The answer is a stale DNS cache causing the application to connect to the old primary’s IP address. After an RDS Multi-AZ failover, the DNS endpoint’s CNAME remains unchanged, but its underlying IP address updates to point to the new writer. If the application caches this resolved IP, it will attempt to connect to the old primary—now a replica or unavailable node—even though the RDS console shows the instance as Available. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this question tests your understanding of how RDS Multi-AZ failover DNS caching can silently break connectivity, a common trap where engineers assume the endpoint itself changes. The key insight is that the DNS record is a CNAME, not an A record; applications must always resolve the endpoint fresh on each connection. Memory tip: “CNAME stays the same, IP flips—don’t cache the flip.”
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. The database instance fails and AWS automatically fails over to the standby. After the failover, the application cannot connect to the database. The engineer checks the RDS console and sees that the instance status is Available. What is the MOST likely cause of the connectivity 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.
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 database's DNS endpoint for the old primary, which is no longer the writer.
After an RDS Multi-AZ failover, the DNS endpoint for the DB instance remains the same but its underlying IP address changes to point to the new primary (formerly the standby). If the application caches the IP address of the old primary or uses a direct connection to the old writer endpoint, it will attempt to connect to a node that is no longer the writer. The correct practice is to always connect using the RDS instance endpoint (CNAME), which automatically resolves to the current writer, and to avoid caching the resolved IP address. Since the instance status is 'Available', the new primary is ready, so the issue is a stale connection target.
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 security group for the RDS instance has changed during failover.
Why it's wrong here
Security groups remain the same.
✓
The application is using the database's DNS endpoint for the old primary, which is no longer the writer.
Why this is correct
After failover, the writer endpoint points to the new primary, but if the application caches the old endpoint, it may fail.
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 DNS record for the RDS endpoint has not propagated to the application's DNS resolver.
Why it's wrong here
RDS DNS updates quickly and is not usually the issue.
✗
The database instance is still in the process of failover and is not yet accepting connections.
Why it's wrong here
The status is Available, so failover is complete.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a failed Multi-AZ failover or a DNS propagation delay, when in reality the instance is healthy and DNS updates quickly, but the application's cached IP address from the old primary is the root cause.
Detailed technical explanation
How to think about this question
Under the hood, RDS Multi-AZ uses synchronous replication to a standby in a different Availability Zone. During failover, AWS updates the DNS CNAME record for the DB instance endpoint to point to the new primary's private IP address within seconds. However, many applications use connection pooling or DNS caching libraries (e.g., JVM's InetAddress cache) that ignore TTL and cache IP addresses for minutes or hours. This is a common cause of 'stale DNS' connectivity failures after failover, and the fix is to either configure the application to not cache DNS or to use a proxy like RDS Proxy that handles failover transparently.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 application is using the database's DNS endpoint for the old primary, which is no longer the writer. — After an RDS Multi-AZ failover, the DNS endpoint for the DB instance remains the same but its underlying IP address changes to point to the new primary (formerly the standby). If the application caches the IP address of the old primary or uses a direct connection to the old writer endpoint, it will attempt to connect to a node that is no longer the writer. The correct practice is to always connect using the RDS instance endpoint (CNAME), which automatically resolves to the current writer, and to avoid caching the resolved IP address. Since the instance status is 'Available', the new primary is ready, so the issue is a stale connection target.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company is using Amazon RDS for MySQL with Multi-AZ deployment. The database experiences a failover due to an availability zone outage. After the failover, the application team reports that the database endpoint is not resolving to the new primary. What is the most likely reason?
medium
A.The RDS CNAME record was not updated by AWS after the failover.
B.The application is using the read replica endpoint instead of the primary endpoint.
C.The application is using a Route 53 health check that failed and redirected traffic away from the endpoint.
✓ D.The application is using a cached DNS resolution that points to the old primary.
Why D: Option D is correct because after an RDS Multi-AZ failover, the DNS CNAME record for the DB instance is updated to point to the new primary in the standby AZ. However, if the application or its DNS resolver has cached the previous DNS resolution, it will continue to use the old IP address, which is no longer reachable. This is a common issue that can be resolved by reducing the TTL on the DNS record or implementing retry logic with DNS re-resolution in the application.
Last reviewed: Jun 11, 2026
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.