DEA-C01 Data Operations and Support Practice Question
A company uses AWS DMS to migrate a 2 TB Oracle database to Amazon RDS for PostgreSQL. The migration completes successfully, but data validation shows some tables have missing rows. The task is configured for ongoing replication using change data capture (CDC). What is the MOST likely cause of the missing rows?
⚠ Common exam trap
DEA-C01 often tests the misconception that DMS CDC works on any table, when missing primary keys silently break change capture and cause missing rows.
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
✓
Source tables missing primary keys
AWS DMS requires a primary key or unique index on source tables to reliably identify and apply row changes during CDC. Without a primary key, DMS cannot uniquely match rows for updates and deletes, and it may also fail to capture all inserts during the initial load plus CDC handoff, resulting in missing rows.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Source database archive log retention period too short
Why it's wrong here
If archive logs are purged before DMS reads them, CDC changes are lost, producing missing rows in the target. It tempts because it is a genuine CDC failure mode, but the stem's completed full load with ongoing replication points to log retention rather than task configuration.
- ✗
Large objects (LOBs) not supported by the target
Why it's wrong here
AWS DMS does replicate LOB columns to PostgreSQL; unsupported LOBs cause task failure or column exclusion, not silent row loss in specific tables. It is tempting because LOB handling is a known DMS limitation, but missing rows during CDC typically stem from replication instance or task configuration issues.
- ✓
Source tables missing primary keys
Why this is correct
DMS applies CDC changes using the source table's primary key to identify and update the correct target rows. Tables lacking a primary key cannot be matched reliably, so updates and deletes are dropped or misapplied, producing missing rows in the PostgreSQL target after replication.
- ✗
Insufficient storage on the DMS replication instance
Why it's wrong here
Storage exhaustion halts or aborts a replication task outright, surfacing as task failures in CloudWatch, not as silently missing rows after a successful load. Replication instance storage sizing matters when staging large LOBs or cached changes, so it would be the culprit if the task had stopped with a full-disk error.
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JA
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
This DEA-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DEA-C01 exam.