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?
⚠ Common exam trap
The trap is confusing redo logs (crash recovery) with undo tablespace (read consistency), leading candidates to increase redo log size when the actual fix is undo retention.
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.
ORA-1555 'snapshot too old' occurs when Oracle cannot reconstruct a consistent read image because the required undo data has been overwritten. Increasing the UNDO tablespace size and raising UNDO_RETENTION gives Oracle more undo history to retain, allowing the long-running DMS full-load read to complete without losing its snapshot. This directly addresses the root cause of insufficient undo retention on the source.
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
Disabling supplemental logging removes the row-change metadata DMS needs for ongoing replication, breaking the task rather than resolving snapshot expiry. It is tempting as a way to reduce source overhead, but ORA-1555 requires extending undo retention or shortening the read window.
- ✗
Increase the size of the redo logs on the source database.
Why it's wrong here
Redo log size governs redo retention, not undo retention; ORA-1555 arises when the undo tablespace cannot retain the read-consistent image the DMS extract needs. Enlarging redo logs is tempting because redo sizing genuinely fixes log-switch and archiving bottlenecks, but here the fix is increasing undo retention or undo tablespace size.
- ✗
Enable batch optimized apply on the DMS task.
Why it's wrong here
Batch optimized apply changes target-side transaction grouping, leaving the source's undo retention untouched, so ORA-1555 persists. It is tempting because it speeds replication, but the error is source snapshot expiry; increasing undo retention or the task's read window addresses it.
- ✓
Increase the UNDO tablespace size and set UNDO_RETENTION to a higher value.
Why this is correct
ORA-1555 occurs when Oracle cannot reconstruct a consistent read image because undo data was overwritten before the DMS read completed. Enlarging the UNDO tablespace and raising UNDO_RETENTION preserves that undo long enough for the long-running extraction.
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Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
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