C100DBA Replication Practice Question
What happens to the replica set if a majority of voting members are permanently lost?
⚠ Common exam trap
Candidates incorrectly assume the replica set remains fully functional as long as one node is alive, ignoring the requirement for a majority to maintain cluster consistency.
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 set stops accepting writes to maintain consistency.
A replica set requires a majority of its voting members to be online to elect a new primary and perform write operations with majority concern. If the majority is lost, the remaining nodes enter a read-only state, as they cannot reach consensus. This is a critical failure scenario that requires immediate administrative intervention, such as reconfiguring the set or restoring the failed nodes.
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 set automatically switches to a single-node mode to remain functional.
Why it's wrong here
MongoDB does not automatically degrade to single-node mode when a majority is lost, as this would violate the consistency requirements for write operations. The set remains in a read-only state to prevent inconsistent data from being written while the cluster lacks a quorum for confirming those operations.
- ✓
The set stops accepting writes to maintain consistency.
Why this is correct
Without a majority, the remaining nodes cannot achieve consensus for write operations. To prevent split-brain and maintain consistent data, the replica set stops accepting writes. This ensures that no data is committed until the cluster can confirm that a quorum of nodes has successfully processed the operation.
- ✗
The secondary nodes elect a new primary regardless of the missing majority.
Why it's wrong here
The voting protocol strictly requires a majority. A node cannot become primary without receiving votes from more than half of the configured voting members. This is the core mechanism that guarantees durability and consistency in the face of network partitions or hardware failures within the replica set infrastructure.
- ✗
The primary node continues to accept writes for a short period.
Why it's wrong here
If the primary itself is part of the minority, it will immediately step down and stop accepting writes. The entire goal of the consensus protocol is to ensure that no writes occur unless a majority can guarantee the operation. Once a majority is lost, all write activity effectively ceases.
About these practice questions
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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 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.