DEA-C01 Data Operations and Support Practice Question
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?
Answer choices
Why each option matters
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.
The ORA-1555 'snapshot too old' error occurs when a long-running query (such as Change Data Capture (CDC) in AWS DMS) needs to read consistent data from undo segments, but the undo information has been overwritten or retained for too short a period. Increasing the UNDO tablespace size and setting UNDO_RETENTION to a higher value ensures that undo data is preserved longer, allowing CDC to read consistent snapshots without encountering this error. Option A (disabling supplemental logging) would prevent CDC from capturing changes, thus is incorrect. Option B (increasing redo logs) does not address the undo retention issue. Option C (enabling batch optimized apply) may improve apply performance but does not resolve the source-side undo problem.
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.
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