The correct answer is that the ECS service's minimum healthy percent was set to 100, which caused the desired count reduction to zero to be rejected during the update. This failure occurs because the minimum healthy percent configuration dictates the proportion of tasks that must remain running during a deployment; when set to 100, the ECS service cannot reduce the desired count below the current running count without violating the requirement that all tasks stay healthy. CloudFormation detects this stalled progress and, after waiting indefinitely for the deployment to complete, cancels the update with the 'Resource update cancelled' error and rolls back. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how ECS deployment parameters interact with CloudFormation stack updates, often appearing as a trap where candidates overlook that a 100% minimum healthy percent prevents any task reduction. A useful memory tip: "100% healthy means zero room to shrink"—if you cannot reduce tasks, your update cannot proceed.
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. 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.
Exhibit
Refer to the exhibit.
# AWS CloudFormation stack event log
2024-03-01 12:00:00 UTC UPDATE_IN_PROGRESS AWS::ECS::Service MyService
2024-03-01 12:01:00 UTC UPDATE_FAILED AWS::ECS::Service MyService Resource update cancelled
2024-03-01 12:01:00 UTC UPDATE_ROLLBACK_IN_PROGRESS AWS::ECS::Service MyService
2024-03-01 12:05:00 UTC UPDATE_ROLLBACK_COMPLETE AWS::ECS::Service MyService
A DevOps engineer updates an ECS service via CloudFormation. The stack update fails with the message 'Resource update cancelled'. The engineer notices that the ECS service's desired count was temporarily reduced during the update. What is the most likely cause of the failure?
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 ECS service's minimum healthy percent was set to 100, causing the desired count reduction to zero to be rejected.
The error 'Resource update cancelled' occurs because CloudFormation detected that the ECS service update was not progressing as expected. When the minimum healthy percent is set to 100, the deployment process cannot reduce the desired count to zero (or below the current running count) without violating the requirement that 100% of the tasks remain healthy. This causes the update to be cancelled as CloudFormation waits indefinitely for the deployment to complete, eventually timing out and rolling back.
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 ECS service's minimum healthy percent was set to 100, causing the desired count reduction to zero to be rejected.
Why this is correct
CloudFormation reduces desired count to 0 during update; if min healthy percent is 100, it fails.
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 ECS service's target group had an unhealthy instance that prevented the deregistration.
Why it's wrong here
Unhealthy instances cause deployment issues, not cancellation.
✗
The ECS service deployment circuit breaker was triggered due to a timeout.
Why it's wrong here
Circuit breaker is for failed deployments, not for cancelled updates.
✗
The ECS service did not have the required IAM role to call ecs:UpdateService.
Why it's wrong here
This would cause an access denied error, not cancellation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the 'Resource update cancelled' error with a permissions or circuit breaker issue, but the key clue is the temporary reduction in desired count, which directly points to a minimum healthy percent constraint that prevents the service from scaling down to zero.
Detailed technical explanation
How to think about this question
The minimum healthy percent parameter in an ECS service defines the lower limit on the number of tasks that must remain healthy during a deployment. When set to 100, it means that during a rolling update, the service cannot reduce the running count below 100% of the desired count, effectively preventing any task replacement unless new tasks are started first. CloudFormation's 'Resource update cancelled' error often occurs when a deployment is stuck because the service cannot meet the minimum healthy percent requirement, leading to a timeout after the configured deployment maximum percent is also considered.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Incident and Event Response — This question tests Incident and Event Response — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ECS service's minimum healthy percent was set to 100, causing the desired count reduction to zero to be rejected. — The error 'Resource update cancelled' occurs because CloudFormation detected that the ECS service update was not progressing as expected. When the minimum healthy percent is set to 100, the deployment process cannot reduce the desired count to zero (or below the current running count) without violating the requirement that 100% of the tasks remain healthy. This causes the update to be cancelled as CloudFormation waits indefinitely for the deployment to complete, eventually timing out and rolling back.
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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Question Discussion
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