C100DBA Replication Practice Question
A three-member replica set has default settings, and all three members are healthy. A DBA needs to perform an index build on the primary during business hours. Which concern should the DBA consider regarding replica set behavior?
⚠ Common exam trap
The trap here is assuming that index builds only happen on the primary and are later replicated through the oplog, when in fact modern MongoDB coordinates the build across all data-bearing members before committing.
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 index build will be applied to all data-bearing members, and the primary waits for secondaries to finish before committing the index.
In MongoDB 4.4 and later, index builds on replica sets are coordinated: the primary initiates the build, secondaries build concurrently, and the primary waits for all data-bearing secondaries to finish before committing. This ensures index consistency across the set. The operation can be resource-intensive and may impact replication lag temporarily, but it does not roll back or step down due to lag.
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 index build will cause the primary to step down if any secondary has a replication lag greater than 10 seconds.
Why it's wrong here
Replication lag does not trigger an automatic primary stepdown during an index build. The primary may step down if it cannot see a majority of voting members, but lag alone is not a stepdown condition. The primary continues to coordinate the index build as long as it maintains majority connectivity.
- ✗
The index build will only be performed on the primary, and secondaries will build the index lazily during their next election.
Why it's wrong here
Indexes are not built lazily on secondaries during elections. Replica set members maintain identical indexes, and index builds are propagated through the oplog. The primary coordinates the build across all data-bearing members, not just itself, so secondaries do not wait for an election to create the index.
- ✓
The index build will be applied to all data-bearing members, and the primary waits for secondaries to finish before committing the index.
Why this is correct
In MongoDB 4.4 and later, index builds are coordinated across all data-bearing replica set members. The primary starts the build and waits for the secondaries to complete their builds before committing the index, ensuring the index is consistent across the set. This behavior affects the duration and resource usage during the build.
- ✗
The index build will be rolled back on the secondaries if any secondary falls behind.
Why it's wrong here
Index builds are not rolled back merely because a secondary falls behind. Starting in MongoDB 4.4, index builds on replica sets occur simultaneously on all data-bearing members, and the primary waits for secondaries to finish before committing. If a secondary lags, it may delay commit, but there is no automatic rollback of the index build due to lag.
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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.