Write concern levels (w: 1, majority) in MongoDB - Step-by-Step Execution

Concept Flow - Write concern levels (w: 1, majority)

Client sends write request

↓

Write concern level checked

↓

w: 1

↓

Write to 1 node

↓

Send ack to client

↓

End

The client sends a write request with a write concern level. If w:1, the primary node acknowledges after writing. If w:majority, the write is acknowledged after most nodes confirm.

Execution Sample

MongoDB

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

Two writes: one waits for primary node ack (w:1), the other waits for majority nodes ack (w:majority).

Execution Table

Step	Write Concern	Nodes Written To	Acknowledgment Sent	Client Receives
1	w: 1	Primary node only	After primary writes	Write success ack
2	w: majority	Primary + majority of replicas	After majority write	Write success ack
3	End	-	-	All writes acknowledged

💡 Writes complete after required nodes acknowledge based on write concern level.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
writeConcern	undefined	w: 1	w: majority	w: majority
nodesWritten	0	1 (primary)	majority	majority
acknowledged	false	true	true	true

Key Moments - 2 Insights

Why does w:1 acknowledge faster than w:majority?

Because w:1 waits only for the primary node to confirm the write (see execution_table row 1), while w:majority waits for most nodes to confirm (row 2), which takes longer.

What happens if a replica node is down during w:majority write?

The write waits until a majority of nodes confirm. If not enough nodes are available, the write may fail or timeout (implied by nodesWritten in variable_tracker).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the client receive acknowledgment for w:1?

A Step 1

B Step 2

C Step 3

D Never

💡 Hint

Check the 'Client Receives' column for w:1 in execution_table row 1.

According to variable_tracker, how many nodes are written to after step 2?

A 1 node

B No nodes

C Majority of nodes

D All nodes

💡 Hint

Look at 'nodesWritten' variable after Step 2 in variable_tracker.

If the writeConcern was changed from w:1 to w:majority, what changes in the execution_table?

A Acknowledgment sent after primary node only

B Acknowledgment sent after majority nodes write

C Client receives no acknowledgment

D Write never completes

💡 Hint

Compare rows for w:1 and w:majority in execution_table under 'Acknowledgment Sent'.

Concept Snapshot

Write Concern Levels in MongoDB:
- w:1 means write acknowledged after primary node writes.
- w:majority means write acknowledged after majority of nodes write.
- Higher w means stronger durability but slower acknowledgment.
- Use w:1 for speed, w:majority for safety.
- Write fails if required nodes don't acknowledge.

Full Transcript

This visual execution shows how MongoDB write concern levels work. When a client sends a write with w:1, only the primary node must confirm the write before the client gets an acknowledgment. For w:majority, the write must be confirmed by the primary and most replica nodes before acknowledgment. The execution table traces these steps, showing nodes written to and when the client receives confirmation. The variable tracker shows how writeConcern, nodesWritten, and acknowledged status change during execution. Key moments clarify why w:1 is faster and what happens if replicas are down. The quiz tests understanding of these steps and differences between write concern levels.

Practice

1. What does the write concern w: 1 mean in MongoDB?

easy

A. The write is acknowledged by all servers in the cluster.

B. The write is acknowledged by only one server before returning success.

C. The write is acknowledged by the majority of servers before returning success.

D. The write is not acknowledged by any server.

Solution

1. Step 1: Understand write concern w: 1

   This means the write operation waits for acknowledgment from only one server.

2. Step 2: Compare with other write concerns

   w: majority waits for most servers, so w: 1 is less strict and faster.

3. Final Answer:

   The write is acknowledged by only one server before returning success. -> Option B

4. Quick Check:

   Write concern w: 1 = one server acknowledgment [OK]

Hint: Remember: w:1 means wait for one server only [OK]

Common Mistakes:

• Confusing w:1 with w:majority
• Thinking w:1 waits for all servers
• Assuming w:1 means no acknowledgment

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

easy

A. { writeConcern: { w: majority } }

B. { writeConcern: { w: "1" } }

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

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

Solution

1. Step 1: Identify correct syntax for string values

   Write concern values like 'majority' must be strings enclosed in quotes.

