How to Start a Session in MongoDB: Simple Guide

To start a session in MongoDB, use the startSession() method on the client or database object. This creates a session object that you can use to perform operations with transaction support or causal consistency.

Syntax

The startSession() method is called on the MongoDB client instance. It returns a session object that you use to group operations.

• client.startSession(): Starts a new session.
• session.startTransaction(): Begins a transaction within the session (optional).
• session.endSession(): Ends the session when done.

javascript

const session = client.startSession();

Example

This example shows how to start a session, perform a simple operation, and then end the session.

javascript

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

async function run() {
  const uri = 'mongodb://localhost:27017';
  const client = new MongoClient(uri);

  try {
    await client.connect();
    const session = client.startSession();

    // Use the session for operations
    const db = client.db('testdb');
    const collection = db.collection('testcollection');

    // Start a transaction (optional)
    session.startTransaction();

    await collection.insertOne({ name: 'Alice' }, { session });

    // Commit the transaction
    await session.commitTransaction();

    console.log('Document inserted within session transaction');

    // End the session
    session.endSession();
  } finally {
    await client.close();
  }
}

run().catch(console.dir);

Output

Document inserted within session transaction

Common Pitfalls

Common mistakes when starting sessions in MongoDB include:

• Not calling session.endSession() after finishing, which can cause resource leaks.
• Using sessions without transactions when transactions are needed for atomicity.
• Forgetting to pass the session object to operations inside the session.

javascript

/* Wrong: Not passing session to operation */
await collection.insertOne({ name: 'Bob' });

/* Right: Pass session to operation */
await collection.insertOne({ name: 'Bob' }, { session });

Quick Reference

Method	Description
startSession()	Starts a new client session
startTransaction()	Begins a transaction in the session
commitTransaction()	Commits the current transaction
abortTransaction()	Aborts the current transaction
endSession()	Ends the session and frees resources

Key Takeaways

Use client.startSession() to begin a session in MongoDB.

Always pass the session object to operations inside the session.

Call session.endSession() to properly close the session.

Use transactions within sessions for atomic multi-operation tasks.

Avoid forgetting to commit or abort transactions when using sessions.