Read concern and write concern in transactions in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a transaction with a read concern of 'majority'.

MongoDB

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

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

Amajority

Blinearizable

Clocal

Davailable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'local' which reads from the node's local data and may not be majority committed.

Using 'available' which is not a valid read concern level.

The read concern 'majority' ensures the transaction reads data that has been acknowledged by a majority of replica set members.

2fill in blank

medium

Complete the code to set the write concern to 'w: 1' in a transaction.

MongoDB

session.startTransaction({ writeConcern: { w: [1] } });

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

Amajority

B0

C1

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' which means no acknowledgment.

Using 'all' which is not a valid write concern value.

Write concern 'w: 1' means the write must be acknowledged by the primary node only.

3fill in blank

hard

Fix the error in the transaction options to correctly specify read concern level.

MongoDB

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

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

AmajorityLevel

BMAJORITY

Cmajor

Dmajority

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase letters which cause errors.

Using incomplete or incorrect words.

The read concern level must be exactly 'majority' in lowercase.

4fill in blank

hard

Fill both blanks to start a transaction with read concern 'snapshot' and write concern 'majority'.

MongoDB

session.startTransaction({ readConcern: { level: '[1]' }, writeConcern: { w: '[2]' } });

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

Asnapshot

Blocal

Cmajority

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing read concern levels that are not supported in transactions.

Using write concern values that are not strings.

Read concern 'snapshot' provides a consistent view of data during the transaction. Write concern 'majority' ensures writes are acknowledged by most nodes.

5fill in blank

hard

Fill all three blanks to start a transaction with read concern 'local', write concern '1', and specify a max commit time of 1000 milliseconds.

MongoDB

session.startTransaction({ readConcern: { level: '[1]' }, writeConcern: { w: '[2]' }, maxCommitTimeMS: [3] });

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

Amajority

B1

C1000

Dlocal

Attempts:

3 left

💡 Hint

Common Mistakes

Using string quotes around maxCommitTimeMS value.

Confusing read concern levels.

This transaction uses read concern 'local' for reading the node's data, write concern '1' for primary acknowledgment, and sets a max commit time of 1000 milliseconds to limit commit duration.

Practice

1. What does readConcern control in a MongoDB transaction?

easy

A. How fresh or confirmed the data you read is during the transaction

B. How many servers confirm your data changes before commit

C. The speed of the transaction execution

D. The size of the data being written

Solution

1. Step 1: Understand the role of readConcern

   Read concern defines the level of isolation and freshness of data read during a transaction.

2. Step 2: Differentiate from writeConcern

   Write concern controls confirmation of writes, not reads.

3. Final Answer:

   How fresh or confirmed the data you read is during the transaction -> Option A

4. Quick Check:

   readConcern = data freshness [OK]

Hint: Read concern is about data freshness, write concern is about confirmation [OK]

Common Mistakes:

• Confusing readConcern with writeConcern
• Thinking readConcern controls write confirmations
• Assuming readConcern affects transaction speed

2. Which of the following is the correct way to specify writeConcern in a MongoDB transaction?

easy

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

B. { readConcern: { w: 'majority' } }

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

D. { writeConcern: 'readConcern' }

Solution

1. Step 1: Recall writeConcern syntax

   Write concern uses the key w to specify acknowledgement level, e.g., 'majority'.

2. Step 2: Check options for correctness

   { writeConcern: { w: 'majority' } } correctly uses writeConcern: { w: 'majority' }. Others misuse keys or mix readConcern and writeConcern.

3. Final Answer:

   { writeConcern: { w: 'majority' } } -> Option A

4. Quick Check:

   writeConcern uses w key [OK]

Hint: writeConcern uses w, readConcern uses level [OK]

Common Mistakes:

• Using 'r' instead of 'w' for writeConcern
• Mixing readConcern and writeConcern keys
• Passing writeConcern as a string instead of object

3. Given this transaction code snippet, what is the effective read concern level?

const session = client.startSession();
session.startTransaction({ readConcern: { level: 'snapshot' }, writeConcern: { w: 'majority' } });
// transaction operations
await session.commitTransaction();

medium

A. local

B. majority

C. linearizable

D. snapshot

Solution

1. Step 1: Identify readConcern in transaction options

   The transaction is started with readConcern: { level: 'snapshot' }.

2. Step 2: Understand readConcern levels

   'snapshot' means the transaction reads a consistent snapshot of data at start time.

3. Final Answer:

   snapshot -> Option D

4. Quick Check:

   readConcern level = snapshot [OK]

Hint: Check readConcern.level in startTransaction options [OK]

Common Mistakes:

• Confusing writeConcern level with readConcern level
• Assuming default readConcern instead of specified
• Mixing up 'majority' with readConcern levels

4. Identify the error in this transaction code snippet:

const session = client.startSession();
session.startTransaction({ readConcern: { level: 'majority' }, writeConcern: { w: 1 } });
// transaction operations
await session.commitTransaction();

medium

A. writeConcern w: 1 is invalid

B. Missing session.endSession() call

C. readConcern level 'majority' is invalid

D. startTransaction syntax is incorrect

Solution

1. Step 1: Check for session lifecycle management

   The code does not call session.endSession() after commitTransaction(). This is necessary to properly release session resources and avoid leaks.

2. Step 2: Verify other components

   readConcern level 'majority' is valid; writeConcern { w: 1 } is valid (acknowledgement from primary); syntax is correct.

3. Final Answer:

   Missing session.endSession() call -> Option B

4. Quick Check:

   endSession after commit/abort [OK]

Hint: Always call session.endSession() after commitTransaction or abortTransaction [OK]

Common Mistakes:

• Thinking readConcern 'majority' is invalid for transactions
• Assuming writeConcern w:1 is invalid
• Believing session.endSession() is optional

5. You want a transaction that reads the most recent confirmed data and ensures all writes are acknowledged by a majority of servers. Which combination of readConcern and writeConcern should you use?

hard

A. readConcern: 'snapshot', writeConcern: { w: 0 }

B. readConcern: 'local', writeConcern: { w: 1 }

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

D. readConcern: 'linearizable', writeConcern: { w: 'majority' }

Solution

1. Step 1: Understand readConcern 'majority'

   This level reads data confirmed by a majority of servers, ensuring recent confirmed data.

2. Step 2: Understand writeConcern { w: 'majority' }

   This ensures writes are acknowledged by a majority of servers before commit.

3. Step 3: Evaluate other options

   readConcern: 'local', writeConcern: { w: 1 } uses weaker writeConcern; C uses no write acknowledgement; D uses linearizable readConcern which is stricter than needed.

4. Final Answer:

   readConcern: 'majority', writeConcern: { w: 'majority' } -> Option C

5. Quick Check:

   Majority read and write concern = readConcern: 'majority', writeConcern: { w: 'majority' } [OK]

Hint: Use 'majority' for both read and write for confirmed data [OK]

Common Mistakes:

• Choosing writeConcern w:1 for majority confirmation
• Confusing 'snapshot' with 'majority' readConcern
• Ignoring writeConcern w:0 means no acknowledgement