Causal consistency concept in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is causal consistency in MongoDB?

Causal consistency means that operations that are related in cause and effect order are seen by clients in the same order. It ensures that if one operation depends on another, the dependent operation is always seen after the first.

intermediate

How does causal consistency differ from eventual consistency?

Eventual consistency means updates will be seen eventually but not necessarily in order. Causal consistency guarantees that related updates are seen in the correct order, preserving cause and effect.

beginner

Which MongoDB feature supports causal consistency?

MongoDB supports causal consistency through sessions with causal consistency enabled. This allows clients to read their own writes and maintain operation order.

beginner

Why is causal consistency important in distributed databases?

It helps avoid confusing results by ensuring that operations that depend on each other are seen in the right order, making data easier to understand and trust.

intermediate

What happens if causal consistency is not used in a multi-region MongoDB deployment?

Clients might see updates out of order, causing confusion like reading a reply before the original message or missing intermediate changes.

What does causal consistency guarantee in MongoDB?

A Operations are never delayed

B All operations are instantly visible everywhere

C Operations related by cause and effect are seen in order

D Data is always fully synchronized

Causal consistency ensures that operations that depend on each other are seen in the correct order.

Which MongoDB feature enables causal consistency?

A Sessions with causal consistency enabled

B Sharding

C Indexing

D Aggregation pipeline

Sessions with causal consistency enabled allow clients to maintain operation order.

What problem can occur without causal consistency in distributed systems?

A Faster queries

B Seeing dependent operations out of order

C Data loss

D Automatic backups

Without causal consistency, clients may see operations in the wrong order, causing confusion.

Causal consistency is stronger than which consistency model?

A No consistency

B Strong consistency

C Immediate consistency

D Eventual consistency

Causal consistency is stronger than eventual consistency because it preserves operation order.

In MongoDB, what does enabling causal consistency in a session allow?

A Reading your own writes in order

B Faster writes

C Ignoring network delays

D Automatic schema changes

It allows clients to read their own writes and maintain the correct order of operations.

Explain causal consistency in MongoDB and why it matters for applications.

Think about how users expect to see their actions reflected in the right order.

Describe how MongoDB sessions support causal consistency and what happens if it is not used.

Consider the role of sessions in tracking operation order.

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