How to Commit a Transaction in MongoDB: Simple Guide

To commit a transaction in MongoDB, you first start a session and a transaction using startSession() and startTransaction(). After performing your operations, call commitTransaction() on the session to save all changes atomically.

Syntax

Here is the basic syntax to commit a transaction in MongoDB using the Node.js driver:

• const session = client.startSession(); - starts a new session.
• session.startTransaction(); - begins the transaction.
• Perform your database operations inside the transaction.
• await session.commitTransaction(); - commits all operations atomically.
• session.endSession(); - ends the session.

javascript

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

Example

This example shows how to transfer money between two accounts using a transaction. It commits the transaction to save both debit and credit operations together.

javascript

const { MongoClient } = require('mongodb');

async function run() {
  const client = new MongoClient('mongodb://localhost:27017');
  await client.connect();
  const session = client.startSession();

  try {
    session.startTransaction();

    const accounts = client.db('bank').collection('accounts');

    // Debit 100 from account A
    await accounts.updateOne({ accountId: 'A' }, { $inc: { balance: -100 } }, { session });

    // Credit 100 to account B
    await accounts.updateOne({ accountId: 'B' }, { $inc: { balance: 100 } }, { session });

    await session.commitTransaction();
    console.log('Transaction committed successfully');
  } catch (error) {
    await session.abortTransaction();
    console.error('Transaction aborted due to error:', error);
  } finally {
    await session.endSession();
    await client.close();
  }
}

run();

Output

Transaction committed successfully

Common Pitfalls

Common mistakes when committing transactions in MongoDB include:

• Not starting a session before starting a transaction.
• Forgetting to pass the session object to all operations inside the transaction.
• Not handling errors properly, which can leave transactions open.
• Trying to commit a transaction after the session has ended.

Always use try/catch/finally to commit or abort transactions and end the session.

javascript

/* Wrong way: Missing session in operations */
const session = client.startSession();
session.startTransaction();
await collection.insertOne({ name: 'test' }); // Missing { session }
await session.commitTransaction();
session.endSession();

/* Right way: Pass session to operations */
const session = client.startSession();
session.startTransaction();
await collection.insertOne({ name: 'test' }, { session });
await session.commitTransaction();
await session.endSession();

Quick Reference

Remember these key points when committing transactions in MongoDB:

• Always start a session with startSession().
• Begin the transaction with startTransaction().
• Pass the session to all database operations inside the transaction.
• Commit the transaction with commitTransaction() to save changes.
• Abort the transaction with abortTransaction() on errors.
• End the session with endSession() to free resources.

Key Takeaways

Start a session and transaction before performing operations to use transactions in MongoDB.

Always pass the session object to all operations inside the transaction to include them in the commit.

Use commitTransaction() to save all changes atomically and abortTransaction() to cancel on errors.

Handle errors with try/catch and always end the session to avoid resource leaks.

Transactions ensure multiple operations succeed or fail together, keeping data consistent.