C100DBA Replication Practice Question
When adding a new member to an existing replica set, what is the most common reason for the initial sync process to fail if the data set is extremely large?
⚠ Common exam trap
Candidates often blame network latency or hardware issues. They overlook that the primary's oplog is a fixed size and will overwrite history if the sync takes too long.
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 oplog size on the primary is smaller than the duration of the sync.
Initial sync often fails for large data sets because the oplog on the primary node is too short. If the syncing node takes longer to copy the initial data than the time it takes for the primary to overwrite the necessary oplog entries, the syncing node will fall too far behind and be forced to restart the process, causing an infinite loop of sync failures.
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 primary node lacks sufficient RAM for the new member.
Why it's wrong here
Initial sync is a disk-intensive and network-intensive process for the new member. While the primary needs to serve data, the primary's RAM usage does not typically cause sync failures. The primary simply reads from its data files and transmits them to the new member over the network protocol.
- ✓
The oplog size on the primary is smaller than the duration of the sync.
Why this is correct
If the oplog is too small, the primary will roll over its log entries before the new member finishes copying all data. Once the primary wraps around, the new member loses the pointer to the operations it needs to replay, causing it to fall behind and fail the initial sync process.
- ✗
The new member's hardware is not identical to the primary.
Why it's wrong here
MongoDB replica sets do not require identical hardware. A new member can be added with different hardware specifications, such as more or less RAM or different CPU speeds. The sync process will still function correctly as long as the new member can store the data and reach the primary.
- ✗
The replica set name does not match on the new member.
Why it's wrong here
If the replica set name did not match, the node would never be able to join the set in the first place. It would report a configuration mismatch error immediately upon startup, rather than failing during the initial data synchronization process, which is a later stage of joining the cluster.
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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.