Session and transaction syntax in MongoDB

Introduction

Sessions and transactions help you group multiple database actions so they happen together or not at all. This keeps your data safe and correct.

When you want to update several documents and ensure all updates succeed or none do.

When transferring money between two accounts and you need both debit and credit to happen together.

When inserting related data in multiple collections and want to avoid partial data.

When you want to keep your database consistent during complex operations.

When you want to retry operations safely if something goes wrong.

Syntax

MongoDB

const session = client.startSession();
session.startTransaction();
try {
  // database operations here
  await session.commitTransaction();
} catch (error) {
  await session.abortTransaction();
} finally {
  session.endSession();
}

Use startSession() to begin a session.

Wrap your operations between startTransaction() and commitTransaction() or abortTransaction().

Examples

Starts a session and transaction, inserts a document, commits the transaction, then ends the session.

MongoDB

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

Updates two documents inside a transaction. If any update fails, the transaction is aborted.

MongoDB

const session = client.startSession();
try {
  session.startTransaction();
  await collection.updateOne({name: 'Alice'}, {$set: {age: 30}}, {session});
  await collection.updateOne({name: 'Bob'}, {$set: {age: 25}}, {session});
  await session.commitTransaction();
} catch (e) {
  await session.abortTransaction();
} finally {
  session.endSession();
}

Sample Program

This program starts a session and transaction, inserts a user, updates the user's age, then commits the transaction. If any step fails, it aborts the transaction.

MongoDB

async function runTransaction(client) {
  const session = client.startSession();
  try {
    session.startTransaction();
    const coll = client.db('testdb').collection('users');
    await coll.insertOne({name: 'John', age: 28}, {session});
    await coll.updateOne({name: 'John'}, {$set: {age: 29}}, {session});
    await session.commitTransaction();
    console.log('Transaction committed');
  } catch (error) {
    await session.abortTransaction();
    console.log('Transaction aborted');
  } finally {
    session.endSession();
  }
}

// Assume client is connected MongoClient
runTransaction(client);

Important Notes

Always end your session with endSession() to free resources.

Transactions only work on replica sets or sharded clusters, not standalone servers.

Pass the session object to all operations inside the transaction.

Summary

Sessions group operations for transactions.

Transactions ensure all-or-nothing execution.

Use startSession(), startTransaction(), commitTransaction(), and abortTransaction() properly.

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