DEA-C01 Data Operations and Support Practice Question
A data engineer is using AWS Database Migration Service (AWS DMS) to migrate an on-premises Oracle database to Amazon Redshift. The migration uses a full load plus change data capture (CDC). During the CDC phase, the engineer notices that some updates are not being applied to the target Redshift tables. The DMS task logs show no errors. What is the MOST likely cause?
⚠ Common exam trap
The trap here is assuming that target-side constraints like primary keys or LOB settings cause missing updates, when the root cause is source-side CDC configuration.
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
✓
The source Oracle database is not configured for supplemental logging, so DMS cannot capture all column changes.
Oracle CDC in AWS DMS relies on supplemental logging to capture column-level changes. If supplemental logging is not enabled, the redo logs lack the necessary information for DMS to reconstruct updates, leading to missed changes without errors. Enabling supplemental logging at the database or table level resolves the issue.
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 Redshift target table does not have a primary key defined, causing DMS to skip updates.
Why it's wrong here
AWS DMS does not require a primary key on the Redshift target to apply updates. DMS uses the source table's primary key to identify rows for update or delete. The absence of a target primary key does not cause DMS to skip updates; it may affect performance or duplicate handling but not silently drop changes.
- ✗
The DMS replication instance is undersized, causing it to drop change events under load.
Why it's wrong here
An undersized replication instance can cause performance issues or task failures, but DMS does not silently drop change events. If the instance is overwhelmed, the task would typically log errors or lag behind, not apply only some updates without any error messages.
- ✓
The source Oracle database is not configured for supplemental logging, so DMS cannot capture all column changes.
Why this is correct
For Oracle CDC, DMS requires supplemental logging to be enabled at the database or table level. Without it, redo logs may not contain enough information to reconstruct updates, especially for columns not in the primary key. This can cause updates to be missed silently, as DMS cannot capture the full before/after image.
- ✗
The DMS task is configured with 'Limited LOB mode' and large LOB columns are being truncated, causing updates to be ignored.
Why it's wrong here
Limited LOB mode truncates large LOB data, but it does not cause updates to be ignored entirely. The update would still be applied with truncated LOB values. The symptom of missing updates without errors points to CDC capture issues, not LOB handling.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
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