The answer is asynchronous replication lag, which is the most likely cause of stale data from an RDS Read Replica. Amazon RDS for MySQL uses asynchronous replication, meaning the primary instance commits writes first and then sends binary log events to the replica. This introduces replication lag—a delay between when a write occurs on the primary and when it appears on the replica. If your application reads from the replica before that lag is resolved, it will see older, stale data. On the AWS Certified Database Specialty DBS-C01 exam, this question tests your understanding of the fundamental trade-off: read replicas boost read performance but sacrifice read-after-write consistency due to asynchrony. A common trap is assuming replicas are synchronous like Multi-AZ failover; they are not. Remember the memory tip: “Replicas are for reads, not recency—lag is the cost of scale.”
DBS-C01 Deployment and Migration Practice Question
This DBS-C01 practice question tests your understanding of deployment and migration. 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 deploying a new web application that uses Amazon RDS for MySQL. To improve read performance, they add a read replica. However, the application reports stale data. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The replication is asynchronous and there is replication lag.
Amazon RDS for MySQL uses asynchronous replication for read replicas. The primary instance commits writes and then sends the binary log (binlog) events to the replica, which applies them. This asynchronous nature introduces replication lag, meaning the replica may not have the most recent writes. If the application reads from the replica before the lag is resolved, it will see stale (older) data.
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 is using a connection pool that caches connections to the primary.
Why it's wrong here
Connection pooling does not cause stale data.
✗
The read replica is in a different AWS Region.
Why it's wrong here
Cross-region replicas have more lag but stale data can happen even in same region.
✓
The replication is asynchronous and there is replication lag.
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The application is reading from the replica before the write is committed on the primary.
Why it's wrong here
Write is committed on primary; replica lag is the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse asynchronous replication with synchronous replication, assuming the replica always has the latest data, or they incorrectly attribute the issue to geographic distance or connection pooling rather than the fundamental replication lag inherent in MySQL's async model.
Detailed technical explanation
How to think about this question
Replication lag in Amazon RDS for MySQL is measured in seconds and can be monitored via the `ReplicaLag` CloudWatch metric. The lag occurs because the replica's SQL thread applies binlog events sequentially, and heavy write traffic on the primary or a slow replica instance can increase lag. In real-world scenarios, a common mitigation is to use the `SHOW REPLICA STATUS` command to check `Seconds_Behind_Master` and implement read-after-write consistency by routing critical reads to the primary.
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.
Deployment and Migration — This question tests Deployment and Migration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The replication is asynchronous and there is replication lag. — Amazon RDS for MySQL uses asynchronous replication for read replicas. The primary instance commits writes and then sends the binary log (binlog) events to the replica, which applies them. This asynchronous nature introduces replication lag, meaning the replica may not have the most recent writes. If the application reads from the replica before the lag is resolved, it will see stale (older) data.
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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