C100DBA Replication Practice Question
A MongoDB DBA is investigating replication lag on a secondary member named rs0-2. The DBA runs rs.status() and notices that the 'optimeDate' of rs0-2 is several hours behind the primary's 'optimeDate'. The DBA wants to quickly determine the exact time difference. Which field in the rs.status() output should the DBA compare to calculate the replication lag?
⚠ Common exam trap
A common mix-up: candidates confuse network latency metrics like 'pingMs' or heartbeat information with actual replication lag, which is based on oplog application timestamps.
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 'optimeDate' field of each member.
Replication lag is calculated by comparing the 'optimeDate' of the secondary with that of the primary. The 'optimeDate' field shows the timestamp of the last oplog entry applied by each member, so the difference indicates how far behind the secondary is.
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 'optimeDate' field of each member.
Why this is correct
The 'optimeDate' field in rs.status() provides the human-readable date of the last oplog entry applied by each member. Subtracting the secondary's 'optimeDate' from the primary's 'optimeDate' gives the replication lag. This is the standard method to quickly assess lag.
- ✗
The 'pingMs' field of each member.
Why it's wrong here
'pingMs' measures the network round-trip time between members, which is a measure of network latency, not replication lag. A high pingMs might contribute to lag, but it does not directly indicate how far behind the secondary is in applying operations.
- ✗
The 'lastHeartbeat' field of each member.
Why it's wrong here
'lastHeartbeat' indicates the last time a heartbeat was received from a member, which is used for health monitoring, not for measuring replication lag. It does not reflect the timestamp of the last applied operation, so it cannot be used to calculate replication delay.
- ✗
The 'optime' field of each member.
Why it's wrong here
The 'optime' field contains a timestamp and term, but it is not a human-readable date. Comparing 'optime' values requires converting the timestamp, which is less straightforward than comparing dates. The 'optimeDate' field is designed for easy comparison of replication lag.
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.