Oplog and replication mechanism in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the oplog in MongoDB replication?

The oplog (operation log) is a special capped collection that records all changes to the data in the primary node. Secondary nodes use it to replicate these changes and stay in sync.

beginner

How does MongoDB replication keep data consistent across nodes?

MongoDB replication uses the oplog to copy all write operations from the primary node to secondary nodes. This ensures all nodes have the same data over time.

intermediate

What type of collection is the oplog in MongoDB?

The oplog is a capped collection, meaning it has a fixed size and works like a circular buffer, overwriting old entries when full.

intermediate

What happens if a secondary node falls behind and misses oplog entries?

If a secondary node falls too far behind and oplog entries it needs are overwritten, it must perform a full resync from the primary to catch up.

beginner

Why is the oplog important for MongoDB's replication mechanism?

The oplog allows MongoDB to replicate data efficiently by recording only changes, not full data snapshots, enabling fast and incremental updates to secondaries.

What does the oplog in MongoDB store?

A All write operations on the primary node

B Full copies of the database

C User authentication data

D Backup snapshots

The oplog stores all write operations (inserts, updates, deletes) performed on the primary node.

What type of collection is the oplog in MongoDB?

A Capped collection

B Regular collection

C View

D Temporary collection

The oplog is a capped collection with a fixed size that overwrites old entries when full.

If a secondary node misses oplog entries, what must it do?

A Restart the primary node

B Wait for the primary to resend entries

C Nothing, it will catch up automatically

D Perform a full resync from the primary

If oplog entries are lost, the secondary must do a full resync to get current data.

How does MongoDB replication ensure data consistency?

A By manual data export/import

B By sharing the same disk storage

C By copying oplog entries from primary to secondaries

D By using a single node only

Replication copies oplog entries to keep all nodes consistent.

What is the main advantage of using an oplog for replication?

A Storing full backups

B Efficient incremental data updates

C Encrypting data

D Faster query execution

The oplog records only changes, allowing efficient incremental replication.

Explain how MongoDB uses the oplog to replicate data between primary and secondary nodes.

Think about how changes on one node get copied to others.

Describe what happens when a secondary node falls behind and misses oplog entries in MongoDB replication.

Consider the fixed size of the oplog and its impact.

Practice

1. What is the primary purpose of the oplog in MongoDB replica sets?

easy

A. To manage database indexes

B. To store user authentication details

C. To record all data changes for replication

D. To backup data to external storage

Solution

1. Step 1: Understand the role of oplog in MongoDB

   The oplog is a special log that records every data change in a replica set.

2. Step 2: Connect oplog to replication

   Replication uses the oplog to copy changes from the primary to secondary servers.

3. Final Answer:

   To record all data changes for replication -> Option C

4. Quick Check:

   Oplog records changes [OK]

Hint: Oplog logs changes for syncing replicas [OK]

Common Mistakes:

• Confusing oplog with user data storage
• Thinking oplog manages backups
• Assuming oplog stores indexes

2. Which of the following is the correct command to view the oplog entries in a MongoDB replica set?

easy

A. db.oplog.rs.find()

B. db.oplog.find()

C. db.replica.oplog()

D. db.oplogrs.find()

Solution

1. Step 1: Recall the oplog collection name

   The oplog is stored in the local database under the collection named oplog.rs.

2. Step 2: Identify the correct query syntax

   To view entries, use db.oplog.rs.find() in the mongo shell.

3. Final Answer:

   db.oplog.rs.find() -> Option A

4. Quick Check:

   Correct oplog collection query [OK]

Hint: Oplog collection is oplog.rs in local DB [OK]

Common Mistakes:

• Using wrong collection name like oplog or oplogrs
• Trying to call oplog as a function
• Missing the dot between oplog and rs

3. Given a MongoDB replica set, what will happen if the oplog on the primary server is deleted accidentally?

medium

A. Secondaries will stop replicating and may become stale

B. Secondaries will continue replicating normally

C. Primary will automatically recreate the oplog with old data

D. The replica set will elect a new primary immediately

Solution

1. Step 1: Understand oplog role in replication

   The oplog stores all changes that secondaries use to replicate data.

2. Step 2: Consequence of oplog deletion on primary

   If oplog is deleted, secondaries lose the source of changes and stop replicating, causing stale data.

3. Final Answer:

   Secondaries will stop replicating and may become stale -> Option A

4. Quick Check:

   Oplog missing = replication stops [OK]

Hint: No oplog means no replication updates [OK]

Common Mistakes:

• Assuming oplog auto-recreates with old data
• Thinking secondaries replicate without oplog
• Confusing oplog deletion with primary election

4. You run db.oplog.rs.find().limit(1) on a secondary node and get an error. What is the most likely cause?

medium

A. The oplog collection does not exist on secondaries

B. You need to switch to the local database first

C. The oplog is empty on secondaries

D. You must run the command on the primary node

Solution

1. Step 1: Check database context for oplog

   The oplog is stored in the local database, so you must switch to it before querying.

2. Step 2: Understand error cause

   If you run the command without use local, the shell looks in the wrong database and errors.

3. Final Answer:

   You need to switch to the local database first -> Option B

4. Quick Check:

   Oplog in local DB = switch DB first [OK]

Hint: Always use 'local' DB to query oplog [OK]

Common Mistakes:

• Trying to query oplog in wrong database
• Assuming oplog only exists on primary
• Thinking oplog is empty on secondaries

5. In a MongoDB replica set, how does the oplog help maintain data consistency during failover when the primary goes down?

hard

A. Oplog automatically merges conflicting writes from multiple primaries

B. Oplog backs up data externally to restore after failover

C. Oplog prevents any writes during failover to avoid conflicts

D. Secondaries use oplog entries to catch up and elect a new primary with latest data

Solution

1. Step 1: Understand failover in replica sets

   When primary fails, secondaries elect a new primary to continue operations.

2. Step 2: Role of oplog in failover

   Secondaries use oplog entries to apply all changes and ensure they have the latest data before election.

3. Step 3: Why other options are incorrect

   Oplog does not merge conflicting writes or prevent writes during failover; it also does not backup externally.

4. Final Answer:

   Secondaries use oplog entries to catch up and elect a new primary with latest data -> Option D

5. Quick Check:

   Oplog syncs data for failover [OK]

Hint: Oplog helps secondaries catch up before election [OK]

Common Mistakes:

• Thinking oplog merges conflicting writes
• Assuming oplog blocks writes during failover
• Confusing oplog with backup system