C100DBA Replication Practice Question
What occurs when a replica set member experiences a 'rollback'?
⚠ Common exam trap
Candidates frequently mistake rollback for a simple 'undo' operation or a database crash recovery, failing to realize it involves moving data to local files for manual administrator intervention.
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 node discards data to align with the new primary and saves it to rollback files.
Rollback happens when a node was previously a primary, performed writes that were never replicated to the majority, and then rejoined the set after a new primary was elected. To maintain consistency, the old primary must discard those unacknowledged writes to match the new primary's state. These discarded writes are saved to a local BSON file for manual inspection and potential recovery by administrators.
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 node automatically merges its divergent data with the primary.
Why it's wrong here
MongoDB does not attempt to merge divergent data automatically, as this could lead to application-level conflicts and integrity issues. Instead, the node discards the inconsistent data to align itself with the current majority-confirmed state of the replica set, ensuring that all nodes eventually reach a single consensus.
- ✗
The node is permanently removed from the replica set until manually re-added.
Why it's wrong here
Rollback is an automated process for reconciliation. After the node discards its divergent data and catches up with the new primary, it automatically resumes its role as a secondary. It does not require manual intervention to be re-added, though administrators should inspect the rollback files for data loss.
- ✓
The node discards data to align with the new primary and saves it to rollback files.
Why this is correct
During a rollback, the node reverts its local state to the point of divergence from the current primary. The discarded operations are written to separate BSON files on the local filesystem. This process ensures the cluster returns to a consistent state while preserving the orphaned data for forensic analysis.
- ✗
The node initiates a global election to force a return to its old state.
Why it's wrong here
Rollback does not trigger an election to restore the previous state. The consensus protocol has already decided on a new primary. The node must accept the new reality of the cluster state and perform the rollback to become a functional, consistent secondary again, regardless of its previous status.
About these practice questions
Courseiva writes every C100DBA question from scratch — 222 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
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 MongoDB exam blueprint
This C100DBA practice question is part of Courseiva's free MongoDB 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 C100DBA exam.