Read concern and write concern in transactions in MongoDB - Step-by-Step Execution

Concept Flow - Read concern and write concern in transactions

Start Transaction

↓

Set Read Concern

↓

Read Data

↓

Set Write Concern

↓

Write Data

↓

Commit Transaction

↓

End Transaction

This flow shows how a transaction sets read and write concerns before reading and writing data, then commits to ensure data consistency.

Execution Sample

MongoDB

session.startTransaction({readConcern: {level: 'snapshot'}, writeConcern: {w: 'majority'}});
const doc = collection.findOne({}, {session});
collection.insertOne({name: 'Alice'}, {session});
await session.commitTransaction();

Starts a transaction with snapshot read concern and majority write concern, reads a document, inserts a new one, then commits.

Execution Table

Step	Action	Read Concern	Write Concern	Result
1	Start transaction	snapshot	majority	Transaction started with specified concerns
2	Read document	snapshot	majority	Reads data consistent with snapshot
3	Insert document	snapshot	majority	Document staged for write
4	Commit transaction	snapshot	majority	Writes applied and committed
5	End transaction	N/A	N/A	Transaction ends, session ready for next

💡 Transaction ends after commit; read and write concerns ensure data consistency and durability.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
readConcern	none	snapshot	snapshot	snapshot	snapshot	N/A
writeConcern	none	majority	majority	majority	majority	N/A
transactionState	none	started	reading	writing	committing	ended
dataRead	none	none	document data	document data	document data	document data
dataWritten	none	none	none	new document staged	new document committed	new document committed

Key Moments - 3 Insights

Why do we set readConcern before reading data in a transaction?

Read concern defines the consistency level of data read. Setting it before reading ensures the transaction sees a stable snapshot of data, as shown in step 2 of the execution table.

What happens if writeConcern is not set in a transaction?

Without write concern, writes may not be guaranteed durable or replicated. Setting writeConcern before writes (step 3) ensures data is safely written and replicated before commit (step 4).

Why must commitTransaction happen after setting concerns and performing operations?

Commit applies all staged writes atomically. It uses the set writeConcern to ensure durability. This final step (step 4) confirms all changes are saved consistently.

Visual Quiz - 3 Questions

Look at the execution table, what is the readConcern level during the data read step?

A majority

B snapshot

C local

D none

💡 Hint

Check the 'Read Concern' column at step 2 in the execution table.

At which step does the transaction apply the writes to the database?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Result' column for when writes are committed in the execution table.

If writeConcern was set to 'w:1' instead of 'majority', how would the write durability change?

A Writes would not be acknowledged at all

B Writes would be more durable, acknowledged by all nodes

C Writes would be less durable, acknowledged by only one node

D Writes would fail immediately

💡 Hint

Refer to the meaning of writeConcern values in the variable_tracker and key moments.

Concept Snapshot

Transactions in MongoDB use readConcern to control data consistency during reads
and writeConcern to ensure durability of writes.
Set readConcern before reading data to get a stable snapshot.
Set writeConcern before writes to guarantee replication.
Commit applies all writes atomically with the specified concerns.

Full Transcript

This visual execution trace shows how MongoDB transactions use readConcern and writeConcern. The transaction starts with readConcern set to 'snapshot' and writeConcern set to 'majority'. This means reads see a consistent snapshot of data, and writes are replicated to a majority of nodes before commit. The transaction reads a document, stages a new document for insertion, then commits. The commit applies all writes atomically with the specified durability. Variables like readConcern and writeConcern remain constant during the transaction. Key moments include why readConcern is set before reading, why writeConcern ensures durability, and why commit applies all changes. The quizzes test understanding of these steps and their effects on data consistency and durability.