The answer is that an I/O bottleneck on the writer instance is the most likely cause of high replica lag in Aurora MySQL. When the writer experiences insufficient I/O capacity, it cannot commit transactions and write redo logs to the shared storage volume quickly enough, forcing the replica to wait for these updates before applying them. This directly manifests as elevated ReadReplicaLag even when CPU utilization remains low, since the bottleneck is storage throughput rather than compute. On the AWS Certified Database Specialty DBS-C01 exam, this question tests your understanding that replica lag is not always a network or compute issue—it often points to I/O constraints on the writer, especially during peak write-heavy periods. A common trap is assuming high CPU or SELECT queries cause lag, but Aurora replicas handle reads independently. Remember the memory tip: “Low CPU, high lag? Check the writer’s I/O bag.”
DBS-C01 Monitoring and Troubleshooting Practice Question
This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. 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 is running an Amazon Aurora MySQL-Compatible DB cluster. The application is experiencing increased latency during peak hours. The DB cluster's CPU utilization is at 40%, but the ReadReplicaLag metric is high. Which of the following 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The writer instance is experiencing I/O bottlenecks, causing slow replication
Option A is correct because high replica lag indicates the replica is falling behind, often due to insufficient I/O capacity on the writer or replica. Option B is wrong because read replicas handle SELECT queries. Option C is wrong because binary log retention does not affect replica lag directly. Option D is wrong because the DB cluster parameter group is shared.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 writer instance is experiencing I/O bottlenecks, causing slow replication
Why this is correct
I/O bottlenecks on the writer can delay writing to the binary log, increasing replica lag.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The read replica is receiving too many write requests
Why it's wrong here
Read replicas do not accept write requests in Aurora.
✗
The DB cluster parameter group is misconfigured
Why it's wrong here
Parameter group misconfiguration is less likely to cause replica lag.
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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. NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated. 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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DBS-C01 NAT questions on configuration and troubleshooting.
Monitoring and Troubleshooting — This question tests Monitoring and Troubleshooting — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The writer instance is experiencing I/O bottlenecks, causing slow replication — Option A is correct because high replica lag indicates the replica is falling behind, often due to insufficient I/O capacity on the writer or replica. Option B is wrong because read replicas handle SELECT queries. Option C is wrong because binary log retention does not affect replica lag directly. Option D is wrong because the DB cluster parameter group is shared.
What should I do if I get this DBS-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DBS-C01 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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