The answer is that the source Oracle database is not configured to retain archived redo logs for a sufficient period. This is the most likely cause because AWS DMS Change Data Capture (CDC) relies entirely on reading the source database’s transaction logs—specifically Oracle’s archived redo logs—to identify and apply incremental changes to the target. If these logs are purged or overwritten before DMS has a chance to process them, any changes made during that gap are simply lost, resulting in missing data on the target. On the AWS Certified Data Engineer Associate DEA-C01 exam, this question tests your understanding of DMS CDC prerequisites and common misconfigurations; a frequent trap is confusing network issues (like VPC peering) with log retention problems. Remember: DMS is a log reader, not a query engine for CDC—if the logs are gone, the changes are gone. Memory tip: “Logs lost, changes lost—keep your redo logs long enough for DMS to coast.”
DEA-C01 Data Ingestion and Transformation Practice Question
This DEA-C01 practice question tests your understanding of data ingestion and transformation. 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 AWS DMS to migrate data from an on-premises Oracle database to Amazon RDS for PostgreSQL. The migration is ongoing with continuous replication. The data engineer notices that some changes are not being captured in the target database. 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 source Oracle database is not configured to retain archived redo logs for a sufficient period.
Option A is correct because DMS uses the source database's transaction logs (redo logs) for CDC; if they are not retained, DMS cannot read changes. Option B is wrong because VPC peering affects network connectivity, not CDC. Option C is wrong because DMS uses its own task logs, not CloudTrail. Option D is wrong because table mapping would cause missing tables, not missing changes.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 VPC peering connection between on-premises and AWS is down.
Why it's wrong here
Would cause connectivity error, not missing changes.
✗
The DMS task's table mapping is incorrectly configured.
Why it's wrong here
Would cause specific tables to be missing, not changes.
✗
DMS is not publishing task logs to CloudWatch Logs.
Why it's wrong here
CloudWatch logging is for monitoring, not capturing changes.
✓
The source Oracle database is not configured to retain archived redo logs for a sufficient period.
Why this is correct
DMS requires archived logs to capture changes; if logs are purged, changes are lost.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related DEA-C01 ACL questions on filtering logic and placement.
Data Ingestion and Transformation — This question tests Data Ingestion and Transformation — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The source Oracle database is not configured to retain archived redo logs for a sufficient period. — Option A is correct because DMS uses the source database's transaction logs (redo logs) for CDC; if they are not retained, DMS cannot read changes. Option B is wrong because VPC peering affects network connectivity, not CDC. Option C is wrong because DMS uses its own task logs, not CloudTrail. Option D is wrong because table mapping would cause missing tables, not missing changes.
What should I do if I get this DEA-C01 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related DEA-C01 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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Question Discussion
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