to store session state externally (e.g., ElastiCache), then deploy it across multiple AZs with an Application Load Balancer.
Why this is correct
Correct: Externalizing session state allows the application to be stateless, enabling horizontal scaling and high availability.
Related concept
Read the scenario before looking for a memorised answer.
✗
Migrate the application to a larger instance type.
Why it's wrong here
Incorrect: A larger instance still represents a single point of failure.
✗
Create a standby EC2 instance and use an Elastic IP to fail over manually.
Why it's wrong here
Incorrect: Manual failover is slow and not highly available.
✗
Use Route 53 health checks to route traffic to a secondary instance if the primary fails.
Why it's wrong here
Incorrect: Without externalizing state, session data is lost on failover; also, secondary instance is not automatically provisioned.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often assume that simply adding a second instance or using DNS failover (Route 53) is sufficient for high availability, overlooking the critical requirement to externalize session state for stateful applications.

Detailed technical explanation

How to think about this question

Under the hood, an ALB operates at Layer 7 and can distribute traffic based on request attributes, but it does not inherently share session state between instances. ElastiCache (Memcached or Redis) provides a low-latency, in-memory data store that can be accessed by all instances, enabling sticky sessions (session affinity) to be decoupled from instance identity. In a real-world scenario, if the application uses cookies for session tracking, the ALB can use a custom cookie or the AWS-generated cookie to route requests to the same instance, but with external storage, even if that instance fails, another instance can retrieve the session data from ElastiCache and continue the user's session transparently.

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.

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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: Refactor the application to store session state externally (e.g., ElastiCache), then deploy it across multiple AZs with an Application Load Balancer. — Option A is correct because it addresses the core challenge of making a stateful web application highly available. By storing session state externally in ElastiCache, the application becomes stateless from a networking perspective, allowing any EC2 instance to handle any request. Deploying these instances across multiple Availability Zones (AZs) behind an Application Load Balancer (ALB) provides fault tolerance and automatic traffic distribution, eliminating the single point of failure.

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.

About these practice questions

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Same concept, more angles

1 more ways this is tested on SOA-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 company is running a stateful web application on a single EC2 instance in a public subnet. The instance stores user sessions locally. The company wants to improve availability without rewriting the application. Which design should they use?

hard

A.Create a second EC2 instance in a different AZ and use Route 53 with health checks.
B.Use an Auto Scaling group across multiple AZs but keep sessions on instance.
✓ C.Deploy an Application Load Balancer across multiple AZs, move session storage to ElastiCache, and use an Auto Scaling group.
D.Use an Application Load Balancer with sticky sessions and an Auto Scaling group in a single AZ.

Why C: Option C is correct because it addresses the core issue of stateful sessions without rewriting the application. By moving session storage to ElastiCache (a centralized, external data store), the application becomes stateless from the instance's perspective, allowing an Auto Scaling group across multiple Availability Zones (AZs) and an Application Load Balancer (ALB) to distribute traffic seamlessly. This design improves availability by enabling horizontal scaling and fault tolerance, as any instance can handle any request since sessions are stored externally.

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