Write concern and data durability in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the write concern to acknowledge writes on the primary only.

MongoDB

db.collection.insertOne({name: 'Alice'}, {writeConcern: {w: [1])

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

A1

B0

Cmajority

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using w: 0 causes no acknowledgment and no guarantee of durability.

Using w: majority waits for multiple nodes, not just primary.

Setting w: 1 means the write is acknowledged by the primary only.

2fill in blank

medium

Complete the code to ensure the write is acknowledged by the majority of replica set members.

MongoDB

db.collection.insertOne({score: 100}, {writeConcern: {w: "[1]"}})

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

A0

B1

Cmajority

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using w: 1 only waits for primary, not majority.

Using w: 0 means no acknowledgment.

Using w: 'majority' ensures the write is confirmed by most nodes, increasing durability.

3fill in blank

hard

Fix the error in the write concern option to wait for journaling confirmation.

MongoDB

db.collection.insertOne({item: 'book'}, {writeConcern: {w: 1, j: [1])

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

Atrue

Bfalse

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using j: false disables journaling acknowledgment, risking data loss.

Using j: 1 or 0 is invalid; it must be boolean true or false.

Setting j: true waits for the write to be committed to the journal, ensuring durability.

4fill in blank

hard

Fill both blanks to set write concern to wait for 2 nodes and enable journaling.

MongoDB

db.collection.insertOne({status: 'active'}, {writeConcern: {w: [1], j: [2])

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

A2

Btrue

Cfalse

Dmajority

Attempts:

3 left

💡 Hint

Common Mistakes

Using j as a number instead of boolean.

Using w as 'majority' when number 2 is required.

Setting w: 2 waits for acknowledgment from 2 nodes, and j: true waits for journaling.

5fill in blank

hard

Fill all three blanks to set write concern with majority, journaling enabled, and a timeout of 5000 ms.

MongoDB

db.collection.insertOne({task: 'backup'}, {writeConcern: {w: "[1]", j: [2], wtimeout: [3])

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

A1

Btrue

C5000

Dmajority

Attempts:

3 left

💡 Hint

Common Mistakes

Using wtimeout as a boolean or string instead of a number.

Setting j to false disables journaling.

Using w: 'majority' waits for most nodes, j: true enables journaling, and wtimeout: 5000 sets a 5-second timeout.

Practice

1. What does the writeConcern option in MongoDB control?

easy

A. How MongoDB confirms that data is saved to the database

B. The format of data stored in MongoDB

C. The speed of data retrieval from MongoDB

D. The size limit of documents in MongoDB

Solution

1. Step 1: Understand the role of write concern

   Write concern tells MongoDB how to confirm that a write operation is saved and durable.

2. Step 2: Differentiate from other options

   It does not control data format, retrieval speed, or document size.

3. Final Answer:

   How MongoDB confirms that data is saved to the database -> Option A

4. Quick Check:

   Write concern = data save confirmation [OK]

Hint: Write concern = confirmation of data saved [OK]

Common Mistakes:

• Confusing write concern with data format
• Thinking it controls read speed
• Assuming it limits document size

2. Which of the following is the correct way to specify a write concern that waits for acknowledgment from the primary in MongoDB?

easy

A. { writeConcern: { w: -1 } }

B. { writeConcern: { w: 0 } }

C. { writeConcern: { w: 'majority' } }

D. { writeConcern: { w: 1 } }

Solution

1. Step 1: Recall write concern values

   w:1 means wait for acknowledgment from the primary node only.

2. Step 2: Check other options

   w:0 means no acknowledgment; w:'majority' waits for majority nodes; w:-1 is invalid.

3. Final Answer:

   { writeConcern: { w: 1 } } -> Option D

4. Quick Check:

   w:1 = primary ack wait [OK]

Hint: w:1 waits for primary acknowledgment [OK]

Common Mistakes:

• Using w:0 which means no acknowledgment
• Confusing 'majority' with primary only
• Using invalid negative values

3. Consider this MongoDB insert command with write concern:

db.collection.insertOne({name: 'Alice'}, {writeConcern: {w: 'majority', wtimeout: 5000}})

What happens if the majority of nodes do not acknowledge the write within 5 seconds?

medium

A. The write operation succeeds immediately without waiting

B. The write operation retries indefinitely until acknowledged

C. The write operation fails with a timeout error

D. The write operation ignores the write concern and writes anyway

Solution

1. Step 1: Understand w:'majority' and wtimeout

   The write waits for majority nodes to confirm within 5000 ms (5 seconds).

2. Step 2: Outcome if timeout exceeded

   If majority nodes don't respond in time, MongoDB returns a timeout error and the write is considered failed.

3. Final Answer:

   The write operation fails with a timeout error -> Option C

4. Quick Check:

   w:'majority' + wtimeout = fail if no timely ack [OK]

Hint: wtimeout limits wait; timeout causes failure [OK]

Common Mistakes:

• Assuming write succeeds immediately
• Thinking MongoDB retries forever
• Ignoring write concern on timeout

4. You wrote this code to insert a document with write concern:

db.collection.insertOne({item: 'book'}, {writeConcern: {w: 2, wtimeout: '3000'}})

But it throws an error. What is the problem?

medium

A. wtimeout must be a number, not a string

B. w:2 is invalid; only 0,1, or 'majority' allowed

C. insertOne does not accept writeConcern option

D. The document field 'item' is not allowed

Solution

1. Step 1: Check writeConcern syntax

   wtimeout expects a number (milliseconds), not a string.

2. Step 2: Validate other parts

   w:2 is valid if the replica set has at least 2 nodes; insertOne accepts writeConcern; 'item' field is valid.

3. Final Answer:

   wtimeout must be a number, not a string -> Option A

4. Quick Check:

   wtimeout type must be number [OK]

Hint: wtimeout needs number, not quotes [OK]

Common Mistakes:

• Using string for wtimeout
• Thinking w:2 is invalid always
• Assuming insertOne disallows writeConcern

5. You want to ensure that a write is saved to disk on the primary and replicated to at least two secondary nodes before confirming success. Which write concern setting achieves this?

hard

A. { writeConcern: { w: 1, j: true } }

B. { writeConcern: { w: 3, j: true } }

C. { writeConcern: { w: 'majority', j: false } }

D. { writeConcern: { w: 2, j: false } }

Solution

1. Step 1: Understand w and j options

   w:3 means wait for acknowledgment from primary plus two secondaries (total 3 nodes). j:true means wait for write to be committed to journal (disk) on primary.

2. Step 2: Check other options

   B waits only for primary with journaling; C waits for majority but no journaling; D waits for 2 nodes but no journaling.

3. Final Answer:

   { writeConcern: { w: 3, j: true } } -> Option B

4. Quick Check:

   w:3 + j:true = 3 nodes + disk write [OK]

Hint: w:number = nodes ack; j:true = disk commit [OK]

Common Mistakes:

• Ignoring journaling for durability
• Using w:'majority' without knowing node count
• Assuming w:2 means two secondaries only