Session and transaction syntax in MongoDB - Step-by-Step Execution

Concept Flow - Session and transaction syntax

Start Session

↓

Begin Transaction

↓

Perform Operations

↓

Commit Transaction?

No→Abort Transaction

↓

End Session

↓

Commit Transaction

↓

End Session

This flow shows starting a session, beginning a transaction, performing operations, then either committing or aborting the transaction, and finally ending the session.

Execution Sample

MongoDB

const session = db.getMongo().startSession();
session.startTransaction();
db.collection.insertOne({item: 'apple'}, {session});
await session.commitTransaction();
session.endSession();

This code starts a session, begins a transaction, inserts a document within the transaction, commits it, and ends the session.

Execution Table

Step	Action	Session State	Transaction State	Operation Result
1	Start session	Active	No transaction	Session started
2	Start transaction	Active	Transaction started	Transaction begun
3	Insert document with session	Active	Transaction started	Document inserted (pending commit)
4	Commit transaction	Active	Transaction committed	Transaction committed successfully
5	End session	Ended	No transaction	Session ended

💡 Session ended after committing the transaction

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
session.state	None	Active	Active	Active	Active	Ended
transaction.state	None	No transaction	Transaction started	Transaction started	Transaction committed	No transaction

Key Moments - 2 Insights

Why does the insert operation need the session parameter?

The insert operation includes the session parameter to ensure it is part of the ongoing transaction, as shown in step 3 of the execution_table.

What happens if commitTransaction is not called?

If commitTransaction is not called, the transaction remains uncommitted and can be aborted, meaning changes won't be saved, as implied by the branch in the concept_flow.

Visual Quiz - 3 Questions

Look at the execution_table, what is the transaction state after step 2?

A No transaction

B Transaction committed

C Transaction started

D Transaction aborted

💡 Hint

Refer to the 'Transaction State' column in row for step 2 in execution_table

At which step does the session end?

A Step 5

B Step 4

C Step 3

D Step 2

💡 Hint

Check the 'Session State' column in execution_table for when it changes to 'Ended'

If the transaction is aborted instead of committed, what would be the transaction state after step 4?

A Transaction committed

B Transaction aborted

C Transaction started

D No transaction

💡 Hint

Consider the alternative path in concept_flow where commitTransaction is not called

Concept Snapshot

MongoDB sessions start with startSession().
Use startTransaction() to begin a transaction.
Operations must include the session to be transactional.
Commit with commitTransaction(), or abort with abortTransaction().
End session with endSession().

Full Transcript

This visual execution shows how MongoDB sessions and transactions work step-by-step. First, a session is started, then a transaction begins. Operations like inserts include the session to be part of the transaction. After operations, the transaction is either committed or aborted. Finally, the session ends. Tracking session and transaction states helps understand the flow and ensures data consistency.

Practice

1. What is the main purpose of using startSession() in MongoDB?

easy

A. To commit changes to the database

B. To begin a session that groups operations for transactions

C. To abort a running transaction

D. To create a new database

Solution

1. Step 1: Understand the role of sessions

   Sessions in MongoDB group multiple operations so they can be executed as a single unit.

2. Step 2: Identify what startSession() does

   This method starts a new session to enable transactions or grouped operations.

3. Final Answer:

   To begin a session that groups operations for transactions -> Option B

4. Quick Check:

   startSession() starts a session [OK]

Hint: Sessions group operations; startSession() begins it [OK]

Common Mistakes:

• Confusing startSession() with commitTransaction()
• Thinking startSession() commits or aborts transactions
• Assuming startSession() creates a database

2. Which of the following is the correct syntax to start a transaction in a MongoDB session named session?

easy

A. session.beginTransaction()

B. session.transactionStart()

C. session.startTransaction()

D. session.openTransaction()

Solution

1. Step 1: Recall the MongoDB transaction method

   The correct method to start a transaction on a session object is startTransaction().

2. Step 2: Check each option's syntax

   Only session.startTransaction() matches the official MongoDB API.

