Why consistency levels matter in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read a document with strong consistency using MongoDB's read concern.

MongoDB

db.collection.findOne({}, {readConcern: {level: '[1]'}})

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

Amajority

Blocal

Cavailable

Dlinearizable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'local' which may read uncommitted data.

Choosing 'available' which is weaker consistency.

The majority read concern ensures that the read reflects the most recent acknowledged writes, providing strong consistency.

2fill in blank

medium

Complete the code to write a document with acknowledged write concern in MongoDB.

MongoDB

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

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Amajority

Bunacknowledged

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' which means no acknowledgment.

Using 'unacknowledged' which is not a valid w value.

Write concern w: 1 means the write is acknowledged by the primary node, ensuring the write is saved.

3fill in blank

hard

Fix the error in the code to ensure a read uses linearizable consistency.

MongoDB

db.collection.find({}, {readConcern: {level: '[1]'}})

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

Alinearizable

Bmajority

Clocal

Davailable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'majority' which is strong but not linearizable.

Using 'local' which can read stale data.

The linearizable read concern guarantees the strongest consistency by reading the most recent acknowledged write.

4fill in blank

hard

Fill both blanks to set a write concern that waits for majority and a timeout of 5000ms.

MongoDB

db.collection.updateOne({name: 'Bob'}, {$set: {age: 30}}, {writeConcern: {w: '[1]', wtimeout: [2])

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

Amajority

B1000

C5000

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using '1' instead of 'majority' for w.

Setting timeout too low or too high.

Setting w: 'majority' waits for most nodes to confirm the write, and wtimeout: 5000 sets a 5-second timeout for this acknowledgment.

5fill in blank

hard

Fill all three blanks to create a read with majority level, a write with w=1, and a write timeout of 2000ms.

MongoDB

db.collection.find({}, {readConcern: {level: '[1]'}}); db.collection.insertOne({item: 'book'}, {writeConcern: {w: [2], wtimeout: [3])

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

Amajority

B1

C2000

Dlocal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'local' for read concern which is weaker.

Setting wtimeout too high or missing it.

The read uses majority for strong consistency, the write uses w: 1 for acknowledgment from primary, and wtimeout: 2000 sets a 2-second timeout.