The answer is the application using old master user credentials that were changed on the primary but not replicated to the standby. This is the most likely cause of an RDS Multi-AZ failover access denied credentials error because, during a failover, the standby instance promotes to primary with its own local user database; any credential changes made to the original primary after the last replication checkpoint are not automatically synced to the standby, leaving the new primary with outdated or missing user entries. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how Multi-AZ failover handles authentication state versus storage state—a common trap is assuming credentials are replicated like data, but they are not. Remember the memory tip: “Data syncs, users don’t—update your secrets after failover.”
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.
A company uses Amazon RDS for MySQL with Multi-AZ deployment. During an incident, the primary DB instance becomes unreachable. The failover to the standby instance succeeds, but application connections are failing with 'Access denied for user'. What is the most likely cause?
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: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The application is using the old master user credentials that were changed on the primary but not replicated to the standby
Option A is correct because after failover, the standby instance may have different credentials if not using the same secret. Option B is incorrect because DNS propagation does not cause access denied errors. Option C is incorrect because the security group is associated with the RDS instance, not the endpoint. Option D is incorrect because the standby instance has the same storage as the primary.
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 DNS CNAME for the RDS endpoint has not propagated to the application's DNS resolver
Why it's wrong here
DNS issues cause connection timeouts, not access denied.
✗
The standby instance has a different storage configuration than the primary
Why it's wrong here
Storage is replicated; the standby has the same configuration.
✓
The application is using the old master user credentials that were changed on the primary but not replicated to the standby
Why this is correct
Credentials are not replicated across Multi-AZ; they must be the same.
Clue confirmation
The clue words "most likely", "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The security group for the RDS instance does not allow inbound traffic from the application's new IP address
Why it's wrong here
Security groups are attached to the RDS instance, which retains the same endpoint.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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 application is using the old master user credentials that were changed on the primary but not replicated to the standby — Option A is correct because after failover, the standby instance may have different credentials if not using the same secret. Option B is incorrect because DNS propagation does not cause access denied errors. Option C is incorrect because the security group is associated with the RDS instance, not the endpoint. Option D is incorrect because the standby instance has the same storage as the primary.
What should I do if I get this DOP-C02 question wrong?
Identify which DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "most likely", "primary". 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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