2. Step 2: Check each option

   { writeConcern: { w: 'majority' } } uses quotes correctly. { writeConcern: { w: majority } } misses quotes. { writeConcern: { w: majority'' } } has syntax error with extra quotes. { writeConcern: { w: "1" } } sets w to string '1', not 'majority'.

3. Final Answer:

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

4. Quick Check:

   Use quotes for string write concern values [OK]

Hint: Write concern strings need quotes like 'majority' [OK]

Common Mistakes:

• Omitting quotes around 'majority'
• Using invalid quotes or syntax
• Confusing w:1 string with w:'majority'

3. Consider this MongoDB write operation with write concern:

db.collection.insertOne({name: 'Alice'}, {writeConcern: {w: 'majority'}})

What does this guarantee about the write?

medium

A. The write is confirmed by the majority of replica set members before success.

B. The write is not confirmed by any server before success.

C. The write is confirmed by at least one server before success.

D. The write is confirmed by all servers including secondaries and arbiters.

Solution

1. Step 1: Understand w: 'majority' write concern

   This means the write waits for acknowledgment from most members of the replica set.

2. Step 2: Compare with other options

   The write is confirmed by at least one server before success. is w:1. The write is not confirmed by any server before success. means no acknowledgment. The write is confirmed by all servers including secondaries and arbiters. is incorrect because arbiters do not store data and are not counted.

3. Final Answer:

   The write is confirmed by the majority of replica set members before success. -> Option A

4. Quick Check:

   w: 'majority' means majority confirmation [OK]

Hint: Majority means most data-bearing members confirm write [OK]

Common Mistakes:

• Thinking arbiters count towards majority
• Confusing majority with all servers
• Assuming w:majority is same as w:1

4. You wrote this code:

db.collection.insertOne({item: 'book'}, {writeConcern: {w: majority}})

But it throws an error. What is the problem?

medium

A. The write concern value majority must be a string in quotes.

B. The writeConcern option is misspelled.

C. The insertOne method does not accept writeConcern option.

D. The document to insert is missing required fields.

Solution

1. Step 1: Check syntax of write concern value

   The value majority must be a string, so it needs quotes: 'majority'.

2. Step 2: Verify other parts

   writeConcern is spelled correctly, insertOne accepts writeConcern, and document is valid.

3. Final Answer:

   The write concern value majority must be a string in quotes. -> Option A

4. Quick Check:

   Write concern strings need quotes [OK]

Hint: Always quote string write concern values like 'majority' [OK]

Common Mistakes:

• Forgetting quotes around 'majority'
• Assuming writeConcern is misspelled
• Thinking insertOne disallows writeConcern

5. You want to ensure your MongoDB writes are safe and confirmed by most servers, but you also want to avoid waiting for all servers. Which write concern should you choose and why?

hard

A. Use w: 1 because it waits for only one server, making writes fastest.

B. Use w: 0 because it does not wait, making writes fastest but unsafe.

C. Use w: majority because it waits for most servers, balancing safety and speed.

D. Use w: all because it waits for every server, ensuring maximum safety.

Solution

1. Step 1: Understand the trade-offs

   w: 1 is fast but less safe. w: majority waits for most servers, giving good safety without waiting for all.

2. Step 2: Evaluate options

   w: all waits for every server, which can be slow. w: 0 is unsafe as it doesn't wait for acknowledgment.

3. Final Answer:

   Use w: majority because it waits for most servers, balancing safety and speed. -> Option C

4. Quick Check:

   Majority write concern balances safety and speed [OK]

Hint: Majority write concern balances safety and speed well [OK]

Common Mistakes:

• Choosing w:1 for safety
• Assuming w:all is always best
• Ignoring performance impact of write concern