DBS-C01 Deployment and Migration Practice Question
A company is migrating a 3 TB SQL Server database to Amazon RDS for SQL Server. The migration must be completed with less than 15 minutes of downtime. The company has set up AWS DMS with ongoing replication. During the final cutover, the DMS task fails with a 'Target error: Cannot insert duplicate key' error. What is the most likely cause?
⚠ Common exam trap
Test-takers frequently confuse a target-side duplicate key error with a source-side primary key violation or think that truncating the target will fix all replication issues, but the real cause is the identity column's explicit insert restriction specific to SQL Server.
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 target table has an identity column that is not set to allow insert of explicit values.
The error 'Cannot insert duplicate key' during DMS full load or ongoing replication indicates that the target table has an identity column that does not allow explicit value insertion. By default, SQL Server identity columns are set to NOT FOR REPLICATION = OFF, meaning DMS cannot insert rows with explicit identity values, causing duplicate key violations when the source has identity values that conflict with the target's auto-generated sequence. To resolve this, you must set the identity column to allow explicit inserts (e.g., SET IDENTITY_INSERT ON) or configure DMS to use 'Do nothing' for identity handling.
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 target table has an identity column that is not set to allow insert of explicit values.
Why this is correct
If identity insert is off, DMS cannot insert source values, causing duplicate key errors.
- ✗
The DMS task is not set to truncate the target before loading.
Why it's wrong here
Truncate option is for full load, not ongoing replication.
- ✗
The source database transaction log is full.
Why it's wrong here
Transaction log full would cause DMS to fail, but not with duplicate key error.
- ✗
The source database has a primary key violation.
Why it's wrong here
Primary key violations on source would be unlikely to cause target duplicate key error during ongoing replication.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DBS-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 DBS-C01 exam.