C100DBA Replication Practice Question

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 oplog is a temporary file used during initial sync to transfer data from the primary to a new secondary.

  Why it's wrong here

  Initial sync uses the oplog to apply operations that occurred during the data copy phase, but the oplog is a persistent capped collection, not a temporary file. It exists on all data-bearing members and continuously records writes. It is not created solely for initial sync.

• ✗

  The oplog is a cache that stores frequently accessed documents to improve read performance.

  Why it's wrong here

  The oplog is not a cache. Caching is handled by the WiredTiger storage engine and the filesystem cache. The oplog's role is to record write operations for replication. It does not store documents for read performance; it stores idempotent operations that can be replayed.

• ✓

  The oplog is a capped collection that records all write operations and is used by secondaries to replicate data from the primary.

  Why this is correct

  The oplog (operations log) is a capped collection in the local database that records all write operations. Secondaries continuously read the oplog from their sync source and apply the operations to their own data sets, ensuring replication. It is essential for maintaining data consistency across replica set members.

• ✗

  The oplog is a log file on disk that records all queries and read operations for auditing purposes.

  Why it's wrong here

  The oplog does not record read operations; it only records write operations. Auditing of read operations is handled by separate mechanisms like the audit log, not the oplog. The oplog's purpose is replication, not auditing. It is a capped collection, not a text log file.