Read concern levels (local, majority, snapshot) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the local read concern level in MongoDB guarantee?

The local read concern returns the most recent data on the node that processes the query, without waiting for replication to other nodes. It may include uncommitted or rolled-back data.

beginner

Explain the majority read concern level in MongoDB.

The majority read concern returns data that has been acknowledged by a majority of replica set members, ensuring the data is durable and won't be rolled back.

intermediate

What is the purpose of the snapshot read concern level?

The snapshot read concern provides a consistent view of data at a single point in time across the entire replica set, useful for transactions to read a stable dataset.

beginner

Which read concern level is best for reading the most up-to-date data without waiting for replication?

The local read concern is best for reading the most recent data on the node without waiting for replication.

intermediate

Why would you use majority read concern instead of local?

You use majority to ensure the data read is durable and acknowledged by most nodes, preventing reading data that might be rolled back, unlike local which may read uncommitted data.

Which read concern level returns data acknowledged by most replica set members?

A local

B linearizable

C snapshot

D majority

The majority read concern returns data that has been acknowledged by a majority of replica set members.

What does the snapshot read concern provide?

A A consistent view of data at a single point in time

B The most recent data on a single node

C Data that is not yet replicated

D Data only from the primary node

Snapshot read concern provides a consistent view of data at a single point in time across the replica set.

Which read concern level might return data that could be rolled back?

A local

B snapshot

C majority

D linearizable

Local read concern may return data that is not yet replicated and could be rolled back.

For a transaction requiring a stable dataset, which read concern is recommended?

A majority

B snapshot

C local

D available

Snapshot read concern is recommended for transactions to read a stable, consistent dataset.

Which read concern level does NOT wait for replication to other nodes?

A majority

B linearizable

C local

D snapshot

Local read concern returns data immediately from the node without waiting for replication.

Describe the differences between local, majority, and snapshot read concern levels in MongoDB.

Think about data freshness, durability, and consistency.

When would you choose majority read concern over local in a MongoDB application?

Consider the risk of reading uncommitted or unstable data.

Practice

1. Which MongoDB read concern level provides the fastest read but may return data that is not yet acknowledged by majority of nodes?

easy

A. local

B. majority

C. snapshot

D. linearizable

Solution

1. Step 1: Understand read concern levels

   local returns the most recent data on the node but does not wait for replication.

2. Step 2: Compare with other levels

   majority waits for data to be acknowledged by majority nodes, snapshot is for transactions, and linearizable is stricter and slower.

3. Final Answer:

   local -> Option A

4. Quick Check:

   Fastest read = local [OK]

Hint: Fastest read is always local [OK]

Common Mistakes:

• Confusing local with majority
• Thinking snapshot is fastest
• Assuming linearizable is default

2. Which of the following is the correct syntax to set read concern to majority in a MongoDB query?

easy

A. db.collection.find.readConcern('majority')

B. db.collection.find().readConcern = 'majority'

C. db.collection.find().readConcern('majority')

D. db.collection.find().withReadConcern('majority')

Solution

1. Step 1: Recall MongoDB read concern syntax

   The correct method to set read concern on a query is readConcern('level').

2. Step 2: Check options for syntax correctness

   Some options look similar, but one omits parentheses after find. The correct chaining is db.collection.find().readConcern('majority').

3. Final Answer:

   db.collection.find().readConcern('majority') -> Option C

4. Quick Check:

   Correct method call = db.collection.find().readConcern('majority') [OK]

Hint: Use .readConcern('level') after find() [OK]

Common Mistakes:

• Using assignment instead of method call
• Using non-existent method names
• Placing parentheses incorrectly

3. Given a MongoDB transaction using snapshot read concern, what will be the behavior if a document is updated after the transaction starts but before it reads the document?

medium

A. The transaction reads the updated document reflecting the change.

B. The transaction reads the document state as it was at the start of the transaction.

C. The transaction throws an error due to data inconsistency.

D. The transaction reads the document from the primary node only.

Solution

1. Step 1: Understand snapshot read concern in transactions

   snapshot ensures a consistent view of data as of the start of the transaction.

2. Step 2: Analyze document update timing

   Since the document was updated after the transaction started, the transaction will not see this change.

3. Final Answer:

   The transaction reads the document state as it was at the start of the transaction. -> Option B

4. Quick Check:

   snapshot = consistent start view [OK]

Hint: Snapshot reads see data as of transaction start [OK]

Common Mistakes:

• Assuming transaction sees latest updates
• Thinking snapshot causes errors on updates
• Confusing snapshot with majority read concern

4. You wrote this code snippet to set read concern to majority but it throws an error:

db.collection.find().readConcern = 'majority'

What is the error and how to fix it?

medium

A. No error, code is correct.

B. Error: readConcern must be set on the client, not query.

C. Error: 'majority' is not a valid read concern level.

D. Error: readConcern is a method, use readConcern('majority') instead.

Solution

1. Step 1: Identify the error cause

   Assigning a string to readConcern property is invalid because it is a method, not a property.

2. Step 2: Correct the syntax

   Use method call syntax: readConcern('majority') to set the read concern level.

3. Final Answer:

   Error: readConcern is a method, use readConcern('majority') instead. -> Option D

4. Quick Check:

   Use method call, not assignment [OK]

Hint: Use parentheses for readConcern, not assignment [OK]

Common Mistakes:

• Assigning string instead of calling method
• Setting read concern on wrong object
• Using invalid read concern levels

5. You want to ensure your MongoDB transaction reads a consistent snapshot of data and also that your reads outside transactions see only data confirmed by majority of nodes. Which read concern levels should you use respectively?

hard

A. snapshot for transactions, majority for outside reads

B. local for transactions, snapshot for outside reads

C. majority for transactions, local for outside reads

D. snapshot for transactions, local for outside reads

Solution

1. Step 1: Identify read concern for transactions

   snapshot read concern ensures consistent view inside transactions.

2. Step 2: Identify read concern for outside transactions

   majority ensures reads see data confirmed by majority of nodes, providing safety.

3. Final Answer:

   snapshot for transactions, majority for outside reads -> Option A

4. Quick Check:

   Transaction = snapshot, Outside = majority [OK]

Hint: Use snapshot in transactions, majority outside [OK]

Common Mistakes:

• Using local inside transactions
• Confusing majority with snapshot
• Using local for safe reads