3. Final Answer:

   session.startTransaction() -> Option C

4. Quick Check:

   startTransaction() starts transactions [OK]

Hint: Use startTransaction() to begin transactions on sessions [OK]

Common Mistakes:

• Using beginTransaction() which is not a MongoDB method
• Mixing method names like transactionStart()
• Using openTransaction() which does not exist

3. Given the following code snippet, what will be the output if the transaction commits successfully?

const session = client.startSession();
session.startTransaction();
await collection.insertOne({name: 'Alice'}, {session});
await session.commitTransaction();
session.endSession();
console.log('Transaction complete');

medium

A. No output

B. Transaction aborted

C. SyntaxError

D. Transaction complete

Solution

1. Step 1: Analyze the transaction flow

   The code starts a session, begins a transaction, inserts a document, commits the transaction, and ends the session.

2. Step 2: Determine the output after commit

   Since commitTransaction() is awaited and no errors occur, the console.log will print 'Transaction complete'.

3. Final Answer:

   Transaction complete -> Option D

4. Quick Check:

   Successful commit logs 'Transaction complete' [OK]

Hint: commitTransaction() success leads to 'Transaction complete' log [OK]

Common Mistakes:

• Assuming commitTransaction() throws error without cause
• Thinking session.endSession() prevents console.log
• Confusing commitTransaction() with abortTransaction()

4. Identify the error in the following MongoDB transaction code:

const session = client.startSession();
session.startTransaction();
await collection.insertOne({item: 'book'});
await session.commitTransaction();
session.endSession();

medium

A. Missing session option in insertOne operation

B. startSession() should be called after startTransaction()

C. commitTransaction() should be called before insertOne

D. endSession() should be called before commitTransaction()

Solution

1. Step 1: Review transaction usage in operations

   Operations inside a transaction must include the session option to be part of it.

2. Step 2: Check the insertOne call

   The insertOne call lacks the {session} option, so it is not part of the transaction.

3. Final Answer:

   Missing session option in insertOne operation -> Option A

4. Quick Check:

   Operations need {session} to join transaction [OK]

Hint: Include {session} in operations inside transactions [OK]

Common Mistakes:

• Calling startSession() after startTransaction()
• Calling commitTransaction() before operations
• Ending session before committing transaction

5. You want to ensure that either all writes in a batch succeed or none do in MongoDB. Which sequence of commands correctly implements this using sessions and transactions?

hard

A. const session = client.startSession(); session.startTransaction(); await collection.insertOne(doc1, {session}); await collection.insertOne(doc2, {session}); await session.commitTransaction(); session.endSession();

B. const session = client.startSession(); await collection.insertOne(doc1); await collection.insertOne(doc2); session.startTransaction(); await session.commitTransaction(); session.endSession();

C. const session = client.startSession(); session.startTransaction(); await collection.insertOne(doc1); await collection.insertOne(doc2); session.abortTransaction(); session.endSession();

D. const session = client.startSession(); await session.startTransaction(); await collection.insertOne(doc1, {session}); await collection.insertOne(doc2, {session}); await session.abortTransaction(); session.endSession();

Solution

1. Step 1: Understand transaction requirements

   All operations must be inside a started transaction and include the session option.

2. Step 2: Evaluate each option

   const session = client.startSession(); session.startTransaction(); await collection.insertOne(doc1, {session}); await collection.insertOne(doc2, {session}); await session.commitTransaction(); session.endSession(); correctly starts a session, starts a transaction, performs writes with session, commits, and ends session. Others either miss session in operations, start transaction late, or abort instead of commit.

3. Final Answer:

   const session = client.startSession(); session.startTransaction(); await collection.insertOne(doc1, {session}); await collection.insertOne(doc2, {session}); await session.commitTransaction(); session.endSession(); -> Option A

4. Quick Check:

   Start session, startTransaction, ops with session, commit, endSession [OK]

Hint: Start session, startTransaction, ops with session, commit, endSession [OK]

Common Mistakes:

• Not passing {session} in operations
• Starting transaction after operations
• Aborting instead of committing for success