C100DBA Replication Practice Question
When a secondary node is lagging behind the primary, what is the most significant risk to the replica set if the lag time exceeds the length of the oplog?
⚠ Common exam trap
Candidates often guess that lagging nodes simply retry automatically or drop old data, missing the critical requirement for an expensive full resync.
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 secondary will enter a 'RECOVERING' state and require a full resync.
If a secondary falls behind the primary beyond the duration of the oplog, the secondary must perform an initial sync. This involves cloning the entire dataset, which is resource-intensive and can significantly degrade performance on the primary. Managing oplog size is a critical operational task to ensure that transient network issues do not force expensive full data copies across the network.
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 secondary will trigger an automatic election to become primary.
Why it's wrong here
A lagging secondary is ineligible to become primary because its data is out of sync. Election algorithms prioritize nodes with the most recent data to prevent inconsistencies. Therefore, a secondary that has lost its sync source via oplog truncation is strictly prevented from participating in the election process.
- ✓
The secondary will enter a 'RECOVERING' state and require a full resync.
Why this is correct
When the oplog entries required for synchronization are no longer present on the primary, the secondary can no longer replicate incrementally. It transitions to a recovering state and must perform a full initial sync, which involves deleting existing data and copying the entire dataset from a healthy replica set member.
- ✗
The primary will automatically reduce its write concern to accommodate the lag.
Why it's wrong here
Write concern is a client-side setting or a global configuration. The primary does not dynamically lower its write concern based on the health of secondaries. If a client requests w:'majority', the primary will simply wait until enough nodes respond, potentially causing the primary to stall if the majority isn't reached.
- ✗
The replica set will automatically increase the oplog size to compensate.
Why it's wrong here
MongoDB does not automatically resize the oplog. Administrators must manually manage the oplog size based on expected write volume and the time required to recover from network outages. Relying on the system to manage its own storage resources for the oplog would lead to unpredictable database behavior and disk exhaustion.
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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.