Oplog and replication mechanism in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary role of the oplog in MongoDB replication?

In MongoDB, the oplog is a special collection used in replication. What is its main purpose?

A It stores a log of all write operations to replicate them to secondary nodes.

B It manages network connections between replica set members.

C It holds backup snapshots of the database for recovery.

D It stores user authentication data for replica set members.

💡 Hint

Think about how changes are shared between primary and secondary nodes.

The oplog (operation log) records all write operations on the primary node. Secondary nodes read this log to apply the same changes and stay synchronized.

❓ query_result

intermediate

What does this oplog query return?

Consider this MongoDB query on the oplog collection:

db.oplog.rs.find({op: 'i'}).count()

What does this query return?

A The number of update operations recorded in the oplog.

B The number of insert operations recorded in the oplog.

C The total number of operations in the oplog.

D The number of delete operations recorded in the oplog.

💡 Hint

Look at the filter {op: 'i'} and what 'i' stands for in oplog entries.

In the oplog, the field 'op' indicates the operation type. 'i' means insert, so the query counts all insert operations.

📝 Syntax

advanced

Identify the syntax error in this oplog query

Which option contains a syntax error when querying the oplog for update operations?

db.oplog.rs.find({op: 'u'})

A db.oplog.rs.find({op: 'u'})

B )}'u' :po{(dnif.sr.golpo.bd

C db.oplog.rs.find({op = 'u'})

D b.oplog.rs.find({op: 'u'})

💡 Hint

Check the syntax for specifying query filters in MongoDB.

In MongoDB queries, filters use colon ':' to assign values, not '='. Option C uses '=' which is invalid syntax.

❓ optimization

advanced

How to optimize oplog size for a busy replica set?

You have a busy MongoDB replica set and want to optimize the oplog size to avoid losing operations during replication lag. Which approach is best?

A Set the oplog size to zero to use unlimited space.

B Decrease the oplog size to save disk space.

C Disable the oplog to improve write speed.

D Increase the oplog size to hold more operations before overwriting.

💡 Hint

Think about how oplog size affects replication lag tolerance.

A larger oplog can store more operations, allowing secondaries more time to catch up if they fall behind. Smaller oplogs risk losing operations if secondaries lag too much.

🔧 Debug

expert

Why does this secondary node fail to replicate oplog entries?

A secondary node in a MongoDB replica set is not applying oplog entries and shows this error: Rollback due to missing oplog entries. What is the most likely cause?

A The secondary's oplog window is too small and missed some operations.

B The primary node is offline and not generating oplog entries.

C The secondary has a network firewall blocking all traffic.

D The secondary's oplog collection is corrupted and unreadable.

💡 Hint

Consider what happens if a secondary falls behind too much.

If a secondary falls behind longer than the oplog window (the time span oplog entries are kept), it cannot find the missing operations to catch up and must rollback or resync.