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.