Oplog and replication mechanism in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to query the oplog collection in MongoDB.

MongoDB

db.local.oplog.rs.find([1])

Drag options to blanks, or click blank then click option'

A{ts: 1}

B{op: 'i'}

C{}

D{ns: 'test.collection'}

Attempts:

3 left

💡 Hint

Common Mistakes

Using a filter that restricts results unintentionally.

Leaving the parentheses empty without an object.

The oplog query requires a filter object. An empty filter {} returns all oplog entries.

2fill in blank

medium

Complete the code to find oplog entries for insert operations only.

MongoDB

db.local.oplog.rs.find({op: [1])

Drag options to blanks, or click blank then click option'

A'c'

B'u'

C'd'

D'i'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'u' or 'd' which filter updates or deletes instead of inserts.

Forgetting to put quotes around the operation code.

The op field in the oplog indicates the operation type. 'i' stands for insert.

3fill in blank

hard

Fix the error in the code to query oplog entries after a specific timestamp.

MongoDB

db.local.oplog.rs.find({ts: {$gt: [1])

Drag options to blanks, or click blank then click option'

ATimestamp(1, 0)

BISODate('2023-01-01')

Cnew Date()

DObjectId()

Attempts:

3 left

💡 Hint

Common Mistakes

Using ISODate or Date which do not match the oplog timestamp type.

Using ObjectId which is unrelated here.

The oplog ts field uses MongoDB Timestamp type, so the filter must use a Timestamp object.

4fill in blank

hard

Fill both blanks to create a query that finds oplog entries for updates on a specific namespace.

MongoDB

db.local.oplog.rs.find({op: [1], ns: [2])

Drag options to blanks, or click blank then click option'

A'u'

B'i'

C'test.collection'

D'admin.system'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'i' instead of 'u' for updates.

Using wrong namespace strings.

The op value 'u' filters updates, and ns specifies the namespace like 'test.collection'.

5fill in blank

hard

Fill all three blanks to create a query that finds delete oplog entries on 'logs.events' namespace after a given timestamp.

MongoDB

db.local.oplog.rs.find({op: [1], ns: [2], ts: {$gt: [3])

Drag options to blanks, or click blank then click option'

A'd'

B'logs.events'

CTimestamp(1672531200, 1)

D'admin.system'

Attempts:

3 left

💡 Hint

Common Mistakes

Using string dates instead of Timestamp.

Mixing up operation codes.

Incorrect namespace format.

Use 'd' for delete operations, specify the namespace as 'logs.events', and use a Timestamp object for the timestamp filter.

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