The answer is that the Auto Scaling group was configured to span only two Availability Zones, and the failed AZ contained a majority of the running instances. This is the most likely cause of performance degradation after AZ failure because when an Auto Scaling group is limited to two AZs, losing one removes a disproportionate share of compute capacity, overloading the remaining instances. The application stayed available thanks to the Multi-AZ RDS instance, but the sudden loss of the majority of EC2 capacity forced the surviving instances to handle the full traffic load, causing significant slowdowns. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how ASG AZ count directly impacts fault tolerance and capacity planning. A common trap is assuming three AZs are always configured, but the question explicitly states the group spans only two. Remember the memory tip: “Two AZs, one down — half the compute, full the load.”
DOP-C02 Resilient Cloud Solutions Practice Question
This DOP-C02 practice question tests your understanding of resilient cloud solutions. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 critical application on EC2 instances in an Auto Scaling group across three Availability Zones. The application uses an Amazon RDS Multi-AZ DB instance. During a recent incident, one Availability Zone experienced a complete failure. The application remained available, but performance degraded significantly. What is the most likely cause of the degradation?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Auto Scaling group was configured to span only two Availability Zones, and the failed AZ contained a majority of the running instances
Option C is correct because if the Auto Scaling group is configured to span only two Availability Zones, the failure of one AZ would result in a disproportionate loss of capacity. Since the failed AZ contained a majority of the running instances, the remaining instances in the surviving AZ would be overloaded, causing significant performance degradation. The application remained available because the surviving instances and the Multi-AZ RDS instance continued to operate, but the reduced compute capacity led to degraded performance.
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 Route 53 health checks failed and directed traffic to another Region, increasing latency
Why it's wrong here
No cross-Region failover.
✗
The RDS DB instance failed over to a read replica in a different Region
Why it's wrong here
Multi-AZ stays within Region.
✓
The Auto Scaling group was configured to span only two Availability Zones, and the failed AZ contained a majority of the running instances
Why this is correct
Loss of one AZ halves capacity if only two AZs used.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The EBS volumes in the failed AZ were not available, causing data loss
Why it's wrong here
EBS volumes persist; can be attached to instances in another AZ.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume Multi-AZ RDS or Route 53 health checks are the primary cause of degradation, but the real issue is the Auto Scaling group's AZ configuration and the resulting imbalance in compute capacity after an AZ failure.
Detailed technical explanation
How to think about this question
Auto Scaling groups distribute instances across Availability Zones based on the AZRebalance feature, which maintains balanced capacity. If the group is configured for only two AZs, the loss of one AZ can remove up to 50% or more of the instances, leading to a sudden spike in load on the remaining instances. The RDS Multi-AZ deployment uses synchronous replication to a standby in another AZ, ensuring no data loss and automatic failover, but it does not mitigate compute capacity loss from the Auto Scaling group.
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.
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Auto Scaling group was configured to span only two Availability Zones, and the failed AZ contained a majority of the running instances — Option C is correct because if the Auto Scaling group is configured to span only two Availability Zones, the failure of one AZ would result in a disproportionate loss of capacity. Since the failed AZ contained a majority of the running instances, the remaining instances in the surviving AZ would be overloaded, causing significant performance degradation. The application remained available because the surviving instances and the Multi-AZ RDS instance continued to operate, but the reduced compute capacity led to degraded performance.
What should I do if I get this DOP-C02 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". 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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