The answer is that session data lost after EC2 restart due to local storage is the most likely cause of users being logged out. When a web application stores session data in the local file system of an EC2 instance, that data is ephemeral—it disappears the moment the instance is stopped, restarted, or replaced, whether by Auto Scaling, a hardware failure, or a maintenance event. Since the company minimized code changes and did not migrate session storage to a shared, external service like ElastiCache or DynamoDB, the stateless web tier requirement was never met, causing every instance lifecycle event to wipe user sessions. On the AWS Certified SAP on AWS Specialty PAS-C01 exam, this scenario tests your understanding of the difference between ephemeral local storage and durable, shared session stores; a common trap is assuming RDS handles session persistence, but RDS only manages database state, not web session state. Memory tip: “Local is lost; shared is saved”—if session data lives on the instance, it dies with the instance.
PAS-C01 Migration Practice Question
This PAS-C01 practice question tests your understanding of migration. 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 migrating its on-premises web application to AWS. The application consists of a stateless web tier and a stateful database tier running on a single server. The web tier uses session data stored in the local file system. The database tier runs on MySQL. The company wants to minimize changes to the application code during migration. They plan to use Amazon EC2 for the web server and Amazon RDS for MySQL for the database. After migrating the web server to an EC2 instance and the database to RDS, users report that they are being logged out of the application frequently. What is the most likely cause of this issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The web server's session data is stored locally and is lost when the EC2 instance is replaced or restarted.
The web tier stores session data in the local file system of the EC2 instance. When the instance is replaced or restarted (e.g., due to Auto Scaling, instance failure, or a maintenance event), that local session data is lost. Since the application code was not modified to use a shared session store (like ElastiCache or DynamoDB), users lose their sessions and are logged out. This is the most likely cause because the migration explicitly minimized code changes and did not address the stateless web tier requirement.
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.
✓
The web server's session data is stored locally and is lost when the EC2 instance is replaced or restarted.
Why this is correct
Since the application stores sessions locally, any instance replacement leads to session loss.
Clue confirmation
The clue words "most likely", "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The RDS instance's backup window is causing database connections to drop.
Why it's wrong here
Backups do not cause frequent disconnections; RDS handles backups transparently.
✗
The application's session timeout setting was reduced during migration.
Why it's wrong here
No code changes were made, so timeout settings remain unchanged.
✗
The security group for the web server is blocking inbound session cookies.
Why it's wrong here
Security groups control network traffic, not application-level session cookies.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume session data is automatically preserved across EC2 instance lifecycles, or they may incorrectly blame RDS connectivity or security group rules, when the real issue is the lack of a shared, durable session store for the stateless web tier.
Detailed technical explanation
How to think about this question
Session data stored in the local file system (e.g., /tmp or a custom directory) is ephemeral and tied to the instance's lifecycle. In a typical on-premises single-server setup, this works because the server rarely restarts. On AWS, EC2 instances can be terminated, replaced, or restarted during Auto Scaling events, instance health checks, or patching. To maintain session persistence without code changes, you would need to mount an Amazon EFS file system or use an external session store like ElastiCache for Redis, but the question states the company minimized code changes, so the local file system remains the bottleneck.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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.
Migration — This question tests Migration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The web server's session data is stored locally and is lost when the EC2 instance is replaced or restarted. — The web tier stores session data in the local file system of the EC2 instance. When the instance is replaced or restarted (e.g., due to Auto Scaling, instance failure, or a maintenance event), that local session data is lost. Since the application code was not modified to use a shared session store (like ElastiCache or DynamoDB), users lose their sessions and are logged out. This is the most likely cause because the migration explicitly minimized code changes and did not address the stateless web tier requirement.
What should I do if I get this PAS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely", "minimum / minimize". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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