C100DBA Replication Practice Question
In a replica set, what is the 'oplog' and why is it important?
⚠ Common exam trap
Candidates often confuse the oplog with the journal, believing the oplog is for crash recovery rather than the primary mechanism for asynchronous data replication between nodes.
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
✓
A capped collection that records all write operations for replication.
The oplog (operations log) is a capped collection that stores all write operations applied to the database. It is the fundamental mechanism for replication in MongoDB. Secondaries read from the primary's oplog and apply the same operations locally to stay in sync. Without the oplog, there would be no way to propagate data changes reliably or recover from temporary periods of secondary node disconnection.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A full backup of all data for disaster recovery.
Why it's wrong here
The oplog is not a full database backup; it is an incremental log of changes. While it helps in maintaining replication, it cannot be used alone to restore a database from scratch if the data files are lost. Backups require specialized tools like mongodump or filesystem snapshots for durability.
- ✗
A log of all read operations for performance auditing.
Why it's wrong here
The oplog only tracks write operations that change the data set. Read operations are not logged in the oplog because they do not change the state of the database and therefore do not need to be replicated to other nodes. Audit logs are used for capturing detailed read/write metadata.
- ✓
A capped collection that records all write operations for replication.
Why this is correct
The oplog is indeed a capped collection that stores every modification to the data. Secondaries tail this collection continuously, pulling the operations and applying them to their own datasets. This ensures that all members of the replica set eventually reach the same state as the current primary node.
- ✗
A temporary cache for incoming client write requests.
Why it's wrong here
The oplog is not a write buffer. Write requests are handled by the database engine and applied to the data files, and an entry is then created in the oplog as a result of that operation. It serves as the source of truth for replication, not as a queue.
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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.