C100DBA Server Administration Practice Question
What is the consequence of having a replica set with only two nodes and no arbiter?
⚠ Common exam trap
Candidates assume that two nodes are sufficient for high availability, forgetting that a majority (quorum) of 2 is required, which cannot be achieved if one node fails.
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 replica set will be unable to elect a primary if one node fails.
A replica set requires a majority of nodes to elect a primary. In a two-node set, a failure of one node makes it impossible to reach a majority, as one out of two is not greater than half. This design limitation is critical because it leads to the loss of a primary and the inability to elect a new one, resulting in a read-only database and complete outage for write operations.
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 will automatically double the voting power of the primary.
Why it's wrong here
MongoDB does not allow a single node to dynamically double its voting power to achieve a majority. Every member has exactly one vote, regardless of its role. The voting mechanism is fixed by the configuration, and a majority of the configured members must be reachable to proceed with elections.
- ✓
The replica set will be unable to elect a primary if one node fails.
Why this is correct
With two nodes, a majority is two. If one node fails, only one node remains, which is not a majority of the set. Consequently, the remaining node cannot become or remain a primary, and no new election can occur, effectively stopping all write operations for the application.
- ✗
The replica set will function normally, but writes will be slow.
Why it's wrong here
The replica set will not function normally; it will cease to perform writes entirely. The inability to elect a primary means the database cannot process write requests, which is a major service interruption, not merely a performance issue. Performance is secondary to the complete loss of write capability.
- ✗
The primary will become read-only while the secondary remains idle.
Why it's wrong here
If the primary goes offline, the remaining node does not automatically become read-only; it simply cannot be elected as a primary. The cluster effectively halts all write operations, and the remaining node cannot fulfill the requirements of a primary node, regardless of the read-only or read-write status.
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.