The answer is application-level DNS caching causing downtime after an RDS Multi-AZ failover. When the failover completes in under 30 seconds, the DNS record for the writer endpoint updates to the new primary, but if the application caches DNS resolutions, it continues connecting to the old writer’s IP address, which is now a standby and refuses connections. This mismatch persists until the DNS cache expires or is manually flushed, explaining the full five-minute outage despite the rapid failover. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how DNS TTL and caching interact with RDS failover behavior—a common trap is assuming the database itself caused the delay, when the real bottleneck is client-side resolution. Remember the mnemonic: “Cache the query, lose the primary” to recall that stale DNS entries, not database slowness, are the usual culprit in prolonged failover downtime.
DOP-C02 Resilient Cloud Solutions Practice Question
This DOP-C02 practice question tests your understanding of resilient cloud solutions. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 behind an Application Load Balancer. The application uses an Amazon RDS for PostgreSQL Multi-AZ DB instance. During a recent failover test, the application experienced a 5-minute downtime. The RDS failover completed within 30 seconds. What is the most likely cause of the prolonged downtime?
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 caches DNS resolutions, causing it to connect to the old writer endpoint
The most likely cause is that the application caches DNS resolutions, causing it to continue connecting to the old writer endpoint after failover. When an RDS Multi-AZ failover occurs, the DNS record for the writer endpoint is updated to point to the new primary instance, but the application's cached DNS entry still points to the old IP address. Since the old primary is now a standby and no longer accepts connections, the application experiences downtime until the DNS cache expires (typically 5–60 seconds) or the application refreshes the DNS resolution. The 5-minute downtime suggests the application uses a long DNS TTL or a custom caching layer that delays reconnection.
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 application caches DNS resolutions, causing it to connect to the old writer endpoint
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The RDS Multi-AZ failover took longer than expected due to a large transaction log
Why it's wrong here
Failover is typically fast.
✗
The Application Load Balancer health checks marked all instances as unhealthy during the failover
Why it's wrong here
Unlikely to cause 5-min outage.
✗
The application was using read replicas for writes, which failed during failover
Why it's wrong here
Read replicas are read-only.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the 5-minute downtime must be caused by the database failover itself, but the question explicitly states the failover completed in 30 seconds, so the real issue is application-side DNS caching or stale connection handling.
Detailed technical explanation
How to think about this question
RDS Multi-AZ failover updates the DNS A record for the writer endpoint with a TTL of 5 seconds by default, but many applications or operating systems cache DNS for longer periods (e.g., Java's InetAddress cache defaults to 30 seconds, or custom connection pools may cache resolved addresses indefinitely). The application must implement a retry mechanism with exponential backoff and re-resolve DNS on connection failure to avoid prolonged downtime. In practice, using a connection string that resolves the DNS name at each connection attempt, rather than caching the IP, is critical for high availability.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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 application caches DNS resolutions, causing it to connect to the old writer endpoint — The most likely cause is that the application caches DNS resolutions, causing it to continue connecting to the old writer endpoint after failover. When an RDS Multi-AZ failover occurs, the DNS record for the writer endpoint is updated to point to the new primary instance, but the application's cached DNS entry still points to the old IP address. Since the old primary is now a standby and no longer accepts connections, the application experiences downtime until the DNS cache expires (typically 5–60 seconds) or the application refreshes the DNS resolution. The 5-minute downtime suggests the application uses a long DNS TTL or a custom caching layer that delays reconnection.
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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