C100DBA Replication Practice Question
Which THREE actions occur when a MongoDB secondary node performs an initial sync?
⚠ Common exam trap
Test-takers often assume initial sync copies pre-built indexes directly from the primary, missing the fact that indexes must be built locally.
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
✓
It copies all documents from each collection on the primary.
During initial sync, the node first clones all databases from the primary, then applies all changes recorded in the oplog that occurred during the cloning phase, and finally builds all necessary indexes for the collections. This multi-stage process ensures the new member reaches a consistent state with the primary without requiring an outage or downtime for the existing cluster members.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
It copies all documents from each collection on the primary.
Why this is correct
The first step of initial sync is to copy the data from all databases on the source. The node iterates through each collection and copies all documents. This is necessary to build a baseline state for the new member before it can begin processing the real-time oplog operations from the primary.
- ✗
It immediately becomes the primary to speed up the process.
Why it's wrong here
A node undergoing initial sync is in the 'STARTUP2' state and cannot become primary. It must first complete the data transfer and catch up to the primary's current state. Forcing a node to become primary during sync would be dangerous as it would not have the full dataset required for operations.
- ✓
It applies oplog entries that occurred during the data cloning phase.
Why this is correct
Since the data cloning phase takes time, the primary continues to accept new writes. The new member must track and then apply these oplog entries after the initial clone is finished. This ensures that the new member ends up in a consistent state with the primary despite the ongoing writes.
- ✓
It builds all indexes defined in the collections.
Why this is correct
Building indexes is the final stage of the initial sync. Once the data is copied and the oplog is applied, the node must rebuild all indexes for the collections to ensure queries can be performed efficiently. This ensures that the node is fully functional and ready to serve read traffic.
- ✗
It automatically purges all data from the primary node.
Why it's wrong here
Initial sync only affects the node joining the cluster. It never modifies or deletes data on the primary or any other existing members. Modifying the primary's data during a sync process would be catastrophic and would violate the core principles of data integrity and consistency in a distributed system.
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.