The answer is that the read replica is in a different AWS Region than the source. This is correct because cross-region replication traffic must traverse the public internet or a VPN connection, introducing network latency and potential bandwidth constraints that directly impact the asynchronous MySQL replication process. Since replication relies on a single I/O thread to download binary log events from the source, any network delay causes the replica to fall further behind, resulting in increasing replica lag. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of how cross-region network latency differs from intra-region replication, where traffic stays within AWS’s low-latency backbone. A common trap is to blame the source instance’s workload or storage configuration, but the key clue is the geographic distance between the instances. Memory tip: think “single thread, cross-region dread”—a single I/O thread cannot compensate for high network latency across regions.
DBS-C01 Management and Operations Practice Question
This DBS-C01 practice question tests your understanding of management and operations. 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.
Network Topology
A database administrator runs the above AWS CLI command to troubleshoot replication issues. The DB instance 'mydb' is a read replica of 'my-source-db'. The administrator notices that the replica lag is increasing. 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 read replica is in a different AWS Region than the source.
Option D is correct because when a read replica is in a different AWS Region than the source, the replication traffic must traverse the public internet or a VPN connection, introducing network latency and potential bandwidth constraints. This cross-region lag is a common cause of increasing replica lag, as the asynchronous MySQL replication relies on a single I/O thread to download the binary log events from the source, and any network delay directly impacts the replica's ability to keep up.
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 source DB instance is running a different MySQL version.
Why it's wrong here
The version is the same.
✗
The read replica has Multi-AZ disabled.
Why it's wrong here
Multi-AZ on a replica does not affect replication lag.
✗
The read replica is using a smaller instance class than the source.
Why it's wrong here
The instance class is not specified as smaller; it's db.r5.large.
✓
The read replica is in a different AWS Region than the source.
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume instance size (Option C) is the primary cause of replica lag, but the question explicitly mentions a cross-Region scenario (implied by the AWS CLI command targeting a different Region), making network latency the most likely culprit over compute capacity.
Detailed technical explanation
How to think about this question
Under the hood, MySQL replication uses a single-threaded I/O thread on the replica to fetch binary log events from the source's dump thread. In a cross-Region setup, the network round-trip time (RTT) can be hundreds of milliseconds, and if the source generates binary log events faster than the replica can download them, the lag grows. Additionally, RDS for MySQL uses asynchronous replication, meaning the source commits transactions without waiting for the replica, so any network bottleneck directly manifests as increasing Seconds_Behind_Master.
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.
Management and Operations — This question tests Management and Operations — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The read replica is in a different AWS Region than the source. — Option D is correct because when a read replica is in a different AWS Region than the source, the replication traffic must traverse the public internet or a VPN connection, introducing network latency and potential bandwidth constraints. This cross-region lag is a common cause of increasing replica lag, as the asynchronous MySQL replication relies on a single I/O thread to download the binary log events from the source, and any network delay directly impacts the replica's ability to keep up.
What should I do if I get this DBS-C01 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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