Causal consistency concept in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable causal consistency in a MongoDB session.

MongoDB

const session = client.startSession({ causalConsistency: [1] });

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

Afalse

Btrue

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using false disables causal consistency.

Passing null or undefined does not enable causal consistency.

Setting causalConsistency to true enables causal consistency for the session.

2fill in blank

medium

Complete the code to start a transaction with causal consistency in MongoDB.

MongoDB

session.startTransaction({ readConcern: { level: [1] } });

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

A"available"

B"local"

C"linearizable"

D"majority"

Attempts:

3 left

💡 Hint

Common Mistakes

Using "local" does not guarantee causal consistency.

Using "linearizable" is stricter but not required here.

Using "majority" read concern ensures causal consistency in transactions.

3fill in blank

hard

Fix the error in the code to maintain causal consistency when reading after a write.

MongoDB

await collection.insertOne({ name: "Alice" }, { session: [1] });
const doc = await collection.findOne({ name: "Alice" }, { session: [1] });

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

Aundefined

Bnull

Csession

Dclient

Attempts:

3 left

💡 Hint

Common Mistakes

Not passing the session to the read operation.

Using different session objects for write and read.

Passing the same session object to both operations ensures causal consistency.

4fill in blank

hard

Fill both blanks to create a session with causal consistency and start a transaction with the correct read concern.

MongoDB

const session = client.startSession({ causalConsistency: [1] });
session.startTransaction({ readConcern: { level: [2] } });

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

Atrue

Bfalse

C"majority"

D"local"

Attempts:

3 left

💡 Hint

Common Mistakes

Setting causalConsistency to false disables causal consistency.

Using "local" read concern does not guarantee causal consistency.

Enable causal consistency with true and use "majority" read concern for transactions.

5fill in blank

hard

Fill all three blanks to read a document with causal consistency after a write in the same session.

MongoDB

const session = client.startSession({ causalConsistency: [1] });
await collection.insertOne({ item: "book" }, { session: [2] });
const result = await collection.findOne({ item: "book" }, { session: [3] });

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

Atrue

Bsession

Cfalse

Dclient

Attempts:

3 left

💡 Hint

Common Mistakes

Not enabling causal consistency in the session.

Using different session objects or none at all.

Enable causal consistency with true and use the same session object for both write and read operations.

Practice

1. What does causal consistency ensure in MongoDB sessions?

easy

A. Data is automatically backed up every hour

B. Data is always encrypted during transmission

C. Data changes are seen in the order they happened

D. Data is compressed to save storage space

Solution

1. Step 1: Understand causal consistency meaning

   Causal consistency means users see data changes in the order they occurred.

2. Step 2: Match with options

   Only Data changes are seen in the order they happened describes this behavior correctly.

3. Final Answer:

   Data changes are seen in the order they happened -> Option C

4. Quick Check:

   Causal consistency = ordered data changes [OK]

Hint: Remember: causal consistency means order matters [OK]

Common Mistakes:

• Confusing causal consistency with encryption
• Thinking it means automatic backups
• Assuming it compresses data

2. Which is the correct way to enable causal consistency in a MongoDB session?

easy

A. session = client.startSession({ causalConsistency: true })

B. session = client.startSession({ consistency: 'strong' })

C. session = client.startSession({ causal: 'enabled' })

D. session = client.startSession({ consistencyLevel: 'causal' })

Solution

1. Step 1: Recall MongoDB session syntax

   To enable causal consistency, use the option causalConsistency: true when starting a session.

2. Step 2: Check options for exact syntax

   Only session = client.startSession({ causalConsistency: true }) uses the correct option name and value.

3. Final Answer:

   session = client.startSession({ causalConsistency: true }) -> Option A

4. Quick Check:

   Enable causalConsistency: true in startSession [OK]

Hint: Look for exact option name 'causalConsistency' set to true [OK]

Common Mistakes:

• Using wrong option names like 'consistency' or 'causal'
• Setting values as strings instead of boolean
• Confusing with other consistency levels

3. Given the following code snippet, what will the output be if causal consistency is enabled?

const session = client.startSession({ causalConsistency: true });
session.startTransaction();
await collection.insertOne({ name: 'Alice' }, { session });
await session.commitTransaction();
const doc = await collection.findOne({ name: 'Alice' });
console.log(doc);

medium

A. null

B. { name: 'Alice' }

C. SyntaxError

D. undefined

Solution

1. Step 1: Understand causal consistency effect

   With causal consistency, the read after commit sees the inserted document.

2. Step 2: Analyze code behavior

   After committing the transaction, the findOne query returns the inserted document.

3. Final Answer:

   { name: 'Alice' } -> Option B

4. Quick Check:

   Committed insert visible with causal consistency [OK]

Hint: Committed writes are visible immediately with causal consistency [OK]

Common Mistakes:

• Expecting null because of delayed visibility
• Thinking a syntax error occurs
• Assuming undefined means no document

4. Identify the error in this code snippet that tries to enable causal consistency:

const session = client.startSession({ causalConsistency: 'true' });
await collection.insertOne({ item: 'book' }, { session });

medium

A. The option value should be boolean true, not string 'true'

B. The session object is missing a required parameter

C. The insertOne method does not accept a session option

D. The collection name is invalid

Solution

1. Step 1: Check option type for causalConsistency

   The option causalConsistency must be a boolean, not a string.

2. Step 2: Identify error cause

   Using 'true' as a string causes the option to be ignored or error.

3. Final Answer:

   The option value should be boolean true, not string 'true' -> Option A

4. Quick Check:

   Boolean true required for causalConsistency [OK]

Hint: Use true without quotes for boolean options [OK]

Common Mistakes:

• Passing 'true' as a string instead of boolean
• Assuming insertOne can't use session
• Thinking collection name affects causal consistency

5. You want to ensure users always see the latest updates in the correct order across multiple MongoDB clients. Which approach best applies causal consistency to achieve this?

hard

A. Disable causal consistency and rely on eventual consistency

B. Use standalone queries without sessions for faster reads

C. Enable causal consistency only on the primary node

D. Start sessions with { causalConsistency: true } and share session tokens between clients

Solution

1. Step 1: Understand causal consistency across clients

   To keep updates ordered for users on different clients, sessions with causal consistency must be used and shared.

2. Step 2: Evaluate options

   Only Start sessions with { causalConsistency: true } and share session tokens between clients correctly uses sessions with causalConsistency enabled and shares session tokens to maintain order.

3. Final Answer:

   Start sessions with { causalConsistency: true } and share session tokens between clients -> Option D

4. Quick Check:

   Share causal consistent sessions for ordered updates [OK]

Hint: Share sessions with causalConsistency true for ordered multi-client views [OK]

Common Mistakes:

• Thinking standalone queries guarantee order
• Assuming enabling only on primary is enough
• Confusing causal consistency with eventual consistency