The answer is to increase the UNDO tablespace size and set UNDO_RETENTION to a higher value. This resolves the ORA-1555 snapshot too old error because AWS DMS uses long-running transactions for Change Data Capture (CDC) to read consistent old data from Oracle’s UNDO segments; when those segments are too small or the retention period is too short, the consistent read fails. On the AWS Certified Data Engineer Associate DEA-C01 exam, this question tests your understanding of how CDC replication interacts with Oracle’s undo management—a common trap is confusing redo logs (which handle crash recovery) with undo segments (which handle read consistency). Remember that increasing redo logs won’t fix a snapshot too old error, and disabling CDC or using batch optimized mode avoids the problem rather than solving it. Memory tip: “UNDO for undo, REDO for redo”—snapshot too old is always an undo space or retention issue.
DEA-C01 Data Operations and Support Practice Question
This DEA-C01 practice question tests your understanding of data operations and support. 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 data engineer is troubleshooting a DMS task that is replicating data from an on-premises Oracle database to an RDS for MySQL instance. The task is failing with 'ORA-1555: snapshot too old' error. What is the best course of action?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Increase the UNDO tablespace size and set UNDO_RETENTION to a higher value.
Option A is correct because the snapshot too old error occurs when UNDO segments are too small or retention is too short for the long-running transaction needed by CDC. Increasing undo tablespace size and retention time allows CDC to read consistent old data. Option B is wrong because increasing redo logs does not prevent snapshot too old errors. Option C is wrong because it would lose CDC capability. Option D is wrong because batch optimized mode may not work with CDC and does not address the root cause.
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.
✗
Disable full supplemental logging on the source tables.
Why it's wrong here
Supplemental logging is required for CDC; disabling it would break replication.
✗
Increase the size of the redo logs on the source database.
Why it's wrong here
Redo logs are for recovery, not undo retention.
✗
Enable batch optimized apply on the DMS task.
Why it's wrong here
This improves apply performance but does not fix the undo issue.
✓
Increase the UNDO tablespace size and set UNDO_RETENTION to a higher value.
Why this is correct
This gives the CDC process enough undo to read consistent snapshots.
Clue confirmation
The clue word "best" 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 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. 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. 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 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 Operations and Support — This question tests Data Operations and Support — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Increase the UNDO tablespace size and set UNDO_RETENTION to a higher value. — Option A is correct because the snapshot too old error occurs when UNDO segments are too small or retention is too short for the long-running transaction needed by CDC. Increasing undo tablespace size and retention time allows CDC to read consistent old data. Option B is wrong because increasing redo logs does not prevent snapshot too old errors. Option C is wrong because it would lose CDC capability. Option D is wrong because batch optimized mode may not work with CDC and does not address the root cause.
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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Standard ACLs match source addresses.
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