DOP-C02 Resilient Cloud Solutions Practice Question
A company runs a stateful web application on EC2 instances in an Auto Scaling group. The application uses an Application Load Balancer (ALB) and an Amazon ElastiCache Redis cluster. Users report that after a scaling event, they are logged out and lose session data. What is the most likely cause?
⚠ Common exam trap
The trap here is that candidates may focus on the Auto Scaling group's termination policy or ALB health checks, overlooking that the session data is stored externally in ElastiCache and that its lack of high availability is the root cause of session loss.
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
✓
The ElastiCache cluster is not configured for Multi-AZ and a node failure caused all sessions to be lost
The scenario describes a stateful web application that relies on ElastiCache Redis for session storage. If the ElastiCache cluster is not configured for Multi-AZ, a node failure can cause all cached session data to be lost, logging users out. This is the most likely cause of session loss after a scaling event, as scaling events do not directly affect ElastiCache data persistence.
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 ALB health check interval is too short, causing healthy instances to be marked unhealthy
Why it's wrong here
Even if the ALB health check interval is set too aggressively and temporarily marks an instance as unhealthy, that only affects the load balancer's routing decisions for new requests. It does not delete or invalidate session data, which is stored in the shared ElastiCache layer. The next request is simply sent to a healthy instance, which retrieves the same session from ElastiCache, so user experience is unaffected.
- ✗
The ElastiCache cluster is configured with in-transit encryption, causing session tokens to be invalidated
Why it's wrong here
In-transit encryption on ElastiCache uses TLS to protect data moving between the application and the cache cluster. It does not modify, corrupt, or invalidate any stored session tokens; those tokens remain readable once the TLS connection is established. A session token would only become invalid if the application fails to connect or if the token expires, not because encryption is enabled. Therefore, this configuration cannot explain a sudden loss of all sessions.
- ✗
The Auto Scaling group is using a termination policy that terminates the oldest instance first, which holds active sessions
Why it's wrong here
An Auto Scaling termination policy that selects the oldest instance would remove that EC2 instance, but the active sessions are not held in the instance's local memory—they are persisted in ElastiCache. When the instance is terminated, the ALB routes new requests to a different instance, which loads the same session key from ElastiCache. Since ElastiCache is independent of the EC2 instance lifecycle, terminating any instance, regardless of age, does not destroy session data.
- ✓
The ElastiCache cluster is not configured for Multi-AZ and a node failure caused all sessions to be lost
Why this is correct
An ElastiCache cluster without Multi-AZ deployment means there is only a single primary node with no replica in another Availability Zone. If that node fails, any in-memory session data is lost because there is no replication or automatic failover to a secondary node. Even if ElastiCache replaces the node, the replacement starts empty unless persistence/backups were configured. This exactly matches the symptom of all users being logged out simultaneously.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DOP-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DOP-C02 exam.